PIC controlled op-amp sensitivity?

[Richard S]

Hi everyone.

I am working on a project which has an op-amp (measuring very high temperatures).

The op-amp has a variable sensitivity function, which uses a 100K potentiometer, wired up as shown in the attached image.

I want to make this resistance adjustable by a PIC, but I am not sure how to go about it. I cannot use a digi pot or a DAC, as the potentiometer is not across a fixed voltage, and all the digi pots I have seen require one end to be grounded. Aditionally, the circuit operates of 12V, and I have not seen any digi pots that can run off that.

Does anyone have any idea on how I can make the sensitivity varied? Relays and resistors (in parallel) would work, but be very messy. Replacing the relays with two 4066s would be a little neater, but still not great.

Many thanks

Richard

 

[kpatz]

A FET perhaps? The PIC can control the gate voltage which will control the resistance across the source & drain. Don't know how well that will work in a circuit that floats like that though, since FETs respond to gate-source voltages.

How about keeping the op-amp sensitivity fixed and use a digipot as a divider at the input?

 

[MikeMl]

If you can reconfigure the circuit so that one end of the variable resistance is at ground, it would be a lot easier.

 

[crutschow]

That circuit operates very strangely. You have a variable pot, AC coupled through a capacitor to the + input of IC2, with a resistor between IC2's output and the input. IC2 will stay latched at the supply rail or flip between the two supply rails depending upon the AC signal. Is that what you want?

Where does the - input of IC2 go?