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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: MCP4441 Test Circuit

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:

OPO7C Null Offset Circuit

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  • \$\begingroup\$ Show a circuit of how you connected things please. \$\endgroup\$
    – Andy aka
    Commented Oct 2, 2023 at 13:16
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    \$\begingroup\$ 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. \$\endgroup\$
    – Chester Gillon
    Commented Oct 2, 2023 at 13:45
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    \$\begingroup\$ 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. \$\endgroup\$
    – Hearth
    Commented Oct 2, 2023 at 14:49
  • \$\begingroup\$ @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. \$\endgroup\$
    – ACBlue
    Commented Oct 2, 2023 at 15:02
  • \$\begingroup\$ 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? \$\endgroup\$
    – ACBlue
    Commented Oct 6, 2023 at 12:53

1 Answer 1

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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.

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