A 0.13µm CMOS 78dB SNDR 87mW 20MHz BW CT ΔΣ ADC with VCO-based integrator and quantizer

In this paper we demonstrate a new technique that eliminates the impact of K_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_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_v nonlinearity, but also it is a 1s,-order CT DeltaSigma ADC loop, and it therefore 1s,-order shapes quantization noise.