Design Of A Weak Current Detection Circuit For Chemical Detection Sensors

In 1958,the world's first integrated circuit was born[1],and it developed rapidly in the following decades,and has a very wide range of applications.As a branch of integrated circuits,sensors are windows and media for obtaining information from the outside world,and play an irreplaceable role.Among them,chemical detection sensors can be used to detect molecules of specific chemical substances[2]and have a large application scenario and market prospect.However,after some chemical detection sensors detect molecules,their output electrodes will only generate a current of 10-11 amperes,which is difficult to distinguish with conventional detection methods.By investigating the research results in the field of weak current detection at home and abroad in recent years,this paper designs a weak current detection circuit for chemical detection sensors.The simulation results show that it can detect currents of 10-11 amperes.The equivalent output model of the sensor's sensitive structure is analyzed,and the detection scheme with the transimpedance amplifier as the main body is determined.In order to meet the requirements of the detection circuit for digitization,in view of the low frequency and high accuracy requirements of the sensor output signal,a cascade of integrators feedback(CIFB)?-? modulator and a three-stage cascade are connected after the transimpedance amplifier The comb filter(Cscaded Integrator Comb,CIC)is used as the analog-to-digital conversion module and carries out system-level modeling.Then analyze the noise characteristics of the transimpedance amplifier,and estimate the value of the equivalent input current noise of the system.Analyze the non-ideal factors of the modulator,determine the key points of the modulator circuit design,and ensure that it meets the design requirements.The transimpedance amplifier adopts a three-stage operational amplifier with a TCFC(Transcond with Cap Feedback Comp)structure.Better noise performance can be obtained by increasing the size of the first-stage MOS tube.The simulation results show that the equivalent input noise density at 10Hz is 30.242nV/sqrt(Hz).The pre-amplifier circuit is simulated,and the total equivalent input current noise is 0.8216pA.The modulator uses a large sampling capacitor to reduce the equivalent switching thermal noise,and the first-stage integrator uses chopping technology to reduce 1/f noise.When inputting-1.58dBFS single-ended sinusoidal signal,the signal-to-noise ratio is 104.9dB and the effective number of bits is 17.13.The CIC filter is designed and synthesized.After synthesis,the simulated signal-to-noise ratio is 106.2dB,and the effective number of bits is 17.34,which meets the design requirements.The 0.35?m BCD process is used to complete the layout design of the entire weak current detection circuit.The total area is 4400?m×2050?m.The equivalent input noise density at 10Hz after the TCFC operational amplifier is 30.2455nV/sqrt(Hz),and the total equivalent input current noise is 0.8218pA,which is less than 1/10 of the detection accuracy,which meets the design requirements.After imitating the overall layout of the weak current detection circuit,the output signal-to-noise ratio of the modulator is 98.3dB and the effective number of bits is 16.04.The output signal-to-noise ratio of the CIC filter is 101.1dB,and the effective number of bits is 16.50,which meets the design requirements.