A 120nW 8b sub-ranging SAR ADC with signal-dependent charge recycling for biomedical applications

We present an 8-bit sub-ranging SAR ADC designed for bursty signals having long time periods with small code spread. A modified capacitive-DAC (CDAC) saves previous sample´s MSB voltage and reuses it throughout subsequent conversions. This prevents unnecessary switching of large MSB capacitors as well as conversion cycles, reducing energy consumed in the comparator and digital logic and yielding total energy savings of 2.6×. In 0.18μm CMOS, the ADC consumes 120nW at 0.6V and 100kS/s with 46.9dB SNDR.