Synthesize precision Dpot resistances that aren’t in the catalog

A silly simple and ubiquitous circuit network is a variable resistance consisting of the series connection of a manually adjusted rheostat-connected pot and fixed resistor shown in Figure 1.

Rmax = Rs + Rr Rmin = Rs Iab = (Va – Vb)/R

 Figure 1 Classic variable resistance with the series connection of a manually adjusted rheostat-connected pot and a fixed resistor.

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The availability of pots and resistors spanning ohms to megohms makes optimum choices of Figure 1’s component values obvious and easy. But if an application calls for using a digital potentiometer (Dpot), the situation gets more—ahem—interesting.

Dpots are only available in a relatively narrow range of resistance compared to manual pots. They also suffer from larger wiper resistances and wider tolerances. These limitations make them a dubious choice for implementing precision rheostats if Figure 1’s classic passive topology is solely relied upon. Figure 2 offers an active and more Dpot-friendly alternative.

Here’s how it works.

Rmax = (Rab-1 + Rp-1)-1 Rmin = (Rab-1 + Rp-1 + Rs-1)-1 R = (Rab-1 + Rp-1 + (N/256)Rs-1)-1 Iab = (Va – Vb)/R

Figure 2 Synthetic Dpot evades problems using FET shunt, precision fixed resistors, and op-amp.

Despite the fact we’re implementing a variable resistance, Dpot U1 is operated in potentiometer mode. So, its resistance tolerance (+/-20% for the MCP41xx series) has little negative effect. The precision of Rs and Rp dominate. Likewise, Dpot wiper resistance is rendered purely academic by the pA input current and T ohms input impedance of A1. A1 and Q1 are connected as a programmable current source. Its output is proportional to the Va – Vb voltage differential, thus forming a precise programmable resistance. This relationship makes current Iab linearly proportional to N.

Design equations are for appropriate resistances starting from specified Rab, Rmax, and Rmin are:

1.  Rab > Rmax
2. Rp = (Rmax^-1 – Rab^-1)^-1
3. Rs = (Rmin^-1 – Rab^-1 – Rp^-1)^-1

 Figure 3 shows a typical design example for Rmax = 20k, Rmin = 1k.

Figure 3 Synthetic rheostat design example where Rmax = 20k and Rmin = 1k.

Figure 4 plots R and current per (Va – Vb volts) as functions of N.

Figure 4 Performance of Figure 3’s circuit with values shown, the linear relationship between N and Ia conserves the Dpot’s limited 8-bit resolution.

Note the accurately linear relationship between N and Ia current which does a good job of conserving Dpot limited 8-bit resolution.

A question arises: What if the required Rmax is larger than the Rab resistance of available Dpots? Figure 5 offers a practical (although admittedly somewhat busy) solution that can easily implement an accurate Rmax extending far into the multi-megohm range.

Rmax = Rp Rmin = (Rp-1 + Rs-1)-1 R = (Rp-1 + (N/256)Rs-1)-1

Figure 5 Two buffer amps remove Rab from Rmax equation, allowing an for an Rmax extending far into the megaohms.

Another (stickier) question is: What happens if the polarity of Va – Vb is subject to reversal? Figure 1 can accommodate this without a second thought, but it’s a significant problem for this design idea.

I’ll have to get back to you on that one!

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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