AC to DC converter outputting constant voltage instead of proportional output

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nadirEatsKoukies

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Hi guys,

I've been trying to design a conductivity meter based on the tutorial in this link , The oscillator works perfectly in simulation but the second part of the circuit always outputs a constant ~10.5V DC instead of it being proportional to the resistance of the solution (I replaced the conductive solution by a resistor in simulation), no matter what value resistance I put.

Any idea why?

Here are the circuit and simulation

 

The + input at U7 should be grounded. Right now you have + fdbk
which has the output pinned to the supply rail, roughly speaking.


Regards, Dana.
 

FvM said:
No. It's Sallen-Key, not MFB filter topology.

The OP inputs are already flipped in tutorial Fig.7.
Click to expand...

Agree, my error.

Regards, Dana.
 

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