Research And Design Of Switched Capacitor Analog Front End

With the fast development of technology in integrated circuit(IC),sensor IC has been used in every aspect of our life.The study on system-on-chip(SOC)integrated with signal sampling function becomes very popular for the increase market demand for high integration.In signal sampling system,the analog front end is critical for the signal sampling and conditioning function.Research on switched capacitor(SC)analog front end is carried out in this paper.Two different SC circuit module are designed to satisfy different voltage range of input signal.A fully differential SC analog front end with two stage sampling capacitor is proposed in this paper.With the designed switching sequence,the circuit can greatly reduce the impact of step change in input common mode voltage(ICMV)on the ICMV of the fully differential operational amplifier.The common mode rejection ratio(CMRR)can also be improved.Rail to rail input voltage range can be realized with no need to design a rail-to-rail fully differential operational amplifier.The function of two stage sampling capacitor is analyzed and verification is done by simulating with Verilog A model.Noise and CMRR of the circuit is also analyzed and the equation is derived.The influence of switching sequence is discussed and bottom plate sampling scheme is implemented.The design is completed under HHGrace 0.11μm BCD process.Simulation results show that the ICMV range is rail to rail under 3.3V supply voltage and the input differential voltage range is 3.3Vp-p.The CMRR can achieve 113 d B and the effective number of bit(ENOB)is larger than 13 bit.The noise power spectral density is 348 n V/√Hz@100Hz and the integrated noise from 1Hz to 20 k Hz is54.5μVrms.Design specification can be satisfied.The SOC integrated with the designed circuit is taped out.Some experiment is carried out to test the SC circuit and the basic function of the circuit is verified.To realize the sampling of multi-channel input voltage beyond the 5V power supply voltage,a multi-channel high voltage sampling switch is proposed.The transmission gate sampling switch of the two stage sampling capacitor structure is replaced to handle the input voltage beyond power supply voltage.To improve the drive capability,push-pull output stage is designed in the fully differential operational amplifier.Frequency divide circuit,phase shift circuit and mux circuit is designed to solve the sampling timing problem.The design is completed under HHGrace 0.11μm BCD process and 3 input voltage channel is designed.Simulation results show that each channel can transmit 0～14V ICMV range and 0～12V input differential voltage range.The gain accuracy can be as low as 4.43%.Application demand can be satisfied.