I have a question regarding an analog circuit front end that I am trying to get working for an arduino project. I have part of it working but can't see to get everything working. My basic circuit is a bridge where all the resistors but one are fixed, the one resistor varies a little so this changes the bridge voltage output, I currently feed this through an op-amp to add some gain, I can vary the gain and I usually get around 2 volts out.
However I don't want to measure this output voltage, what I want to do is adjust the bridge voltage so this op-amp output voltage remains constant, when the opamp voltage output drops I need to increase the bridge voltage back up to keep it constant. I've tried doing this two ways, one is to feed the opamp voltage into the sense pin of a voltage regulator (about 1.25 volts) and use the regulator to control the bridge voltage up and down, but this didn't work..? not sure why, could have been a bad component so I might try again.
The other way I tried doing it was to add a difference amplifier where I have a preset reference voltage going to into the + opamp input and the opamp output from the bridge going into the - (inverting) input. This way as the bridge output drops there is less voltage on the inverting input so the voltage output from the difference amplifier goes up. This didn't work either...?
Does anyone know how to make this work? I'd prefer doing it all with opamps as I can get a quad opamp package and do it all with one chip.
P.S. I have a diagram but I can't figure out how to attach it to this message??