A reference generator circuit has a resistor string between the potentials of the power supply voltage that is partitioned into a top string, a middle string, and a bottom string. PFET devices are used to couple the positive power supply voltage a selected node of the top string in response to first control signals and complementary second control signals are used to control NFET devices that couple the ground power supply voltage to a selected node of the bottom string. If a resistor is effectively removed from the top string a corresponding resistor is effectively added in the bottom string keeping the total resistance in the resistor string substantially constant. A pass gate network is used to select between nodes of the middle string as a vernier for generating small step sizes.