Digital Potentiometer Programs and Stabilizes Voltage Reference

The potentiometer portion of a mixed-signal, digitally programmable potentiometer adds variability to an analog circuit, and its digital controls provide programmability. You can use a digital potentiometer in two ways in an analog circuit. You can use it as a two-terminal variable resistance, or rheostat, or as a three-terminal resistive divider. Although both ways bring variability to the analog circuit, the three-terminal implementation usually brings other important characteristics as well. For example, a programmable voltage reference has two implementations. The circuit in Figure 1 is a voltage reference whose output, V_OUT, depends on the 1.25 V reference of the shunt regulator, IC₁, R₁, and the programmable resistance R₂ = pR_POT. The value of p varies from 0 to 1; it represents the proportional setting of the wiper from one end, 0, to the other, 1. For this circuit,

As p varies from 0 to 1, V_OUT varies from 1.25 V to some maximum value established by R₁ and the potentiometer's end-to-end resistance, R_POT. The temperature coefficient of V_OUT is proportional to those of the LM4041CIZ regulator's 1.25 V reference, R₁, and R_POT. The temperature coefficient of the reference voltage and a quality resistor are lower than 100 ppm/°C. However, the temperature coefficient of R_POT is not guaranteed and can run in the hundreds of parts per million per degrees Celsius. Thus, the temperature stability of R_POT has adverse effects on the temperature coefficient of V_OUT.

Figure 1.	In this circuit, the potentiometer’s temperature coefficient adversely affects that of VOUT.

The programmable voltage-reference circuit in Figure 2 uses the potentiometer as a three-terminal device. For this circuit,

This implementation shifts the temperature dependence of V_OUT on the potentiometer from the high temperature coefficient of R_POT to the potentiometer's low ratiometric temperature coefficient of 20 ppm/°C. It also reduces component count and cost and increases programming accuracy. The 15% accuracy of R_POT is the dominant factor in the accuracy of V_OUT in the circuit of Figure 1. In the circuit of Figure 2, the potentiometer's 1% linearity is the dominant factor in the accuracy of V_OUT. For the values shown in Figure 2, the 100-tap CAT5113 digitally programmable potentiometer provides for a variable, temperature-stable V_OUT of 1.25 to 5.5 V (0 ≤ p ≤ 0.77). The measured data correlates to better than 1% with the calculated values.

Figure 2.	This circuit has low component count, low temperature coefficient, and enhanced programming accuracy.

Materials on the topic

1. Datasheet ON Semiconductor CAT5113
2. Datasheet Texas Instruments LM4041CIZ