Design Of A 10-bit VCO-based ADC

As an important part of the brain-to-machine interface,implantable/wearable biomedical electronic devices are responsible for the acquisition,processing and transmission of bioelectrical signals,while analog-to-digital converters(ADCs)is the core module of the signal collecting system.Considering the special application environment of implantable/wearable devices,biomedical electronic devices have certain requirements for the power and area consumiption of the internal ADC.In addition,supply voltage is gradually declining with the reduction of process size.The traditional voltage-based ADCs are increasingly facing serious challenges of quantifying the analog input.Therefore,a 10-bit ADC is designed in this paper based on the voltage controlled oscillator(VCO).The main design approaches and innovations include:1.This ADC is designed based on the analog-time-digital(ATD)approach.The quantization process is completed in time domain after analog-to-time conversion.It can alleviate the challenges of voltage-based quantization under low supply voltage,and reduce the chip area significantly by decreasing the number and size of analog devices in the converter.2.Two identical single-ended input VCO-based ADCs are combined to realize fully differential input structure.Compared with the single-ended input ADC,the differential structure taken in this paper has a good ability to suppress common-mode interference,improve the nonlinearity and optimize the SNR of ADC.3.In order to reduce the chip area,the inverter is used as the delay unit to form the three-stage ring VCO in this paper.The VCO generates oscillation waveform by using the delayed negative feedback of the loop structure.The VCO utilized in this ADC is with simple structure.The oscillation frequency(period)can be changed by adjusting the number of cascaded inverters,without changing the overall structure of the VCO.4.As for the digital circuit,10-bit full subtraction circuit is used to process these two single-ended ADCs' outputs,and realize single-ended output.In order to overcome the shortcomings of traditional subtractor,such as complex circuit structure,largeare area and high power consumption,a new binary subtractor based on Domino logic,which can significantly simplify the circuit structure,reduce power and area consumption,is proposed in this paper.The VCO-based ADC is designeed in 0.18 ?m standard CMOS process,which is designed and simulated by using Cadence and Matlab.The VCO-based ADC has a sampling rate of 25 KS/s.At 1-V supply voltage,the effective number of bits(ENOB)is 9.17 bit.The figure of merit(FoM)is 0.39 pJ/conversion-step and the signal to noise distortion ratio(SNDR)is 57 dB.Including the output drive circuit,the entire power consumption of the ADC is 20.19 ?W.The active area of the 10-bit ADC is 270 ?m ×100?m,which is the most significant advantage of this proposed ADC.The design results show that it is suitable for the application of implantable/wearable biomedical electronics.