A Microchip Inc datasheet covering the MCP4xxx family of digital potentiometers (Dpot) includes an interesting application circuit on datasheet page 15. See Figure 1 for a (somewhat edited) version of their Figure 4-5.
Figure 1 Amplifier with Dpot pseudologarithmic gain control that runs away at zero and 28. Source: Microchip
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As explained in the Microchip accompanying text, the gain range implemented by this circuit begins to change radically when the control setting of the pot approaches 0 or 256. See Figure 2.
Figure 2 Pseudologarithmic gain goes off the chart for codes below 24 and above 232.
As the datasheet puts it: As the wiper approaches either terminal, the step size in the gain calculation increases dramatically. This circuit is recommended for gains between 0.1 and 10 V/V.
This is a sound recommendation. Unfortunately, it involves effectively throwing away some 48 of the 256 8-bit pot settings, amounting to nearly 20% of available resolution. Figure 3 suggests another solution.
Figure 3 Add two fixed resistors to bound the gain range to the recommended limits while keeping full 8-bit resolution.
If we add two fixed resistors, each equal to 1/9th of the pot’s resistance, gain will be limited to the recommended two decades without throwing away any codes or resolution to do so.
The red curve in Figure 4 shows the result.
Figure 4 Two added resistors limits gain to recommended 0.1 to 10 range without sacrificing resolution. The red curve shows the result.
Note that none of this has to do with wiper resistance.
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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