Noise Shaping Techniques for SNR Enhancement in SAR Analog to Digital Converters

In principle, the resolution of a successive approximation (SAR) analog-to-digital converter (ADC) can be increased by 1 bit by quadrupling the area and the power dissipation. In practice, due to the increased size of the capacitor array and consequent higher mismatch, the resolution is limited to 12–13 bits. Calibration or trimming can be used to improve the resolution beyond this, but are expensive in area and test time. This work presents a 12-bit SAR ADC embedded in a first-order noise shaping loop to obtain a 16-bit resolution while maintaining sample-by-sample correspondence. Using a sub-quantization DAC in parallel with the LSB DAC allows the conversion cycles to be shorter than the first, increasing the effective sampling rate. Dynamic element matching is used in the 6-bit MSB DAC. The integrator in the loop filter is implemented with minimal overhead using one of the stages of the multistage preamplifier in the comparator. Measurements of a 0.6 μm prototype show 0.7 LSB INL and 0.3 LSB DNL at the 16-bit level and 92 dB peak SNDR for 2.5 kHz input at a 250 kS/s sampling rate.