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 1.25V 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, V_OUT=1.25V(1+pR_POT/R₁). As p varies from 0 to 1, V_OUT varies from 1.25V 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 1.25V 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. 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 5.5V (0≤p≤0.77). The measured data correlates to better than 1% with the calculated values.

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