DAC fine-tunes reference output

Adding a DAC and op amp makes a precision adjustable reference.

Fons Janssen, Maxim Integrated Products, Bilthoven, Netherlands; Edited by Paul Rako and Fran Granville -- EDN, August 25, 2011

Data converters must have a stable reference voltage to accurately measure or generate analog signals. Such references offer many guaranteed levels of precision and stability. Their variety of output-voltage levels is much smaller, which manufacturers specify as standard values, such as 2.048, 2.500, or 4.096V. You sometimes need to dynamically calibrate the reference, fine-tune its output value, or generate a slightly different value. For instance, when you measure a voltage with a resistive divider, you could adjust the reference voltage to compensate for an error in the divider.

The buffer amplifier isolates V_OUT from the load, so the only current flowing through R₁ is that from the current source. You can implement this idea with a highly stable voltage reference, a current DAC, and a low-offset op amp (Figure 2). If you choose a value of 4.7 kΩ for R_FS1, the full-scale DAC current is 0.981 mA, as the data sheet states. With a value of 10Ω for R₁, this current value yields a tuning range of ±0.981 mA×10Ω=±9.81 mV, divided into 31 steps of 0.654 mV each.

If you use a 2.048V reference, the following equation calculates the error:
where DAC represents the DAC’s decimal equivalent-output value (−15≤DAC≤15). Thus, the DAC introduces a maximum error of 0.5969 mV, yielding a total of roughly 1 mV when you combine it with the initial accuracy of the voltage reference itself.

Because the DAC has an operating-temperature range of −40 to +85°C, you use the voltage reference’s drift specification in that same range, ±3 ppm/°C. IC makers often use the box method to specify temperature drift (Reference 1). You can then calculate this maximum reference-voltage drift over the temperature range of −40 to +85°C:
The drifts in the DAC, R_FS1, and of R₁ cause the drift in the second term (I_DAC×R₁). The IC vendor specifies drift in the DAC as a typical value of ±75 ppm/°C. You assume ±25 ppm/°C for the resistors. These values yield a typical drift:

---

Reference

Fry, David, “Calculating the Error Budget in Precision Digital-to-Analog Converter (DAC) Applications,” Application Note 4300, Maxim Integrated Products, Sept 25, 2008.