VCO-Based 0- $$\varDelta \varSigma $$ MASH ADC

Abstract

As discussed in Chap. 4, to address the issue of limited voltage headroom and to take advantage of the reduced CMOS gate delay in scaled CMOS technologies, VCO-based highly-digital ADCs have recently emerged in recent years. However, the performance of VCO-based ADCs is limited by the inherent nonlinearity of the VCOs. As shown in Fig. 7.1, a VCO-based ADC can be utilized as a quantizer in a single-loop \(\varDelta \varSigma \) modulator. The preceding loop filter suppresses both the quantization noise and the nonlinearity, however, a high loop gain results in a high power consumption and potential instability problems. As discussed in Chap. 3, in traditional \(\varDelta \varSigma \) ADC design, compared to single-loop structures, MASH architectures can offer a higher-order noise shaping with less instability concerns. This structure can also be implemented with VCO-based quantizers as will be demonstrated.