A non-pipelined ADC with a GM-R amplifier

Abstract

A GM-R amplifier is proposed as a residual amplifier to implement a 16-bit two-stage non-pipelined ADC in intermittent signal acquisition applications. Unlike common capacitive feedback amplifiers, residual amplifiers utilize resistor ratios to generate constant gain for small input amplitude ranges and wide temperature ranges. The input circuit of the proposed amplifier consists of two source followers and a bridge resistor, thus converting residual voltages to currents, and the transconductance is the reciprocal of the bridge resistor. The amplifier offset voltage is corrected by a one-time foreground offset calibration, and the gain error is calibrated with the capacitor weights in the first stage. Redundancy is adopted between the two stages to accommodate the residual offset voltage after calibrating the offset voltage of a comparator and the gain variation. A design prototype is implemented in 180 nm CMOS technology, occupies a 0.69 mm2 footprint, and consumes 3.0 mW from 1.8/5 V power supplies. After calibration, it achieves the SNR of 88.2 dB, SNDR of 87.6 dB, and THD of −95.6 dB at 200 kHz sampling rate. The measured differential nonlinearity (DNL) and integral nonlinearity (INL) are −0.62/+0.74 LSB and −1.39/+1.50 LSB, respectively.