Method and apparatus for enabling wide input common-mode range in sar adcs with no additional active circuitry

Abstract

A differential successive approximation register (SAR) analog-to-digital converter (ADC) with wide input common-mode range adds one step to its conversion process. No additional circuitry is required for full rail-to-rail common mode voltage operation. In a first step the top-plate nodes vcp and vcn may be reset to a fixed voltage vcm. Then in a next step sampling may be performed while leaving vcp and vcn floating but shorted. Whereby a single node vx is formed, which provides for simple capacitive voltage division. Thereafter a standard sequential SAR bit-by-bit analog-to-digital conversion is performed. the voltage at node vx will follow vcmin during the entire sampling phase, with a limitation in rate of change only limited by the RC time constant of the shorting switch and the sampling capacitors. This will have much higher bandwidth than any active OTA-based tracking circuit.