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Generating a bipolar offset for an op amp
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netdudeuk:
Hi

I have the below circuit (courtesy of Louis, Scullcom Hobby Electronics).

It is a work in progress but adjusting RV7 does indeed allow a positive / negative offset to be added to the output.

The problem is that I'm using a precision trimmer to do this and even if I used a regular pot, it would probably be a bit hit and miss in terms of what the offset actually is.

So, what I would like to do is either drive the junction of C21 / R15 with a DAC or the like (so that I get to say exactly what the output voltage / offset is) or use a regular pot with an ADC to read in the value at that junction.

The preferred option would be to do the former.  It wouldn't matter if I needed to use a relay to switch the polarity.

Any thoughts please ?

Thanks
cvanc:
I'm not quite sure what your goal is?

Are you trying to just null this circuit to have a low DC offset?  (If yes, consider an op-amp based DC servo loop.)

Or are you trying to *modulate* the DC offset like it was a signal?
ledtester:
It's for controlling the DC offset of a function generator circuit. I imagine the OP wants to control it digitally.

Schematic is attached. Relevant part of the circuit is bottom left of center.

netdudeuk:

--- Quote from: ledtester on August 16, 2019, 02:31:20 pm ---It's for controlling the DC offset of a function generator circuit. I imagine the OP wants to control it digitally.

Schematic is attached. Relevant part of the circuit is bottom left of center.

--- End quote ---

Thanks guys.

Yes, that's where I have taken that block of hardware from and yes, I would like to drive it digitally.

However, I am using my own DDS device.
MarkF:
I found that I could replace the adjustment pot and series with just a DAC and an op-amp to shift the +4V output to +/-2V output.

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