A gain calibration system for an amplifier includes an amplifier having inputs and outputs and an analog to digital converter having inputs and an output. There is a voltage supply for providing a plurality of output voltages. A first switching circuit couples at least one of the output voltages to the inputs of the amplifier in a first phase. A second switching circuit couples the outputs of the amplifier to the inputs of the analog to digital converter in the first phase and couples the sum of all of the at least one output voltages to the input of the analog to digital converter in a second phase. A processor responsive to the outputs of the analog to digital converter in the first and second phases calculates a calibration factor to accommodate for amplifier gain error.