Design Of 24-bit High Precision Sigma Delta ADC

With the rapid development of semiconductor and microelectronic technology,the design and application of analog to digital converter(ADC)are paid more and more attention in academic and industrial circles.Compared with the traditional Nyquist ADC,Sigma-Delta ADC uses oversampling and noise shaping technology to reduce the quantization noise power in the signal frequency band,which can effectively improve the signal-to-noise ratio of the system.It has the advantages of high precision,high reliability and compatibility with digital CMOS technology.It is widely used in aerospace,measurement and control,communication and other high-precision signal processing fields.In this paper,a kind of 24-bit high-precision Sigma-Delta ADC for EEG detection is designed,which mainly includes programmable gain amplifier(PGA)module,Sigma-Delta modulator module,digital decimation filter module and SPI interface module.The PGA module uses low noise operational amplifier combined with switched resistor feedback network to realize automatic control of PGA gain,and uses chopper stabilization technology to reduce the noise of operational amplifier.Sigma-Delta modulator is designed with a second-order feedforward multi-bit quantization structure,which can achieve high-precision quantization at low order and ensure the stability of the modulator.The digital decimation filter adopts a three-stage cascade integrated comb-filter structure,the internal register word length is 26-bits,and the hardware implementation is simple,which can effectively reduce the chip area and power consumption.SPI interface module sends ADC conversion results in serial mode,which is convenient for ADC to communicate with external host computer.The circuit level design and simulation of Sigma-Delta ADC are carried out by using TSMC 0.18?m CMOS technology,and the overall layout design of ADC is completed.The PGA module can achieve 1?24 times gain adjustment,the input reference noise is less than 2.3n V/sqrt(Hz)in the range of 0?70Hz,and the total harmonic distortion is less than-92 d B.The signal-to-noise harmonic distortion ratio of Sigma-Delta modulator is 136.5d B,and the significant bit is 22.39 bit.The digital decimation filter can achieve 128 times down sampling,the passband ripple is less than 0.03 db,and the stopband attenuation is more than 120 d B.The post simulation results of Sigma-Delta ADC are as follows: under the conditions of sampling frequency of 1.024-MHz,input signal frequency of 62.5-Hz and normalized amplitude of 0.79,the signal-to-noise harmonic distortion ratio of the ADC as a whole reaches 118.0d B,significant bits of 19.31 bit,DNL range of-0.60?0.70 LSB and INL range of-1.38?0.61 LSB.The simulation results show that the 24-bit high-precision Sigma-Delta ADC can meet the design requirements.