An Over-60dB True Rail-to-Rail Performance Using Correlated Level Shifting and an Opamp with 30dB Loop Gain

This work introduces correlated level shifting (CLS) that simultaneously decreases the error due to finite opamp gain and allows true rail-to-rail operation. An extra phase is needed, but, surprisingly, there is no speed penalty when compared to a high-gain opamp solution because CLS does not require full settling during the next-to-last phase. In fact, the increased signal swing means that the same SNR can be achieved using smaller sampling capacitors. Thus, it could be argued that CLS can provide accurate results at a higher speed than using a traditional 2-phase operation with a high-gain opamp. For example, with CLS, the performance is maintained over the whole 0.9V supply, whereas a standard configuration has only a 0.6V swing. Thus, the standard configuration requires 2.25x larger sampling capacitors to achieve the same signal to kT/C noise ratio.