Design And Implementation Of A Sigma Delta Modulator With High Accuracy And Low Power Consumption

With the increasing application of MEMS technology in capacitive sensors,there is a high demand for capacitive detection circuits with a wide range.Sigma-delta analog-to-digital converter(ADC)is very suitable for applications with low signal bandwidth and high precision because of its high accuracyThis paper introduces the basic principle of sigma-delta analog-to-digital converter,including the structure of sigma-delta analog-to-digital converter,oversampling and noise shaping technology.The high-order sigma-delta modulator and the multi-bit quantized sigma-delta modulator are also discussed.At the same time,the principle of capacitance detection using traditional sigma-delta modulation technology based on OTA is discussed.However,because the Miller compensation capacitance in multi-stage OTA needs to change with the change of detection capacitance,otherwise it will cause the stability of the loop,which limits the detection range of the capacitance of sigma-delta modulator based on OTABased on the problems mentioned above,a sigma-delta analog-to-digital converter based on comparator and current source is proposed for wide capacitance range detection.The circuit adopts a second-order single bit structure and replaces the traditional OTA with a comparator and a current source to avoid the problem of loop stability caused by the change of detection capacitance in the traditional OTA,which greatly enlarges the detection range of capacitance In addition,in order to further expand the scope of capacitance detection,an adjustable capacitor is added to the first stage integrator.In addition,in order to determine the detection time reasonably,an automatic self-adaptive time adjustment scheme is proposed to determine the discharge time by pre-charging and discharging.In order to reduce the low frequency 1/f noise,the chopper stabilization technology is adopted in the design of the comparator,and the one-way comparator which can automatically turn off is used to save power.In addition,the comparator of the two-stage integrator is multiplexed due to the different working phases of the two-stage integratorThe second-order single-bit sigma-delta ADC designed in this paper has been fabricated in GlobalFoundries 0.18?m CMOS process.Under 1.5V supply voltage and 128 OSR,the power spectral density of the sigma-delta modulator is tested.The results show that the chopper stabilization technique can effectively suppress low frequency noise and increase the SNR by about 10 dB at low frequencies.In addition,in order to verify the wide-range detection of this design,each Cs corresponding to Cr=6pF,Cr=100pF and Cr=10nF are tested.The result shows that the designed sigma-delta based on the comparator and current source replaced OTA technology.The analog-to-digital converter can extend the detection range of the capacitor to the range of lp-10nF,and maintain the resolution at around 13bit with a small FoM value.