1. A weak current measuring device is characterized by comprising an I-V conversion and amplification circuit, an analog-to-digital conversion circuit, a single chip microcomputer and a display which are connected in sequence; wherein the content of the first and second substances,

the I-V conversion and amplification circuit is used for selecting a corresponding I-V conversion and amplification circuit to convert the weak current signal into a weak voltage signal and perform noise suppression and signal amplification processing according to the type of the weak current signal to be detected; the types of the weak current signals to be detected comprise nano-ampere (nA) level weak current signals, pico-ampere (pA) level weak current signals and flying-ampere (fA) level weak current signals; the I-V conversion and amplification circuit comprises an nA-level I-V conversion and amplification circuit for processing nA-level weak current signals, an pA-level I-V conversion and amplification circuit for processing pA-level weak current signals and an fA-level I-V conversion and amplification circuit for processing fA-level weak current signals;

the analog-to-digital conversion circuit is used for converting the voltage signals subjected to noise suppression and signal amplification into digital signals;

the singlechip is used for analyzing the digital signal to obtain a voltage value and obtaining the current value of the weak current signal to be detected in a preset voltage and weak current mapping table according to the voltage value;

and the display is used for displaying the current value of the weak current signal to be detected and the corresponding voltage value.

2. The weak current measuring device according to claim 1, wherein the nA-stage I-V converting and amplifying circuit includes a nA-stage I-V converting circuit, a first inverting proportional voltage amplifying circuit, and a first common proportional voltage amplifying circuit connected in this order; wherein the content of the first and second substances,

the nA-stage I-V conversion circuit comprises a first operational amplifier IC1 and a first feedback resistor R1; the non-inverting input end of the first operational amplifier IC1 is grounded, the inverting input end is connected to a nA-level weak current signal to be detected, and the output end is connected with the input end of the first inverting proportional voltage amplifying circuit through a resistor R2 and is also reversely connected with the inverting input end of the first inverting proportional voltage amplifying circuit through a first feedback resistor R1;

the first inverting proportional voltage amplifying circuit comprises a second operational amplifier IC2 and a second feedback resistor R3; the non-inverting input terminal of the second operational amplifier IC2 is grounded through a first voltage-dividing resistor R4, the inverting input terminal of the second operational amplifier IC2 is connected to the output terminal of the first operational amplifier IC1 in the nA-level I-V conversion circuit through the resistor R2, and the output terminal of the second operational amplifier IC2 is connected to the inverting input terminal of the first voltage follower through a resistor R5 and is also connected to the inverting input terminal thereof in an inverting manner through the second feedback resistor R3;

the first same-phase voltage amplifying circuit comprises a third operational amplifier IC3 and a third feedback resistor R7; the non-inverting input end of the third operational amplifier IC3 is connected to the output end of the second operational amplifier IC2 in the inverse proportional voltage amplifying circuit through a resistor R5, the inverting input end of the third operational amplifier IC3 is grounded through a second voltage-dividing resistor R6, and the output end of the third operational amplifier IC3 is connected to the input end of the analog-to-digital converting circuit and is also connected to the inverting input end of the third operational amplifier IC7 in the opposite direction.

3. The weak current measuring device as claimed in claim 2, wherein the first operational amplifier IC1, the second operational amplifier IC2 and the third operational amplifier IC3 have the same structure, and are all AD8603 type operational amplifier chips; the amplification factors of the first inverse proportion voltage amplification circuit and the first same proportion voltage amplification circuit are both 10 times.

4. The weak current measuring device as claimed in claim 1, wherein the pA-stage I-V converting and amplifying circuit includes a pA-stage I-V converting circuit, a second reverse phase proportional voltage amplifying circuit, a first voltage following circuit and a second in-phase proportional voltage amplifying circuit connected in sequence; wherein the content of the first and second substances,

the pA-level I-V conversion circuit comprises a fourth operational amplifier IC4 and a first T-shaped feedback network; the non-inverting input end of the fourth operational amplifier IC4 is grounded, the inverting input end of the fourth operational amplifier IC4 is connected to a pA-level weak current signal to be detected, and the output end of the fourth operational amplifier IC4 is connected to the input end of the second inverting proportional voltage amplifying circuit through a resistor R11 and is also reversely connected to the inverting input end of the fourth operational amplifier IC 3526 through the first T-shaped feedback network; the first T-shaped feedback network comprises a resistor R8, a resistor R9 and a first capacitor C9; one end of the resistor R8 is grounded through a resistor R10, and the other end of R8 is connected with one end of the first capacitor C9 and is connected with the inverting input end of the fourth operational amplifier IC 4; one end of the resistor R9 is grounded through a resistor R10, and the other end of R9 is connected with the other end of the first capacitor C9 and is connected with the output end of the fourth operational amplifier IC 4;

the second inverting proportional voltage amplifying circuit comprises a fifth operational amplifier IC5 and a fourth feedback resistor R12; the non-inverting input end of the fifth operational amplifier IC5 is grounded through a third voltage dividing resistor R14, the inverting input end of the fifth operational amplifier IC5 is connected with the output end of a fourth operational amplifier IC4 in the pA-level I-V conversion circuit through a resistor R11, and the output end of the fifth operational amplifier IC5 is connected with the input end of the first voltage follower circuit through a first isolation resistor R13 and is also connected with the inverting input end of the fifth operational amplifier IC 3538 in an inverting manner through a fourth feedback resistor R12;

the first voltage follower circuit comprises a first isolation resistor R13 and a sixth operational amplifier IC 6; one end of the first isolation resistor R13 is connected with the output end of a fifth operational amplifier IC5 in the second inverse proportion voltage amplifying circuit, and the other end of R13 is connected with the non-inverting input end of the sixth operational amplifier IC 6; the output end of the sixth operational amplifier IC6 is connected with the input end of the second in-phase proportional voltage amplifying circuit through a resistor R15 and is also reversely connected with the reverse phase input end of the sixth operational amplifier IC 6;

the second in-phase proportional voltage amplifying circuit comprises a seventh operational amplifier IC7 and a fifth feedback resistor R17; the non-inverting input end of the seventh operational amplifier IC7 is connected to the output end of a sixth operational amplifier IC6 in the first voltage follower circuit through a resistor R15, the inverting input end of the seventh operational amplifier IC7 is grounded through a fourth voltage-dividing resistor R16, and the output end of the seventh operational amplifier IC7 is connected to the input end of the analog-to-digital conversion circuit and is also connected to the inverting input end of the seventh operational amplifier IC7 in the opposite direction through a fifth feedback resistor R17.

5. The weak current measuring device according to claim 4, wherein said first voltage follower circuit further comprises a first low-pass filter circuit; the first low-pass filter circuit is composed of a second capacitor C10, one end of the second capacitor C10 is grounded, and the other end of the second capacitor C10 is connected between the first isolation resistor R13 and the non-inverting input terminal of the sixth operational amplifier IC 6.

