It may be easy to find a precision voltage reference for your application; however, a programmable precision reference is another matter. The circuit in Figure 1 yields a precision reference with an LSB of 62.5 µV. The circuit is a 16-bit DAC using three 8-bit digital potentiometers and three CMOS op amps. Each digital potentiometer operates as an 8-bit multiplying DAC. On the left side of Figure 1, two digital potentiometers, IC_3A and IC_3B, span across V_REF to ground, and the wiper outputs are connected to the noninverting inputs of two amplifiers, IC_4A and IC_4B. In this configuration, the inputs to the amplifiers have high impedance levels, thus isolating the digital potentiometers from the rest of the circuit. The microcontroller, IC₁, programs digital potentiometers IC_3A and IC_3B through its SPI port. If V_REF is equal to 4.096V, the LSB at the outputs of IC_4A and IC_4B is 16 mV.

To make this circuit perform as a 16-bit DAC, a third digital potentiometer, IC_2A, spans across the outputs of the two amplifiers, IC_4A and IC_4B. The programmed setting of IC_3A and IC_3B sets the voltage across this third digital potentiometer. If V_REF is 4.096V, you can program IC_3A and IC_3B such that the output difference of op amps IC_4A and IC_4B is 16 mV. You can achieve high accuracy with this circuit by using a dual digital potentiometer for IC_3A and IC_3B. With the dual structure, the resistances of these two devices match typically within 0.2%. Given the size of the voltage across the third digital potentiometer, the LSB of the complete circuit from left to right is 62.5 µV (V_REF/2¹⁶). Table 1 shows the critical device specifications to obtain optimum performance with this circuit.