A bandgap voltage reference is provided in quasi-parallel with a resistor in the input path of a digital input circuit. Because of the quasi-parallel nature, the current used by the digital input circuit is limited to an amount based on the value of the external resistor. The input current is split between circuitry used to provide the logic signal across the selected isolation barrier and a sink transistor so that the current remains constant. This allows the digital input circuit to accurately limit input current without needing field-side power.