Digital potentiometers enable systems designers to program resistive values in the circuit during initial calibration or even later on, during normal operation of the circuit. This ability permits the dynamic alteration of the circuit conditions, creating a "smart" analog system that can respond to the surrounding environment. In fact, the programmability feature seems all too promising; as usual, suspicious analog engineers will expect that this advantage is not for free. And, they are right.
Initial examination of the temperature performance of today's array of digital potentiometers shows that the devices perform with much less accuracy than the standard mechanical potentiometer or discrete resistor combinations. Certainly, the temperature performance of these digital potentiometers is much less than ideal.
But, the clever designer can take advantage of secondary temperature behavior by using the matching characteristics of these devices. The resistive material of current digital potentiometers is predominantly fabricated with the poly-diffusions (soon to be nichrome) of CMOS processes. Because these resistors are manufactured using poly-diffusions, the resistive elements are not trimmed precisely. Consequently, the initial accuracy of the digital potentiometer from part to part at room temperature is ±30% maximum. The thermal-drift specifications for these types of diffusions are either 800 or 500 ppm/°C, depending on the poly level used for the resistive element. With these specifications, it is easy to see that the simple gain circuit shown will have poor performance over temperature (Fig. 1).
In the circuit in Figure 1, the noninverting gain is established using a standard resistor for R1 and a 256-tap, 100-kΩ digital potentiometer (an MCP42100 from Microchip Technology) positioned as R2. The gain of this circuit is determined by the ratio of R1 and R2 as stated in the formula in Figure 1. The amplifier (an MCP606) is a single-supply CMOS amplifier with a low offset voltage (± 250 µV, max.) and high input impedance (1 pA, typical at 25°C). The lower offset of this amplifier allows for gain changes with a minimal increase in offset errors translating to the output of the amplifier.
The key specifications of the digital potentiometer in Figure 1 in regard to this application are the nominal initial resistance (100 kΩ ±30%, max.) and the change in resistance over temperature (800 ppm/°C, typical). Both of these specifications can cause a significant error in the system, limiting the applications for this circuit. The nominal resistor values of this digital potentiometer are easily calculated using the formulas shown in Figure 2.
An alternative circuit that addresses both the accuracy and temperature-performance issues of the Figure 1 circuit is shown in Figure 3. In this circuit, a dual digital potentiometer is used to fill both resistor positions in the circuit. Since both resistor elements are on the same IC, their nominal matching and temperature-drift characteristics are closely matched.
Now, instead of an initial gain accuracy of ±30%, the initial gain accuracy of the circuit becomes ±1% maximum. The gain accuracy over temperature is also tightened with this topology from the previous typical performance of 800 ppm/°C to 1 ppm/°C.
With a little attention to circuit design detail, the temperature behavior of digital potentiometers can be optimized using these simple design techniques.