Level-Crossing Detection based Low-Power Sigma-Delta ADC for Sensor Applications

A Current Controlled Oscillator (ICO), integrates its information-bearing current-input-signal i
_in
using a capacitor, C. The voltage V
_C
across C, namely the integral, is reset or folded when it reaches a level V
_REF
. If i
_in
(dq/dt) is constant, then V
_C
is a saw-tooth waveform. Since the frequency of the oscillator is defined as the rate of change of phase, the phase wraps itself after every 2π radians or its corresponding voltage V
_REF
. With this property we propose an open-loop continuous-time Sigma-Delta ADC combining level-crossing detection and folding techniques using an ICO. We also propose a technique to increase the effective resolution of the ADC by inserting additional voltage-levels for detection between 0 and V
_REF
. The complete ADC is designed in TSMC 40nm CMOS technology. The simulation results show an ENOB of 12.2-bits consuming 1.5μW of power from a 1V supply corresponding to a Schreier-FOM of 167.2dB and a Walden-FOM of 157fJ per conversion cycle.