A Novel Low Distortion Switch Used in A/D Converter for Directly Sampling Bipolar Voltage Signals

In order to directly and accurately sample a bipolar voltage signal into a unipolar supply A/D converter with normal CMOS technology, a novel low distortion switch is proposed. This novel switch is composed of only four PMOS transistors, one capacitor and one normal switch. It can directly sample a bipolar signal without a conventional used voltage level shift circuit and makes the circuit free of noise, distortion and temperature characteristics problems that are caused by voltage level shift circuit. This switch is one type of high linear bootstrapped switches, of which the source to gate voltage is kept to a constant value to reduce nonlinearity. A sample/hold circuit with this novel switch is implemented in a 12-bit SigmaDelta A/D converter with a standard 0.5 mum mixed mode CMOS process. By simulation, this sample/hold circuit achieves a 120 dB SFDR when sampling a 1 V peak-to-peak, 156 KHz bipolar signal under 10 MS/s sampling rate