DPPs make nonvolatile microvolt DAC
Edited by Bill Travis
Stephen Woodward, University of North Carolina, Chapel Hill, NC -- EDN, October 31, 2002
The availability of a seemingly limitless variety of monolithic DAC chips makes it easy to implement most digital-to-analog-conversion applications with a single off-the-shelf device. Sometimes, an unusual set of requirements necessitates a multichip approach, however. One example of such a requirement is the need for nonvolatility of the DAC's setting in power-up and -down cycles. Another example is the need for output resolution and stability at less than 1 µV. The circuit in Figure 1 combines inexpensive DPPs (digitally programmed potentiometers) from Catalyst Semiconductor (www.catsemi.com) with precision current references from Burr-Brown (www.ti.com) to achieve both nonvolatility and less-than-1-µV performance. Accurate simulation of the signals from high-temperature platinum-rhodium-based thermocouples requires less-than-1-µV performance. These temperature sensors have Seebeck coefficients of only 6 µV/°C. Therefore, only voltage sources with 1-µV-level stability and precision can simulate such sensors.
To achieve such low output drift would normally require the use of active circuit elements, such as chopper-stabilized amplifiers, with offset temperature coefficients not much higher than 1 nV/°C. The circuit in Figure 1 takes a different approach by using current division and a passive and, therefore, inherently drift-free output that needs no amplifiers. Each half of the REF200 sources a 100-µA reference current. The twin currents each connect to the wiper of DPPS, IC1 and IC2. There, they split into two currents (for example, I1 and I2) in a wiper-to-total ratio, K1, which the programmed setting of the DPP determines. I1=K1×100 µA, and I2=(1–K1)×100 µA. I1 passes through the series combination of the 48.7Ω resistor and the 1Ω output resistor and thereby generates the output voltage: V=K1(50Ω×100 µA)=0 to 5 mV as K1 varies from 0 to 1. The operation is straightforward and drift-free. Unfortunately, the resolution with a single potentiometer is inadequate for many precision applications.
IC1, a CAT5113, like other DPPs, offers the versatility of an uncommitted resistance element and nonvolatility of the setting. Its resolution, however, is only 100 steps, which is slightly worse than 7 bits and equivalent to 50 µV in this circuit. You therefore incorporate a second DPP, IC2, in the converter. IC2's output current acts into the 1Ω load for a 50-to-1 resolution enhancement over IC1 alone. IC2 thus adds a 0- to 100-µV contribution to V. Hence, the composite output is V=K1/200+K2/10,000 with a 5-mV span and 1-µV resolution. The circuit is an ideal approach for such applications as the simulation of thermocouple signals in precision temperature-measurement and -control systems.
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