6. The weak current measuring device as claimed in claim 5, wherein said fourth operational amplifier IC4, said fifth operational amplifier IC5, said sixth operational amplifier IC6 and said seventh operational amplifier IC7 have the same structure, and are all AD8603 type operational amplifiers; the amplification factors of the second reverse phase proportional voltage amplification circuit and the second in-phase proportional voltage amplification circuit are both 10 times; in the first T-type feedback network, the resistor R8 is 200M Ω, the resistor R9 is 9.9k Ω, and the first capacitor C9 is 5 pF.

7. The weak current measuring device as claimed in claim 1, wherein said fA-stage I-V converting and amplifying circuit includes an fA-stage I-V converting circuit, a third inverse proportional voltage amplifying circuit, a second voltage follower circuit and a third in-phase proportional voltage amplifying circuit connected in sequence; wherein the content of the first and second substances,

the fA stage I-V conversion circuit comprises an eighth operational amplifier IC8 and a second T-shaped feedback network; the non-inverting input end of the eighth operational amplifier IC8 is grounded, the inverting input end of the IC8 is connected to the fA-level weak current signal to be detected, and the output end of the IC8 is connected to the input end of the third inverting proportional voltage amplifying circuit through a resistor R21 and is also reversely connected to the inverting input end of the third inverting proportional voltage amplifying circuit through the second T-type feedback network; the second T-shaped feedback network comprises a resistor R18, a resistor R19 and a third capacitor C11; one end of the resistor R18 is grounded through a resistor R20, and the other end of the resistor R18 is connected with one end of the third capacitor C11 and is connected with the inverting input end of the eighth operational amplifier IC 8; one end of the resistor R19 is grounded through a resistor R20, and the other end of R19 is connected with the other end of the third capacitor C11 and is connected with the output end of the eighth operational amplifier IC 8;

the third inverting proportional voltage amplifying circuit comprises a ninth operational amplifier IC9 and a sixth feedback resistor R23; the non-inverting input terminal of the ninth operational amplifier IC9 is grounded through a fifth voltage-dividing resistor R22, the inverting input terminal of the ninth operational amplifier IC9 is connected to the output terminal of the eighth operational amplifier IC8 in the fA-stage I-V conversion circuit through a resistor R21, and the output terminal of the ninth operational amplifier IC9 is connected to the input terminal of the second voltage follower circuit through a resistor R24 and is also connected in reverse to the inverting input terminal thereof through the sixth feedback resistor R23;

the second voltage follower circuit comprises a second isolation resistor R24 and a tenth operational amplifier IC 10; one end of the second isolation resistor R24 is connected to the output end of the ninth operational amplifier IC9 in the third inverse proportional voltage amplifying circuit, and the other end is connected to the non-inverting input end of the tenth operational amplifier IC 10; the output end of the tenth operational amplifier IC10 is connected with the input end of the third in-phase proportional voltage amplifying circuit through a resistor R25 and is also reversely connected with the reverse phase input end of the tenth operational amplifier IC 10;

the third in-phase proportional voltage amplifying circuit comprises an eleventh operational amplifier IC11 and a seventh feedback resistor R27; the non-inverting input end of the eleventh operational amplifier IC11 is connected to the output end of a tenth operational amplifier IC10 in the second voltage follower circuit through a resistor R25, the inverting input end of the eleventh operational amplifier IC11 is grounded through a sixth voltage-dividing resistor R26, and the output end of the eleventh operational amplifier IC11 is connected to the input end of the analog-to-digital conversion circuit and is also connected to the inverting input end of the circuit in the opposite direction through a seventh feedback resistor R27.

8. The weak current measuring device according to claim 7, wherein said second voltage follower circuit further comprises a second low-pass filter circuit; the second low-pass filter circuit is composed of a fourth capacitor C12, one end of the fourth capacitor C12 is grounded, and the other end is connected between the second isolation resistor R24 and the non-inverting input terminal of the tenth operational amplifier IC 10.

9. The weak current measuring device according to claim 8, wherein said eighth operational amplifier IC8, said ninth operational amplifier IC9, said tenth operational amplifier IC10 and said eleventh operational amplifier IC11 are identical in structure and are all AD549 type operational amplifiers; the amplification factors of the third reverse-phase proportional voltage amplifying circuit and the third in-phase proportional voltage amplifying circuit are both 10 times; in the second T-type feedback network, the resistor R18 is 200M Ω, the resistor R19 is 99k Ω, and the third capacitor C11 is 5 pF.

10. The weak current measuring device according to claim 1, wherein the single chip microcomputer is an ATC89C51 type CPU chip.

Background

In recent years, the weak current detection technology has been widely applied to professional technologies such as medical field, measurement technology field, signal processing field, and circuit design field, and has promoted the vigorous development of the related professional technologies to some extent, such as medical science, physics, chemistry, electrochemistry, and biomedicine, which all develop new vitality due to the great progress of the weak current signal detection technology.

The weak current signal detection technology is a scientific technology which focuses on reducing the influence of noise on a detection signal and increasing the signal to noise ratio, and the recently appeared scientific technology is to use electronic knowledge and a signal measurement method to find out effective small current signals from a plurality of invalid signals. When detecting weak current signals, the weak current is difficult to directly detect, so that the weak current signals are generally converted into other signals which are easier to measure, and the magnitude of the weak current signals can be reversely deduced simply through the signals which are easier to measure.

However, when the weak current signal is detected, the noise can generate great interference on the weak current signal, which can affect the normal detection of the signal. The amplifying circuit not only amplifies weak current signals to be detected, but also amplifies various noises in the circuit, so that the accuracy of the final result of weak current detection is influenced. Therefore, it is very critical to reduce noise and precisely amplify the weak current signal to be measured in the detection process of the weak current signal. The noise of the weak current signal is very small when the weak current signal is detected, and the signal is weaker than the noise, so that the effective current signal cannot be detected only by increasing the signal, and the current signal to be detected can be detected only by increasing the size of the weak current signal under the condition of effectively reducing the noise. Therefore, the root cause, the propagation path and the propagation rule of the noise generation are analyzed, so that the noise can be specifically and effectively suppressed by adopting various effective methods.

At present, the price of the existing weak current measuring device is generally higher, and the noise interference is larger in the measuring process, so that the measuring has errors, and the measuring precision is low.

Disclosure of Invention

The technical problem to be solved by the embodiments of the present invention is to provide a weak current measuring device, which can solve the problem of measurement error caused by noise interference in the existing weak current measuring device, improve the measurement accuracy, and is low in price.

