A 2nd Order 1-bit Complex Switched Capacitor Sigma-Delta ADC with 90dB SNDR in a 180kHz Bandwidth

Abstract

A complex switched capacitor sigma-delta ADC is described. The ADC is used in the VLIF RX path of a GSM/GPRS/EDGE phone and has an SNDR of 90dB in a 180kHz bandwidth, with the VLIF centered at 123kHz. The use of a complex noise transfer function allows for a more optimal use of noise shaping. The ADC is 2 nd order, 1-bit, with a sampling rate of 52MHz implemented in 90nm CMOS.

Digital Complex Delta–Sigma Modulators With Highly Configurable Notches for Multi-Standard Coexistence in Wireless Transmitters

“…In addition, complex transfer functions have been studied also for Sigma-Delta Modulator (SDM)-based ADCs which can be used in the intermediate frequency (IF) stage of a receiver (RX). Reference [13] presents a 2 nd order single-bit Complex SDM (CSDM) ADC to be used in a very low IF RX path in GSM/ GPRS/ EDGE phone applications for improved noise shaping (complex NTF), whereas only one integrator stage is complex. The coefficients of the architecture were obtained with a DSM specific design tool, because the custom complex zero is placed extremely close to the signal band (z = -0.02j), and has very little impact on the poles position.…”

“…5), as it can be seen from the STF and NTF 12) (13) (14The differences between the transfer functions of the two architectures (highlighted in Eq. (10)(11) and (13)(14)) show the transition from a DSM to a CDSM by moving a zero from DC (z = 1) to a custom position (z = r -jc), without affecting the optimization of the poles, i.e. the denominators of the STF and NTF are equivalent for ax = a4 + r -1.…”