The AD7703 is a 20-bit ADC that uses a S-D conversion technique. The analog input is continuously sampled by an analog modulator whose mean output duty cycle is proportional to the input signal. The modulator output is processed by an on-chip digital filter with a six-pole Gaussian response, which updates the output data register with 16-bit binary words at word rates up to 4 kHz. The sampling rate, filter corner frequency, and output word rate are set by a master clock input that may be supplied externally, or by a crystal controlled on-chip clock oscillator. The inherent linearity of the ADC is excellent and endpoint accuracy is ensured by self-calibration of zero and full scale, which may be initiated at any time. The self-calibration scheme can also be extended to null system offset and gain errors in the input channel.