A 50 MHz bandwidth 54.2 dB SNDR reference-free stochastic ADC using VCO-based quantizers

A reference-free stochastic ADC is proposed by utilizing both spatial averaging and oversampling noise-shaping schemes. By implementing multiple VCO-based quantizers in parallel, stochastic spatial averaging for quantization errors is inherently obtained. In addition, 1st-order noise shaping of a VCO-based quantizer is achieved in an open-loop oversampling configuration. By resolving a faster conversion rate, this open-loop structure eliminates biasing, loop filter, sample-and-hold, and external reference, and it consists of only delay cells and digital logic. The proof-of-concept prototype which includes eight VCO-based quantizers and spatial averaging estimator is implemented in a 0.18 μm CMOS process, demonstrating 54.2 dB and 45.4 dB SNDR for 50 MHz and 100 MHz bandwidths, with 116 mW power consumption. Measurement results reveal that the eight channel stochastic ADC provides an average 9 dB SQNR improvement due to the spatial averaging.