16-bit adjustable reference uses 8-bit digital potentiometers
Edited by Bill Travis
Bonnie Baker, Tucson, AZ -- EDN, December 25, 2003
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, IC3A and IC3B, span across VREF to ground, and the wiper outputs are connected to the noninverting inputs of two amplifiers, IC4A and IC4B. 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 IC3A and IC3B through its SPI port. If VREF is equal to 4.096V, the LSB at the outputs of IC4A and IC4B is 16 mV.
To make this circuit perform as a 16-bit DAC, a third digital potentiometer, IC2A, spans across the outputs of the two amplifiers, IC4A and IC4B. The programmed setting of IC3A and IC3B sets the voltage across this third digital potentiometer. If VREF is 4.096V, you can program IC3A and IC3B such that the output difference of op amps IC4A and IC4B is 16 mV. You can achieve high accuracy with this circuit by using a dual digital potentiometer for IC3A and IC3B. 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 (VREF/216). Table 1 shows the critical device specifications to obtain optimum performance with this circuit.
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I found an error in a formula in Figure 1 of the article "16-bit adjustable reference uses 8-bit digital potentiometers."
The formula reads
V3 = V4 = (V1 - V2)(R3B) / (R3A + R3B)
The formula should read
V3 = V4 = [(V1)(R3B) + (V2)(R3A)] / (R3A + R3B)
Clifford Greenblatt - 2003-31-12 05:48:00 PST -
V1 and V2 each have +/- 1/2 lsb accuracy of the 8-bit pots. V1-V2 will have +/- 1 lsb = 16 mv of random variation. I don't see how this circuit has 16-bit accuracy.
Tim D. Conway - 2003-30-12 13:25:00 PST
