# op07c null offset with a digital potentiometer

I am using an op07c in a circuit and can null the offset very well using a 20k pot. I want to replace this pot with a digital pot. I now have a 5k resistance, a 10k digital pot and another 5k resistance in series connected between terminals 1 and 8. I have verified that the digital pot (MCP4441) works correctly without any "load". When I connect the pot to null the offset though, the resistance value measured between 1 and 8 drops to 5k or 7k (should be 20k). The values between the middle pin of the pot and pins 1 and 8 swing wildly from 75k to 280k. What am I doing wrong here?

My pot circuit looks like this:

When I measure between P0A and P0W without any load, I see 9.9k but the value varies wildly when connected to the OPO7C as replacement for the pot shown here:

• Show a circuit of how you connected things please. Oct 2, 2023 at 13:16
• What voltage is V+ on the opamp the wiper of the pot is connected to? The MCP4441 datasheet Absolute Maximum Ratings contains Voltage on all other pins (PxA, PxW, and PxB) with respect to VSS -0.3V to VDD + 0.3V, which implies a problem if the voltages applied to the digital pot are outside of the MCP4441 supply rails. Oct 2, 2023 at 13:45
• Remember that digital potentiometers are usually very restrictive on what voltages they can be used with. It's possible the MCP4441 is far outside its ratings here. Oct 2, 2023 at 14:49
• @Chester Gillon V+ is unfortunately 9V compared to the max 5.5V allowed on the P0W oin. I will have to rework the circuit to accomodate the digital pot and see how it works. I feel a bit silly for having missed such an important detail. Oct 2, 2023 at 15:02
• Modifying my circuit so that V+ stays below VDD gives the desired result. @Chester Gillon Could you post your comment as an answer so that I can accept it as a solution? Oct 6, 2023 at 12:53

## 1 Answer

What voltage is V+ on the opamp the wiper of the pot is connected to?

The MCP4441 datasheet Absolute Maximum Ratings contains Voltage on all other pins (PxA, PxW, and PxB) with respect to VSS -0.3V to VDD + 0.3V, which implies a problem if the voltages applied to the digital pot are outside of the MCP4441 supply rails.