In order to solve the above technical problem, an embodiment of the present invention provides a weak current measuring device, including a current intensity-voltage (I-V) converting and amplifying circuit, an analog-to-digital converting circuit, a single chip microcomputer, and a display, which are connected in sequence; wherein the content of the first and second substances,

the current intensity-voltage (I-V) conversion and amplification circuit is used for selecting a corresponding I-V conversion and amplification circuit to convert a current signal and carrying out noise suppression and signal amplification processing according to the type of a weak current signal to be detected; the types of the weak current signals to be detected comprise nano-ampere (nA) level weak current signals, pico-ampere (pA) level weak current signals and flying-ampere (fA) level weak current signals; the I-V conversion and amplification circuit comprises an nA-level I-V conversion and amplification circuit for processing nA-level weak current signals, an pA-level I-V conversion and amplification circuit for processing pA-level weak current signals and an fA-level I-V conversion and amplification circuit for processing fA-level weak current signals;

the analog-to-digital conversion circuit is used for converting the voltage signals subjected to noise suppression and signal amplification into digital signals;

the singlechip is used for analyzing the digital signal to obtain a voltage value and obtaining the current value of the weak current signal to be detected in a preset voltage and weak current mapping table according to the voltage value;

and the display is used for displaying the current value of the weak current signal to be detected and the corresponding voltage value.

The nA-level I-V conversion and amplification circuit comprises an nA-level I-V conversion circuit, a first inverse proportional voltage amplification circuit and a first same proportional voltage amplification circuit which are connected in sequence; wherein the content of the first and second substances,

the nA-stage I-V conversion circuit comprises a first operational amplifier IC1 and a first feedback resistor R1; the non-inverting input end of the first operational amplifier IC1 is grounded, the inverting input end of the first operational amplifier IC1 is connected to nA-level weak current signals to be detected, and the output end of the first operational amplifier IC1 is connected to the input end of the first inverting proportional voltage amplifying circuit through a resistor R2 and is also reversely connected to the inverting input end of the first operational amplifier IC1 through the first feedback resistor R1;

the first inverting proportional voltage amplifying circuit comprises a second operational amplifier IC2 and a second feedback resistor R3; the non-inverting input end of the second operational amplifier IC2 is grounded through a first voltage-dividing resistor R4, the inverting input end of the second operational amplifier IC2 is connected with the output end of a first operational amplifier IC1 in the nA-stage I-V conversion circuit through a resistor R2, and the output end of the second operational amplifier IC2 is connected with the input end of the first same-phase comparison voltage amplification circuit through a resistor R5 and is also reversely connected with the inverting input end of the second operational amplifier IC2 through a second feedback resistor R3;

the first same-phase voltage amplifying circuit comprises a third operational amplifier IC3 and a third feedback resistor R7; the non-inverting input end of the third operational amplifier IC3 is connected to the output end of the second operational amplifier IC2 in the inverse proportional voltage amplifying circuit through a resistor R5, the inverting input end is grounded through a second voltage-dividing resistor R6, and the output end of the third operational amplifier IC3 is connected to the input end of the analog-to-digital conversion circuit and is also connected to the inverting input end of the third operational amplifier IC7 in the opposite direction.

The first operational amplifier IC1, the second operational amplifier IC2 and the third operational amplifier IC3 are identical in structure and are AD8603 type operational amplifier chips; the amplification factors of the first inverse proportion voltage amplification circuit and the first same proportion voltage amplification circuit are both 10 times.

The pA-level I-V conversion and amplification circuit comprises a pA-level I-V conversion circuit, a second inverse-phase proportional voltage amplification circuit, a first voltage following circuit and a second in-phase proportional voltage amplification circuit which are connected in sequence; wherein the content of the first and second substances,

the pA-level I-V conversion circuit comprises a fourth operational amplifier IC4 and a first T-shaped feedback network; the non-inverting input end of the fourth operational amplifier IC4 is grounded, the inverting input end of the fourth operational amplifier IC4 is connected to a pA-level weak current signal to be detected, and the output end of the fourth operational amplifier IC4 is connected to the input end of the second inverting proportional voltage amplifying circuit through a resistor R11 and is also reversely connected to the inverting input end of the fourth operational amplifier IC4 through the first T-shaped feedback network; the first T-shaped feedback network comprises a resistor R8, a resistor R9 and a first capacitor C9; one end of the resistor R8 is grounded through a resistor R10, and the other end of the resistor R8 is connected with one end of the first capacitor C9 and is connected with the inverting input end of the fourth operational amplifier IC 4; one end of the resistor R9 is grounded through a resistor R10, and the other end of the resistor R9 is connected with the other end of the first capacitor C9 and is connected with the output end of the fourth operational amplifier IC 4;

the second inverting proportional voltage amplifying circuit comprises a fifth operational amplifier IC5 and a fourth feedback resistor R12; the non-inverting input end of the fifth operational amplifier IC5 is grounded through a third voltage-dividing resistor R14, the inverting input end of the fifth operational amplifier IC5 is connected with the output end of a fourth operational amplifier IC4 in the pA-level I-V conversion circuit through a resistor R11, and the output end of the fifth operational amplifier IC5 is connected with the input end of the first voltage follower circuit through a first isolation resistor R13 and is also reversely connected with the inverting input end of the fifth operational amplifier IC5 through a fourth feedback resistor R12;

the first voltage follower circuit comprises the first isolation resistor R13 and a sixth operational amplifier IC 6; one end of the first isolation resistor R13 is connected to the output end of a fifth operational amplifier IC5 in the second inverse proportion voltage amplifying circuit, and the other end is connected to the non-inverting input end of the sixth operational amplifier IC6 and is also grounded through a second capacitor C10; the output end of the sixth operational amplifier IC6 is connected with the input end of the second in-phase proportional voltage amplifying circuit through a resistor R15, and is also reversely and directly connected with the reverse phase input end of the sixth operational amplifier IC 6;

the second in-phase proportional voltage amplifying circuit comprises a seventh operational amplifier IC7 and a fifth feedback resistor R17; the non-inverting input end of the seventh operational amplifier IC7 is connected to the output end of a sixth operational amplifier IC6 in the first voltage follower circuit through a resistor R15, the inverting input end is grounded through a fourth voltage divider resistor R16, and the output end of the seventh operational amplifier IC7 is connected to the input end of the analog-to-digital conversion circuit and is also connected to the inverting input end of the seventh operational amplifier IC 3838 in the opposite direction through a fifth feedback resistor R17.

The first voltage follower circuit further comprises a first low-pass filter circuit; the first low-pass filter circuit is composed of a second capacitor C10, one end of the second capacitor C10 is grounded, and the other end is connected between the first isolation resistor R13 and the non-inverting input terminal of the sixth operational amplifier IC 6.

The fourth operational amplifier IC4, the fifth operational amplifier IC5, the sixth operational amplifier IC6 and the seventh operational amplifier IC7 have the same structure and are AD8603 type operational amplifier chips; in the first T-type feedback network, the resistor R8 is 200M Ω, the resistor R9 is 9.9k Ω, and the first capacitor C9 is 5 pF.

