VCO-Based Wideband Continuous-Time Sigma-Delta Analog-to-Digital Converters

This chapter examines the use of VCO-based quantization within continuous-time, Sigma-Delta ADC circuits. We consider the VCO-based quantizer as an efficient combination of a voltage-to-time converter and a time-to-digital converter, and discuss the advantages it offers in achieving improved quantization noise performance in the ADC. However, the common approach of using frequency as the output of the VCO-based quantizer presents a bottleneck to achieving high SNDR in the ADC due to Kv nonlinearity. We show that using phase as the key output variable removes this nonlinearity barrier. Measured results confirm that 78 dB SNDR performance is achievable with 20 MHz bandwidth while achieving a 330 fJ/conversion step efficiency.

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