Analysis and Design of Continuous-Time Delta-Sigma Converters Incorporating Chopping

Chopping the operational transconductance amplifier (OTA) of the input integrator in a CT Delta Sigma M is a traditional and effective way of addressing flicker noise in such modulators. Unfortunately, chopping leads to aliasing of shaped quantization noise into the signal band and degrades performance. We analyze the mechanisms of shaped-noise aliasing in OTA-RC integrators that use two-stage feedforward-compensated OTAs, and show that aliasing can be largely mitigated by using an finite impulse response feedback digital-to-analog converter with its zeros placed at multiples of twice the chopping frequency. The theory is borne out by measurement results from a single-bit CT Delta Sigma M, which achieves a peak SNDR of 98.5 dB in a 24-kHz bandwidth while consuming only 280 mu W from a 1.8-V supply. Realized in a 180-nm CMOS technology, it achieves a 1/f noise corner of about 3 Hz when chopped at f(s)/24.