Research And Design Of Pseudo Difference Structure Weak Current Amplifying And Adjusting Front-end Circuit

With the rapid development and progress of modern information technology,people have put forward higher requirements for high-precision weak current signal detection.Weak current signal detections are widely used in semiconductor testing,biomedicine,and spectral analysis.This thesis introduces the classic weak current detection method,focuses on explaining and analyzing the basic principle of the negative feedback resistance method and the source of circuit noise,and calculates the current resolution of the negative feedback amplifier.Through analysis and calculation,it is concluded that the weak current amplification scheme of the resistance negative feedback method has inherent defects,and it is difficult to achieve the current resolution we need.After that,the basic principle of the capacitor negative feedback circuit is introduced,the noise source of the capacitor negative feedback scheme is analyzed,and the current resolution of the scheme is calculated.Finally,on the basis of the capacitor negative feedback circuit,a charge integrator is used to amplify the current.In order to suppress the offset and low-frequency noise of the operational amplifier,a correlated double sampling circuit is introduced.In order to reduce the charge injection of the switch,the negative impact of clock feed-through effect on the circuit,and to suppress the output fluctuation caused by the power supply voltage fluctuation(through power supply coupling),a pseudo-differential structure charge integrator circuit is used.In order to make the specific circuit design and parameter adjustment more convenient,a detailed noise analysis is performed on the amplifier conditioning circuit used in this thesis and the current resolution of the circuit is calculated.Charge integrator is the main circuit structure of the weak current front-end amplification and conditioning circuit.The performance of the operational amplifier in the charge integrator is critical,which requires a higher open-loop gain and a wider input common-mode range.Based on the analysis and comparison of operational amplifiers with different structures,a gain-boosting folded cascode operational amplifier is used,and at the same time,source degradation techniques are used to dilute the noise of the operational amplifier.In addition,common-mode fluctuations at the input of the operational amplifier will occur due to the mismatch between the input and the dummy capacitance.Such fluctuations will cause errors in the results through common-mode-to-differential mode conversion.In order to suppress this error,a three-input transconductance amplifier is designed as the input common mode feedback of the circuit.Moreover,the effective output of the charge integrator using the relevant double sampling technology is embodied in a discrete form,which is not conducive to subsequent signal readout,analog-to-digital conversion,and digital processing.Therefore,we add a pair of differential holding capacitors C_H to the back end of the charge integrator to convert the output of the amplifier into a continuous signal.The previous simulation results show that the equivalent input current noise of the overall circuit is 0.063p A/?Hz at 10Hz,which meets the requirements of p A-level weak current detection.