The fA-level I-V conversion and amplification circuit comprises an fA-level I-V conversion circuit, a third inverse proportional voltage amplification circuit, a second voltage follower circuit and a third in-phase proportional voltage amplification circuit which are connected in sequence; wherein the content of the first and second substances,

the fA stage I-V conversion circuit comprises an eighth operational amplifier IC8 and a second T-shaped feedback network; the non-inverting input end of the eighth operational amplifier IC8 is grounded, the inverting input end of the eighth operational amplifier IC8 is connected to the fA-level weak current signal to be detected, and the output end of the eighth operational amplifier IC8 is connected to the input end of the third inverting proportional voltage amplifying circuit through a resistor R21 and is also reversely connected to the inverting input end of the eighth operational amplifier IC8 through the second T-type resistor network; the second T-shaped feedback network comprises a resistor R18, a resistor R19 and a third capacitor C11; one end of the resistor R18 is grounded through a resistor R20, and the other end of the resistor R18 is connected with one end of the third capacitor C11 and is connected with the inverting input end of the eighth operational amplifier IC 8; one end of the resistor R19 is grounded through a resistor R20, and the other end of the resistor R19 is connected with the other end of the third capacitor C11 and is connected with the output end of the eighth operational amplifier IC 8;

the third inverting proportional voltage amplifying circuit comprises a ninth operational amplifier IC9 and a sixth feedback resistor R23; the non-inverting input end of the ninth operational amplifier IC9 is grounded through a fifth voltage-dividing resistor R22, the inverting input end of the ninth operational amplifier IC9 is connected to the output end of the eighth operational amplifier IC8 in the fA-level I-V conversion circuit through a resistor R21, and the output end of the ninth operational amplifier IC9 is connected to the input end of the second voltage follower circuit through a second isolation resistor R24 and is also reversely connected to the inverting input end thereof through a sixth feedback resistor R23;

the second voltage follower circuit comprises a second isolation resistor R24 and a tenth operational amplifier IC 10; one end of the second isolation resistor R24 is connected to the output end of a ninth operational amplifier IC9 in the third inverse proportional voltage amplifying circuit, and the other end is connected to the non-inverting input end of the tenth operational amplifier IC10 and is also grounded through a third capacitor C12; the output end of the tenth operational amplifier IC10 is connected with the input end of the third in-phase proportional voltage amplifying circuit through a resistor R25 and is also reversely connected with the reverse phase input end of the tenth operational amplifier IC 10;

the second voltage follower circuit further comprises a second low-pass filter circuit; the second low-pass filter circuit is composed of a third capacitor C12, one end of the third capacitor C12 is grounded, and the other end is connected between the second isolation resistor R24 and the non-inverting input terminal of the tenth operational amplifier IC 10.

The third in-phase proportional voltage amplifying circuit comprises an eleventh operational amplifier IC11 and a seventh feedback resistor R27; the non-inverting input end of the eleventh operational amplifier IC11 is connected to the output end of a tenth operational amplifier IC10 in the second voltage follower circuit through a resistor R25, the inverting input end is grounded through a sixth voltage-dividing resistor R26, and the output end of the eleventh operational amplifier IC11 is connected to the input end of the analog-to-digital conversion circuit and is also connected to the inverting input end of the eleventh operational amplifier IC11 in the opposite direction through a seventh feedback resistor R27.

The eighth operational amplifier IC8, the ninth operational amplifier IC9, the tenth operational amplifier IC10 and the eleventh operational amplifier IC11 have the same structure and are AD549 type operational amplifier chips; in the second T-type feedback network, the resistor R18 is 200M Ω, the resistor R19 is 99k Ω, and the third capacitor C11 is 5 pF.

The single chip microcomputer is an ATC89C51 type CPU chip.

The embodiment of the invention has the following beneficial effects:

the invention carries out weak current detection based on a transimpedance method, so that a weak current signal to be detected is converted into a voltage signal which is easier to measure, the weak current signal is reversely deduced through the voltage signal which is easier to measure, meanwhile, in an I-V conversion and amplification circuit, a high-value resistor is used as a feedback resistor or a large-resistance T-shaped resistor network formed by small resistance values is used as the feedback resistor to reduce noise, and further, signal amplification and noise suppression are carried out, so that the subsequent signal data can be quickly processed, the problems of the existing weak current detection method are solved, the noise is small, the response time is short, and the signal can be quickly processed.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the invention, and it is within the scope of the invention for those skilled in the art to obtain other drawings based on the drawings without inventive labor.

Fig. 1 is a schematic structural diagram of a weak current measuring apparatus according to an embodiment of the present invention;

FIG. 2 is a circuit connection diagram of the nA-stage current intensity-voltage (I-V) conversion circuit in FIG. 1

FIG. 3 is a schematic circuit connection diagram of the class pA current-to-voltage (I-V) converter circuit of FIG. 1;

FIG. 4 is a circuit connection diagram of the fA stage current intensity-voltage (I-V) conversion circuit of FIG. 1;

FIG. 5 is a schematic diagram of the circuit connection among the analog-to-digital conversion circuit, the single chip and the display in FIG. 1;

fig. 6 is a schematic circuit connection diagram of a power supply circuit in the weak current measuring apparatus according to the embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings.

As shown in fig. 1, in the embodiment of the present invention, the weak current measuring apparatus provided includes an I-V converting and amplifying circuit 1, an analog-to-digital converting circuit 2, a single chip microcomputer 3 and a display 4, which are connected in sequence; wherein the content of the first and second substances,

the I-V conversion and amplification circuit 1 is used for selecting a corresponding I-V conversion and amplification circuit to convert a voltage signal and carrying out noise suppression and signal amplification processing according to the type of a weak current signal to be detected; the types of the weak current signals to be detected comprise nano-ampere (nA) level weak current signals, pico-ampere (pA) level weak current signals and flying-ampere (fA) level weak current signals; the I-V conversion and amplification circuit 1 comprises an nA-level I-V conversion and amplification circuit 11 for processing nA-level weak current signals, a pA-level I-V conversion and amplification circuit 12 for processing pA-level weak current signals and an fA-level I-V conversion and amplification circuit 13 for processing fA-level weak current signals;

the analog-to-digital conversion circuit 2 is used for converting the voltage signals subjected to noise suppression and signal amplification processing into digital signals;

the singlechip 3 is used for analyzing the digital signal to obtain a voltage value and obtaining the current value of the weak current signal to be detected in a preset voltage and weak current mapping table according to the voltage value; it should be noted that the voltage and weak current mapping table is defined in advance and is obtained by mapping according to the magnitudes of weak currents of different levels;

and the display 4 is used for displaying the current value of the weak current signal to be detected and the corresponding voltage value.

