An interesting variation on the theme of digital to analog converters (DACs) is the digital potentiometer (DPOT). In addition to being able to output a voltage proportional to a digital value when used as a multiplying DAC (MDAC), the DPOT can also work as a digitally programmed resistor (rheostat). When used this way, an important parameter that sometimes limits DPOT accuracy is wiper resistance: R_{W}.

Of course, being solid state devices, unlike electromechanical potentiometers DPOTs don’t have a physical resistance element with an actual wiper running around on it. What their “R_{W}” really comprises is the ON resistances of the array of FET switches that select the desired tap on the internal resistor ladder (2^{6} + 1 = 65 taps for a 6 bit pot, 2^{7} + 1 = 129 for 7 bits, 2^{8} + 1 = 257 for 8, etc.). R_{W} effectively appears in series with the selected resistance so that if:

R_{AB} = total resistor ladder resistance = 5k (typical) for exemplar DPOT (MCP4161-502).

N = ladder tap selection setting (0 ≤ N ≤ 256 for 8 bit exemplar).

Then in an ideal world (where all R_{W} = 0), the resulting resistance would be simply:

Unfortunately, in the world of real DPOTs, R_{W} > 0. Consequently:

For the exemplar 8 bit 5k DPOT, R_{W} = 75 ohms (typical, 160 max), setting a minimum (N = 0) R_{AB} = 75/5000 × 256 = ~4 lsb (typical) 8 lsb (max).

Being unable to set R_{AB} < 75 ohms for N = 0 may already be problematic for many applications, but the ill effects of R_{W} > 0 extend to other N. For example, the R_{AB} ladder tempco of the MCP41 series is an excellent 50 ppm/°C (typical), but R_{W}’s tempco is orders of magnitude worse at ~3000 ppm/°C. Therefore, R_{W} dominates net tempco for any N < 230.

Suffice to say, cancellation of R_{W} would make worthwhile improvements to DPOT performance in many precision applications. Figure 1’s topology does this. Here’s how it works.

Figure 1. |
Op-amp A1 actively drives digital pot U1 wiper terminal PW0 to force V_{PBO} = V_{B} whiledrawing negligible current through resistance R _{WB}, thus cancelling the effect of R_{W}. |

The connections of A1’s (+) input to reference voltage V_{B} (typically, but not necessarily, ground), (-) input to U1’s PB0 pin, and its output to U1’s PW0 pin establishes a feedback loop that forces V_{PBO} = V_{B }independently of R_{W}. This, as advertized, wipes out R_{W} effects.

Compensation capacitor C1 probably isn’t absolutely necessary for the part selection shown in Figure 1 for A1 and U1, but if a faster A1 amplifier or a higher R_{AB} resistance DPOT where chosen, it probably would be.