Analog circuits today are typically biased from ±5 V to ±18 V. But the trend is toward lower voltages. For this reason, many components are designed to operate primarily in ±5-V systems. One example is the digitally controlled potentiometer (DCP). Can a low-voltage DCP be employed in a high-voltage circuit such as an amplifier? The answer is yes, if caution is paid to how and where the DCP is used.
This ±12-V inverting amplifier circuit is designed for a nominal gain of ten (Fig. 1). Use of a low-voltage, programmable DCP extends the programmable gain limits from 7.7 to 13.9. Xicor's 256-tap X9250/58 provides a gain resolution of 0.024 over this range. Therefore, it allows accurate gain setting without using expensive, precision resistors.
One feature of the inverting configuration is that the wiper is connected to 0 V or virtual ground. Consequently, any high common-mode voltages on the potentiometer terminals are removed. Another benefit is that the wiper resistance of the DCP is in series with the high input-resistance of the operational amplifier. So any error due to the wiper resistance is eliminated as well.
For the inverting amplifier, the gain is given by:
G = −(R2' + R2)/(R1' + R1)
A noninverting amplifier circuit can't be used since it lacks a virtual ground. The relative values of the potentiometer resistances R1' and R2' compared to R1 and R2 are established by the relative values of the maximum DCP and circuit voltages. For a 12-V circuit and a 5-V DCP, R1' and R2' must be less than five-sevenths of R1 and R2. Clamping-diodes D1 through D4 offer overvoltage protection to the DCP during power-up, power-down, and fault conditions.