In the embodiment of the present invention, the weak current signals to be detected include three types, i.e., a nano ampere (nA) level weak current signal, a pico ampere (pA) level weak current signal, and a flying ampere (fA) level weak current signal, so that the I-V converting and amplifying circuit 1 can flexibly combine and design the nA level I-V converting and amplifying circuit 11, the pA level I-V converting and amplifying circuit 12, and the fA level I-V converting and amplifying circuit 13, that is, the I-V converting and amplifying circuit 1 can be one or more combinations of the nA level I-V converting and amplifying circuit 11, the pA level I-V converting and amplifying circuit 12, and the fA level I-V converting and amplifying circuit 13. At this time, the specific structures and connection relationships of the nA-stage I-V converting and amplifying circuit 11, the pA-stage I-V converting and amplifying circuit 12, and the fA-stage I-V converting and amplifying circuit 13 are designed as follows:

(1) fig. 2 is a schematic circuit connection diagram of the nA-stage I-V conversion and amplification circuit 11. The nA-level I-V converting and amplifying circuit 11 includes a nA-level I-V converting circuit 111, a first inverting proportional voltage amplifying circuit 112, and a first common phase proportional voltage amplifying circuit 113, which are connected in sequence; wherein the content of the first and second substances,

the nA-stage I-V conversion circuit 111 includes a first operational amplifier IC1 and a first feedback resistor R1; the non-inverting input end (+) of the first operational amplifier IC1 is grounded, the inverting input end (-) is connected to a nano-ampere (nA) level weak current signal to be detected, and the output end is connected with the inverting input end of the first inverting proportional voltage amplifying circuit 112 through a resistor R2 and is also reversely connected with the inverting input end (-) of the first inverting proportional voltage amplifying circuit through a first feedback resistor R1;

the first inverting proportional voltage amplifying circuit 112 includes a second operational amplifier IC2 and a second feedback resistor R3; a non-inverting input terminal (+) of the second operational amplifier IC2 is grounded via a first voltage-dividing resistor R4, an inverting input terminal (-) is connected to the output terminal of the first operational amplifier IC1 of the nA-stage I-V conversion circuit 111 via a resistor R2, and an output terminal of the second operational amplifier IC2 is connected to the inverting input terminal of the first common-phase voltage amplification circuit 113 via a resistor R5 and is also connected in reverse to its own inverting input terminal (-) via a second feedback resistor R3;

the first same-phase voltage amplifying circuit 113 includes a third operational amplifier IC3 and a third feedback resistor R7; the non-inverting input terminal (+) of the third operational amplifier IC3 is connected to the output terminal of the second operational amplifier IC2 of the inverse proportional voltage amplifying circuit 112 through a resistor R5, the inverting input terminal (-) is grounded through a second voltage-dividing resistor R6, and the output terminal of the third operational amplifier IC3 is connected to the input terminal of the analog-to-digital converting circuit 2 and also connected to the inverting input terminal (-) thereof in reverse through a third feedback resistor R7.

The first operational amplifier IC1, the second operational amplifier IC2 and the third operational amplifier IC3 have the same structure and are AD8603 type operational amplifier chips; the first inverting proportional voltage amplifying circuit 112 and the first non-inverting proportional voltage amplifying circuit 113 each have an amplification factor of 10. It should be noted that, since the AD8603 type operational amplifier chip has two particularly excellent features of low offset voltage and input bias current, an equivalent large-resistance T-type resistor constructed by a small-resistance high-precision resistor is selected as a feedback resistor in the nA-level I-V conversion circuit 111, for example, the first feedback resistor R1 is 20M Ω, that is, a current feedback type operational amplifier circuit using a high-value resistor as a feedback resistor.

At this time, in the nA-level I-V conversion circuit 111, the input extremely small current signal exchanges the current signal with the voltage signal through a feedback resistor with a high resistance value, and as can be seen from fig. 2, the output weak voltage is a value related to the current of the nano-ampere (nA) level to be measured, the gain is related to the feedback network formed by the resistors, and the voltage is also in phase opposition to the current because the current of the nano-ampere (nA) level to be measured is input from the inverting terminal due to the negative feedback circuit.

Because of the current I of the current signal of the nanoampere (nA) level to be measured_iThe current is very small, so that the first feedback resistor R1 needs to be very large, but the first feedback resistor R1 is not as large as possible, because the larger the feedback resistor is, the larger noise is generated on the whole circuit, and the noise can generate great interference on the detection of the weak current signal, so that the stability of the operational amplifier is reduced; the larger the feedback resistance, the larger the time constant of the circuit, which affects the accuracy and sensitivity of the overall circuit.

Through experiments and debugging, 20M omega is selected as the resistance value of the first feedback resistor R1. If the measured current is 1nA, the voltage generated by conversion of the nA-stage I-V conversion circuit 111 is-20 mV through a resistor of 20M omega serving as a feedback resistor, and the value does not meet the requirement of subsequent A/D conversion, so that voltage amplification is carried out. Experimental tests show that if the amplification factor is 10, the signal-to-noise ratio of the voltage amplification circuit can reach the highest value. According to the invention, by adopting the inverse proportional voltage amplifying circuit 112 with the amplification factor of 10 and the in-phase proportional voltage amplifying circuit 113 with the amplification factor of 10, for a voltage signal obtained by converting a weak current signal with 1nA through the nA-stage I-V converting circuit 111, a voltage with 2V can be obtained through the multi-stage voltage amplifying circuit consisting of the two operational amplifying circuits, and the number value can facilitate subsequent processing.

(2) Fig. 3 is a schematic circuit diagram of the pA stage I-V conversion and amplification circuit 12. The pA-stage I-V converting and amplifying circuit 12 includes a pA-stage I-V converting circuit 121, a second inverse proportional voltage amplifying circuit 122, a first voltage follower circuit 123 and a second in-phase proportional voltage amplifying circuit 124 connected in sequence; wherein the content of the first and second substances,

the pA-stage I-V conversion circuit 121 includes a fourth operational amplifier IC4 and a first T-type resistor network; the non-inverting input end (+) of the fourth operational amplifier IC4 is grounded, the inverting input end (-) is connected to a pico-ampere (pA) level current signal to be measured, and the output end of the fourth operational amplifier IC4 is connected to the input end of the second inverting proportional voltage amplifying circuit 122 and is also reversely connected to the inverting input end (-) of the fourth operational amplifier IC 3578 through the first T-shaped feedback network; the first T-shaped feedback network comprises a resistor R8, a resistor R9 and a first capacitor C9; one end of the resistor R8 is grounded through the resistor R10, and the other end is connected with one end of the first capacitor C9 and is connected with the inverting input (-) of the fourth operational amplifier IC 4; one end of the resistor R9 is grounded through a resistor R10, and the other end of the resistor R9 is connected with one end of the first capacitor C9 and is connected with the output end of the fourth operational amplifier IC 4;

