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Need some help with digital potentiometers

Question asked by aurelion on May 2, 2016
Latest reply on May 2, 2016 by harryh

Greetings to everyone in the engineer zone.

 

I'm an electronic hobbyist, and I'm currently looking for some advice about a circuit I'm working on. I would like to digitally control the gain of a certain dual-supply operational amplifier, but I'm having a difficult time finding the correct digital potentiometer to act as a voltage divider over the negative feedback loop.

 

I have a +5VDC supply line (vs. GND) that could be used to power the digpot, and a +/-9VDC dual voltage supply that I use to power the operational amplifier. The circuit is currently on a breadboard, and the amplifiers are using 100K physical potentiometers that I want to replace with digital variants, but I am having a tough time finding an appropriate model.

 

So what's my problem here? Well it seems that it's the voltage on the A, B and W terminals of any digipot I've tried so far. Why do digital potentiometers limit their A/B/W terminal input voltage to their supply voltage range (i.e. 0 to +5.5V), and where can I find digipots that do not? I always believed that the A, B and W terminals were supposed to be galvanically isolated from the rest of the circuitry. Let me show you an image to get a better idea of what I mean.

 

ez_circuits.jpg

What can I do to solve this, and if isolated digipots aren't an option what other solutions do you propose?

 

Thanks in advance

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