An apparatus is provided. The apparatus comprises a sample and hold circuit, a converter, and an adjustable current circuit. The sample and hold circuit is adapted to receive an analog input signal and to output an amplified signal. The converter is coupled to the sample and hold circuit and that converts the amplified signal to a digital signal. The controller is coupled to the converter and that receives the digital signal. The controller includes a plurality of voltage ranges, wherein each voltage range is associated with a current value, and the controller compares the digital signal to at least one of the voltage ranges to output at least one of the current values. The adjustable current circuit is coupled to the sample and hold amplifier and to the controller so that the adjustable current circuit provides a generally constant operating current that corresponds to the current value output from the controller.