*Chuck Wojslaw*

*EDN*

A key parameter of digitally controlled potentiometers (DCPs) is the number of taps (n) programmable positions of the wiper. This parameter establishes the resolution in programmable-voltage and -current applications. A number of circuit techniques exist for improving the resolution using one or more DCPs with a given number of taps. The circuit in Figure 1 has no theoretical limit on increasing the resolution in programmable-voltage applications. The IC_{3} amplifier circuit is an inverting summer with weighted input-resistor values.

Figure 1. |
The sky’s the limit on resolution in this programmable-voltage circuit. |

The input voltages to the summer are the programmable output voltages of the DCPs. To reduce the effects of loading, IC_{1} and IC_{2} buffer the signals from the potentiometer wipers. For an n-tap potentiometer, the input resistors of the summing amplifier are R and n_{R}, providing a programmable output voltage of

with a resolution of

For the dual 64-tap X9418 DCP and the circuit values shown, the output voltage, V_{OUT}, has n^{2}, or 4096, programmable values. The full-scale value is 2.5391 V, and the smallest programmed voltage is 0.62 mV. You can program the coarse DCP_{1} from 0 to 2.5 V with a resolution of 39 mV, and you can program the fine DCP_{2} from 0 to 39 mV with a resolution of 0.62 mV. This circuit provides the same resolution as a 12-bit D/A converter. Measured data fell within 2 LSBs of calculated values. Adding more potentiometers, buffers, and input resistors provides theoretically unlimited resolution. If you add a third section, the resolution increases to one part in 262,144 (18 bits). You can implement a similar scheme using a noninverting summer circuit. You can use the circuit as a substitute for expensive D/A converters in any application that requires a precise, high-resolution programmable voltage.

See Why JLCPCB Is So Popular: https://jlcpcb.com

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