App note from ON Semiconductors introducing their nano power ADC NCD9801x. Link here (PDF)
The NCD9801x ADC is a differential 12−bit resolution successive approximation register analog−to−digital converter unlike any other SAR ADC available on the market. It uses an innovative design to keep a low input capacitance of 2 pF, easily besting the typical SAR ADC input capacitance. The analog power consumption of the NCD9801x converter can reach nano−Watt levels during conversion and can be scaled dynamically based on the clock rate. These two unique traits allow designers to utilize the NCD9801x in design applications that have previously been unachievable.