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For fun I'm trying to design an epic amplifier with programmable gains from ~1.4 to 256. The opamp being used is a TLV2452.

The digipot A can reach a minimum of 390 Ω. The digipot B can reach a maximum of 100 kΩ.

The voltage source, 3.3 V in the picture, can actually reach much lower values as it's from a real world signal.

enter image description here

The problem that I'm having is that digipot B doesn't seem to work properly. Even when I remove digipot A and replace it with a static resistor, digipot B still shows weird values. It shows negative resistance values over the terminal even when no 3.3 V signal is applied.

I'm confused about how to set the wiper terminals to their correct pins in order for current to flow across them properly. I know the signals that I send to them are working properly because when I isolate each device, the microcontroller changes the resistances correctly. It's only when they get integrated with the opamp - specifically as the forward resistor (like digipot B) that they start to act weird.

For example when the input signal is 1 volt and the gain is at max (256) the opamp will only output 6 V, even when the signal is within the sampling range of my oscilloscope.

Is it possible to use two digital potentiometers in this way?

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  • \$\begingroup\$ What measurement/calculation did you do to obtain a negative resistance? \$\endgroup\$
    – nanofarad
    Commented Oct 10, 2022 at 18:42
  • \$\begingroup\$ what are you asking? \$\endgroup\$
    – jsotola
    Commented Oct 10, 2022 at 20:35

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In the MPC4131 datasheet it says:

The voltage on terminal W must be between VSS and VDD .

You might not be able to have one pin floating (I've seen this in the past with other digital pots). Ground P1A and it should work.

Also, check the voltage on the V- pin to see if your are violating the common mode range of the opamp. Also check it with a scope to see if there are any oscillations, wiper networks are full of switches and do weird things when put in feedback.

If you aren't trying to change the gain of the opamp you might just want to use a DAC and gain the output to get a DC offset which would be much less noisy.

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  • \$\begingroup\$ Thanks but I tried that and it didn't work \$\endgroup\$
    – Mr Man
    Commented Oct 10, 2022 at 20:38
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    \$\begingroup\$ Try taking the wiper connected to GND off of A and see if the problem goes away \$\endgroup\$
    – Voltage Spike
    Commented Oct 10, 2022 at 20:43
  • \$\begingroup\$ Wow that really sucked... It seemed like something weird was happening with what you were saying. I got it to work by messing around with the wiper pin like you suggested. I wound up leaving one of the end nodes disconnected. My advice for someone in a similar situation would be to experiment with all possibilities to connect the high voltage value to the wiper and leave a pin disconnected if needed. However my end circuit and input values ended up being slightly different than what I described in the question. Still, i had to experiment with the digipot cnxns to make it work \$\endgroup\$
    – Mr Man
    Commented Oct 26, 2022 at 17:41
  • \$\begingroup\$ Might be good to post a sch of the answer for future users as an answer \$\endgroup\$
    – Voltage Spike
    Commented Oct 26, 2022 at 17:50

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