the second inverting proportional voltage amplifying circuit 122 includes a fifth operational amplifier IC5 and a fourth feedback resistor R12; the non-inverting input terminal (+) of the fifth operational amplifier IC5 is grounded through the third voltage dividing resistor R14, the inverting input terminal (-) is connected to the output terminal of the fourth operational amplifier IC4 of the pA-class I-V conversion circuit 121 through the resistor R11, the output terminal of the fifth operational amplifier IC5 is connected to the input terminal of the first voltage follower circuit 123 and is also connected to the inverting input terminal (-) thereof in the reverse direction through the fourth feedback resistor R12;

the first voltage follower circuit 123 includes a first isolation resistor R13 and a sixth operational amplifier IC 6; one end of the first isolation resistor R13 is connected to the output end of the fifth operational amplifier IC5 in the second inverse proportional voltage amplifying circuit 122, and the other end is connected to one end of the second capacitor C10 and is connected to the non-inverting input (+) of the sixth operational amplifier IC 6; the output end of the sixth operational amplifier IC6 is connected to the input end of the second non-inverting proportional voltage amplifying circuit 124 through a resistor R15 and is also connected to the inverting input end (-) thereof in the opposite direction;

the second in-phase proportional voltage amplifying circuit 124 includes a seventh operational amplifier IC7 and a fifth feedback resistor R17; the non-inverting input terminal (+) of the seventh operational amplifier IC7 is connected to the output terminal of the sixth operational amplifier IC6 of the first voltage follower circuit 123 through a resistor R15, the inverting input terminal (-) is grounded through a fourth voltage dividing resistor R16, and the output terminal of the seventh operational amplifier IC7 is connected to the input terminal of the analog-to-digital conversion circuit 2 and also connected to the inverting input terminal (-) thereof in reverse through a fifth feedback resistor R17.

It should be noted that the first voltage follower circuit 123 is disposed between the second inverse-phase proportional voltage amplifying circuit 122 and the second in-phase proportional voltage amplifying circuit 124, and is used to isolate the influence of the output impedance of the second inverse-phase proportional voltage amplifying circuit 122 on the input impedance of the second in-phase proportional voltage amplifying circuit 124, and also avoid the influence of the two-stage circuit on the pA-stage I-V conversion circuit 12.

In the embodiment of the present invention, the first voltage follower circuit 123 further includes a first low-pass filter circuit; the first low-pass filter circuit is composed of a second capacitor C10, one end of the second capacitor C10 is grounded, and the other end of the second capacitor C10 is connected between the first isolation resistor R13 and the non-inverting input end (+) of the sixth operational amplifier IC6, so that the adverse effect of noise on a weak current signal to be measured by the whole circuit can be inhibited, and the signal-to-noise ratio which is an important index required by the circuit is improved.

In the embodiment of the invention, the fourth operational amplifier IC4, the fifth operational amplifier IC5, the sixth operational amplifier IC6 and the seventh operational amplifier IC7 have the same structure and are all AD8603 type operational amplifier chips; the amplification factors of the second in-phase proportional voltage amplifying circuit 124 and the second inverse-phase proportional voltage amplifying circuit 122 are both 10 times. It should be noted that, because the AD8603 type operational amplifier has two excellent characteristics of low offset voltage and input bias current, the first T-type feedback network with equivalent large resistance is selected and used in the pA-level I-V conversion circuit 121, that is, the feedback resistor is replaced by the T-type feedback network, and this improvement not only ensures the detection accuracy, but also improves the detection sensitivity, and can detect weaker signals. In the embodiment of the present invention, in the first T-type feedback network, the resistor R8 is 200M Ω, the resistor R9 is 9.9k Ω, the first capacitor C9 is 5pF, and the resistor R10 is 100 Ω.

When a weak current signal is measured, a circuit with a good amplification effect is needed, the feedback resistor needs to be a high-resistance resistor, and the high-resistance resistor means high thermal noise, so that high stability and high accuracy of the whole circuit are greatly affected. In fig. 3, the first T-type feedback network of the present invention can effectively reduce the impedance value, both reducing the thermal noise and improving the signal-to-noise ratio. Moreover, compared with a large resistor, the small impedance has higher precision and better stability, and is more favorable for the measurement of the pA-level current signal by the pA-level I-V conversion circuit 121.

In the embodiment of the invention, the first capacitor C9 is further arranged in the first T-shaped feedback network, the first capacitor C9 is used for reducing the influence of noise on the circuit, and the effect of reducing the noise is more obvious when the value of the first capacitor C9 is larger. Considering that the first capacitor C9 is also an integrating capacitor, which reduces the response speed of the circuit, the larger the value is, the better the value is, and by reasonable analysis, the first capacitor C9 is selected to be 5 pF.

Meanwhile, a first voltage follower circuit 123 is added between the two-stage voltage amplifying circuits (i.e., the second inverse proportional voltage amplifying circuit 122 and the second in-phase proportional voltage amplifying circuit 124). The first voltage follower circuit 123 is used to isolate the influence of the output resistance of the second inverse proportional voltage amplifier circuit 122 on the output resistance of the second in-phase proportional voltage amplifier circuit 124, thereby avoiding the adverse effect of the two-stage circuit on the pA-stage I-V conversion circuit 121. Finally, a low-pass filter (i.e., a second capacitor C10) is added to the first voltage follower circuit 123 to suppress the adverse effect of noise on the pA-class current signal to be measured by the whole pA-class I-V conversion circuit 12, and to improve the signal-to-noise ratio, which is an important index of the circuit requirements.

(3) Fig. 4 is a schematic circuit diagram of the fA-stage I-V conversion and amplification circuit 13. The fA-level I-V converting and amplifying circuit 13 includes an fA-level I-V converting circuit 131, a third inverse proportional voltage amplifying circuit 132, a second voltage follower circuit 133 and a third in-phase proportional voltage amplifying circuit 134 which are connected in sequence; wherein the content of the first and second substances,

the fA-stage I-V conversion circuit 131 includes an eighth operational amplifier IC8 and a second T-type feedback network; the non-inverting input end (+) of the eighth operational amplifier IC8 is grounded, the inverting input end (-) is connected to a current signal of the flying ampere (fA) level to be measured, and the output end is connected to the input end of the third inverting proportional voltage amplifying circuit 132 through a resistor R21 and is also reversely connected to the inverting input end (-) of the eighth operational amplifier IC8 through a second T-shaped feedback network; the second T-shaped feedback network comprises a resistor R18, a resistor R19 and a third capacitor C11; one end of the resistor R18 is grounded through the resistor R20, and the other end is connected with one end of the third capacitor C11 and is connected with the inverting input (-) of the eighth operational amplifier IC 8; one end of the resistor R19 is grounded through the resistor R20, and the other end of the resistor R19 is connected with one end of the third capacitor C11 and is connected with the output end of the eighth operational amplifier IC 8;

