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. |
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.
