AC to DC converter outputting constant voltage instead of proportional output

[nadirEatsKoukies]

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

 

[FvM]

+ and - inputs flipped at U7.

 

[danadakk]

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]

The + input at U7 should be grounded.

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

The OP inputs are already flipped in tutorial Fig.7.

 

[danadakk]

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

The OP inputs are already flipped in tutorial Fig.7.

Agree, my error.

Regards, Dana.