Abstract: An A/D circuit to convert an analog signal to a digital signal. The circuit includes a low noise analog-to-digital conversion chip and a precision voltage reference source. The voltage reference source includes a diode with two terminals and a passive attenuation circuit. The attenuation circuit and the diode are coupled in parallel between the two terminals to provide a voltage reference signal for use by the analog to digital conversion chip. The analog to digital chip uses the voltage reference signal to set the full scale input range for the signal conversion, and the voltage reference signal is attenuated to correspond to the full scale range of the analog signal. The A/D converter circuit has a passive temperature compensation feature to substantially eliminate or reduce the effects of thermal drift and of operating at different temperatures.