A 0.13[mu]m CMOS 78dB SNDR 87mW 20MHz BW CT [Delta Sigma] ADC with VCO-based integrator and quantizer

Abstract

In this paper we demonstrate a new technique that eliminates the impact of K[subscript v] nonlinearity by preserving the integral relationship of the VCO output phase to the input signal. Leveraging the VCO output phase directly precludes the need to span the entire nonlinear K[subscript v] characteristic since small perturbations (in the range of 10s of mV) at the tuning node are sufficient to shift the VCO phase by a substantial amount. Since an open-loop VCO is sensitive to frequency offsets and drift, and easily saturates its phase detector for large input signals, some form of negative feedback is necessary. Here, a multibit DAC subtracts the previously quantized phase value from the VCO input, creating a residue that is integrated during the next clock cycle. This feedback loop not only allows large signals to drive the VCO without incurring distortion from K[subscript v] nonlinearity, but also it is a 1s,-order CT DeltaSigma ADC loop, and it therefore 1s,-order shapes quantization noise.