A programmable gain preamplifier is provided which has a low temperature drift and good dynamic range characteristics. The programmable gain preamplifier may be coupled to an analog to digital converter. The analog to digital converter may be a switched capacitor array analog to digital converter. The analog to digital converter may be a resistor array and capacitor array analog to digital converter. A resistor string having contacts out of the resistor string current path may be utilized with the programmable gain preamplifier or the analog to digital converter or both. The resistor string may be utilized to calibrate the analog to digital converter or the programmable gain preamplifier or both. The resistor string may also be utilized by the analog to digital converter when conversions are being performed. The programmable gain preamplifier provides a programmable gain of the difference between two input signals (Ain and Ain′ for example). One of the input signals (Ain′) may be an estimation of the other input signal (Ain).