the third inverting proportional voltage amplifying circuit 132 includes a ninth operational amplifier IC9 and a sixth feedback resistor R23; a non-inverting input terminal (+) of the ninth operational amplifier IC9 is grounded via a fifth voltage-dividing resistor R22, an inverting input terminal (-) is connected to the output terminal of the eighth operational amplifier IC8 of the fA-stage I-V conversion circuit 131 via a resistor R21, and an output terminal of the ninth operational amplifier IC9 is connected to the input terminal of the second voltage follower circuit 133 via a resistor R24 and is also connected in reverse to its own inverting input terminal (-) via a sixth feedback resistor R23;

the second voltage follower circuit 133 includes a second isolation resistor R24 and a tenth operational amplifier IC 10; one end of the second isolation resistor R24 is connected to the output terminal of the ninth operational amplifier IC9 in the third inverse proportional voltage amplifying circuit 132, and the other end is connected to one end of the fourth capacitor C12 and is connected to the non-inverting input terminal (+) of the tenth operational amplifier IC 10; the output terminal of the tenth operational amplifier IC10 is connected to the input terminal of the third in-phase proportional voltage amplifier circuit 134 via a resistor R25, and is also connected in reverse to its own inverting input terminal (-) to the first operational amplifier IC;

the third in-phase proportional voltage amplifying circuit 134 includes an eleventh operational amplifier IC11 and a seventh feedback resistor R27; the non-inverting input terminal (+) of the eleventh operational amplifier IC11 is connected to the output terminal of the tenth operational amplifier IC10 of the second voltage follower circuit 133 through the resistor R25, the inverting input terminal (-) is grounded through the sixth voltage-dividing resistor R26, and the output terminal of the eleventh operational amplifier IC11 is connected to the input terminal of the analog-to-digital conversion circuit 2 and is also connected in reverse to its own inverting input terminal (-) through the seventh feedback resistor R27.

In the embodiment of the present invention, the structure and connection relationship of the fA-level I-V converting and amplifying circuit 13 in fig. 4 and the pA-level I-V converting and amplifying circuit 12 in fig. 3 are the same, but the design parameters of specific components are different according to the level of the weak current signal to be measured, so that the measurement requirements of the weak current signals of different levels can be met.

In the embodiment of the present invention, the second voltage follower circuit 133 is disposed between the third inverse proportional voltage amplifying circuit 132 and the third in-phase proportional voltage amplifying circuit 134, so as to isolate the influence of the output resistance of the third inverse proportional voltage amplifying circuit 132 on the input resistance of the third in-phase proportional voltage amplifying circuit 134, and avoid the adverse effect of the two-stage circuit on the fA-stage I-V conversion circuit 131. Meanwhile, the second voltage follower circuit 133 further includes a second low-pass filter circuit; the second low-pass filter circuit is composed of a fourth capacitor C12, one end of the fourth capacitor C12 is grounded, and the other end of the fourth capacitor C12 is connected between the second isolation resistor R24 and the non-inverting input (+) of the tenth operational amplifier IC10, so that the adverse effect of noise on the fA-level current signal to be measured by the whole fA-level I-V conversion and amplification circuit 13 can be suppressed, and the signal-to-noise ratio, which is an important index required by the circuit, is improved.

In the embodiment of the present invention, the eighth operational amplifier IC8, the ninth operational amplifier IC9, the tenth operational amplifier IC10, and the eleventh operational amplifier IC11 have the same structure, and are all AD549 type operational amplifier chips; the amplification factors of the third reverse-phase proportional voltage amplifying circuit and the third in-phase proportional voltage amplifying circuit are both 10 times; it should be noted that, because the AD549 operational amplifier has two excellent characteristics of low offset voltage and input bias current, the invention selects a small-resistance resistor to form a second T-type feedback network with equivalent large resistance in the fA-level I-V conversion circuit 131, that is, the feedback resistor is replaced by the T-type network, and this improvement not only can ensure the accuracy of measuring the fA-level current signal, but also can improve the sensitivity of measurement. Meanwhile, the two indexes of the input offset voltage and the drift of the AD549 operational amplifier chip, which influence the operational amplifier performance, are laser adjustment and are better than the AD8063 operational amplifier chip, so that the method is suitable for the requirement of fA level current signal detection. In the embodiment of the present invention, in the second T-type network, the resistor R18 is 200M Ω, the resistor R19 is 99k Ω, the third capacitor C11 is 5pF, and the resistor R20 is 100 Ω.

At this time, when a weak current signal is measured, in order to achieve a good effect, the feedback resistor must be a high resistance resistor, and a high resistance resistor means high thermal noise, which may have a great influence on the high stability and high accuracy of the whole fA-stage I-V conversion and amplification circuit 13. In fig. 4, the second T-type network of the present invention forms a feedback network, which can effectively reduce the resistance of the impedance, thereby reducing the thermal noise and improving the signal-to-noise ratio. Moreover, the small resistance has higher precision and better stability than the large resistance, and is more favorable for measuring the fA-level current signal by the fA-level I-V conversion circuit 131. In the embodiment of the invention, the third capacitor C11 is added on the feedback resistor, the third capacitor C11 is used for reducing the adverse effect of noise on the circuit, and the larger the value of the third capacitor C11 is, the more obvious the noise reduction effect is. Considering that the third capacitor C11 is also an integrating capacitor, which reduces the response speed of the circuit, the larger the value is, the better the value is, and by reasonable analysis, the third capacitor C11 is preferably 5 pF.

Meanwhile, a second voltage follower circuit 133 is added between the two-stage voltage amplifying circuits (i.e., the third inverse proportional voltage amplifying circuit 132 and the third in-phase proportional voltage amplifying circuit 134). The second voltage follower circuit 133 is used to isolate the influence of the output resistance of the third inverse proportional voltage amplifier circuit 132 on the input resistance of the third in-phase proportional voltage amplifier circuit 134, thereby avoiding the adverse effect of the two-stage circuit on the fA-stage I-V conversion circuit 131. The invention also adds a low-pass filter (i.e. a fourth capacitor C12) to the second voltage follower circuit 133 to suppress the adverse effect of noise on the fA-level current signal to be measured by the entire fA-level I-V conversion circuit 13 and to improve the signal-to-noise ratio, which is an important index required by the circuit.

As shown in fig. 5, the circuit connection diagram of the analog-to-digital (a/D) conversion circuit, the single chip and the display is shown. The a/D conversion circuit 2 is an ADC0808 a/D converter, which is a commonly used successive approximation type a/D converter with a resolution of 8 bits.

