0.13pW/Hz Ring VCO-Based Continuous-Time Read-Out ADC for Bio-Impedance Measurement

This brief presents a continuous-time ring voltage-controlled oscillator (VCO)-based second-order $\Delta \Sigma $ analog-to-digital converter (ADC) for bio-impedance measurement. The proposed ADC addresses several limitations of prior works which pre-dominantly use successive approximation register (SAR) ADC, such as kickback noise from sampling circuit and power hungry driver required to drive large sampling capacitance of SAR ADC. A current-reuse architecture is used to reduce power consumption and input-referred noise of the proposed ADC. The ADC is implemented in 65nm CMOS process and used for measuring amplitude and phase of a phantom which models skin impedance. A digital matched filter is used to extract amplitude and phase of the impedance of the phantom. The ADC measures amplitude with a maximum error of 2.8% and measures phase with a maximum error of 0.86° over a frequency range of 1-300 kHz. The ADC has a mean SNR of 67dB with a power efficiency of 0.13pW/Hz which is 9 $\times$ better than state-of-the-art.