The singlechip 3 selects an ATC89C51 type CPU chip, is a low-voltage high-performance CMOS8 bit microprocessor, and is provided with a programmable erasable 2K byte flash read-only memory. There are also 256 bytes of on-chip data memory, 32 bit I/O port lines, 2 16 bit timers, 15 vector two-level interrupt structure, an on-chip oscillator and clock circuits. And the AT89C51 type CPU may operate as static logic down to 0 Hz. While not running, the CPU may terminate operation, but the RAM, timer, interrupt system and serial communication port may still operate. The contents of the RAM are also stored during the power down protection mode, but the oscillator is locked from operation and does not allow any other components to operate, and the hardware reset is not completed. These are advantages of such a microprocessor and are well in accordance with the requirements of the present invention. The ports of the AT89C51 type CPU have VCC, GND, which are power and ground ports. Also P0 (I/O port with 8 bit open drain input output), P1 (I/O port with 8 bit input output with chip having pull-up resistor), P2 (I/O port with 8 bit input output with chip having pull-up resistor), P3 (I/O port with 8 bit input output with chip having pull-up resistor), RST (reset input pin, active high),/PSEN (strobe port determining enable external program memory selection), XTAL1 (input port of inverse oscillation amplifier), XTAL2 (output port corresponding to XTAL 1).

The display 4 is a 7SEG-MPXA-CC Liquid Crystal Display (LCD), which is a 4-bit common cathode 7-segment nixie tube, is lit at high level, and has 8-bit input ports and 4-bit nixie tube gating ports.

The function realized by fig. 5 is to input the analog voltage signal obtained by the previous stage circuit to the ADC0808 type a/D converter chip IC9, convert the analog voltage signal into a digital voltage signal, and perform the subsequent operation. The CLOCK input (CLOCK) of IC9 requires a CLOCK signal of no more than 640kHz, typically 500 kHz; the oscillator frequency of the selected CPU chip IC8 is 12MHz, and the output frequency of the ALE port in the IC8 is one sixth of the oscillation frequency, so that a clock signal of 500kHz can be generated by four-time frequency division. The three ports ADDC, ADDB, and ADDA in IC9 are three-bit address input ports that function to select one of the 8 inputs through. Since IN0 was selected, these three ports are grounded. The chip IC9 operates by inputting a three-bit address and having ALE equal to 1, allowing the address to be normally stored in the address latch, which is then decoded from one of the 8-way analog input ports to the following comparator. The rising edge of the START port input pulse signal will once approach the register and then reset, and the falling edge of this signal will turn on the a/D conversion, during which the EOC port output is always low. When the A/D conversion is successful, the EOC port stops outputting the low voltage and outputs the high voltage, and the result data is stored in the latch. If the output signal needs to input a high level to the OE port, the output tri-state gate can be opened, and the A/D conversion result can be output smoothly.

The connection condition of the voltage display circuit in fig. 5 is that the ports P2.3, P2.4, P2.5 and P2.6 of the CPU chip IC8 are connected to the START, ALE, EOC and OE ports of the chip IC 8. When an 8-bit I/O port of the IC8 is connected with an 8-bit output end of the IC9 and an internal clock of a CPU chip IC8 is adopted, a crystal oscillator of 12MHz is connected with ports of XTAL1 and XTAL2, two ports are grounded through a capacitor respectively, a needed clock signal can be generated, and the capacitor generally adopts 30pF or 33 pF. The EA port is an off-chip program memory access enable signal, low level is applicable, because the circuit only uses on-chip program memory, and is connected to high level. The P00-P08 ports of IC8 are externally connected with 8 pull-up resistors, where a RESPACK-8 exclusion is selected. In addition, the P00 to P08 ports of the IC8 are connected to the A, B, C, D, E, F, G, DP port of the LCD chip. And 4 ports P1.3, P1.2, P1.1 and P1.0 of the IC8 are connected with the gating port of the 4-bit nixie tube. Finally, the voltage value input by the A/D converter can be displayed on the nixie tube through a software program.

Fig. 6 is a schematic diagram of power circuit connection. The chips IC6 and IC7 used by the power supply circuit are LM7805 and LM7905 three-terminal voltage-stabilizing integrated circuits, respectively provide +5V and-5V voltages, and have the characteristics of small volume, high integration level, high load regulation rate and the like, and the circuit is internally provided with a protective circuit of an overcurrent pipe, an overheat pipe and a regulating pipe. 220V alternating current is input into LM7805 and LM7905 chips through a bridge rectifier circuit and a capacitor filter, and finally +5V and-5V voltages are generated.

The weak current measuring device in the embodiment of the invention is subjected to simulation verification, and specifically comprises the following steps: it can be known through simulation that the voltage obtained by the 1nA current conversion is-20 mV, the voltage value transmitted to the analog-to-digital conversion circuit 2 through amplification and reverse phase is 2V, and the voltage value displayed on the four-bit nixie tube is 2V through software control and processing in the singlechip 3. Simulation measurement results of nA-level weak current signals are shown in Table 1:

TABLE 1

As can be seen from table 1, the conversion voltage value and the theoretical value of the weak current to be measured, which are obtained through the nA-stage I-V conversion and amplification circuit 11 in the range of 1nA to 10nA, have errors, but are all less than 1%, and are within the reasonable error range of the system.

Simulation shows that the final conversion result of the current of 1pA is 2.02V, the theoretical value is 2V, the error is only about 1% and is within a reasonable range, and therefore the pA-level weak current signal can be well measured after noise reduction improvement. Simulation measurement results of pA-level weak current signals are shown in table 2:

TABLE 2

As can be seen from table 2, the weak current to be measured has an error between the converted voltage value obtained by the pA-level I-V conversion and amplification circuit 12 and the theoretical value in the range of 1pA to 10 pA. Compared with the nA-stage I-V conversion and amplification circuit, the pA-stage I-V conversion and amplification circuit has the advantages that the distance between the conversion voltage value and the theoretical value is larger, but the distance is smaller than 2%, and the error range is reasonable.

According to simulation, the final conversion result of the current of 10fA is 2.02V, and the theoretical value is 2V, although the error is large, the range of the allowable measurement error is also included, so that the fA level weak current signal can be well measured through the improvement measure of noise reduction. Simulation measurement results of the fA-level weak current signal are shown in table 3:

TABLE 3

As can be seen from table 3, the conversion voltage value and the theoretical value of the weak current to be measured, which are obtained from 10fA to 100fA through the fA-level I-V conversion and amplification circuit 13, have a large error, but the current can also be detected within a reasonable range.

The implementation of the invention has the following beneficial effects:

the invention carries out weak current detection based on a transimpedance method, converts a weak current signal to be detected into a voltage signal which is easy to measure, reversely deduces the size of the weak current signal by measuring the voltage signal, and simultaneously utilizes a high-value resistor as a feedback resistor or a large-resistance T-shaped feedback network formed by small resistance values as the feedback resistor in an I-V conversion and amplification circuit to reduce noise, further amplifies the signal and inhibits the noise so as to quickly process subsequent signal data, thereby overcoming the problems of the existing weak current detection method, and having small noise, quick response time and quick signal processing.

The above disclosure is only for the purpose of illustrating the preferred embodiments of the present invention, and it is therefore to be understood that the invention is not limited by the scope of the appended claims.