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Sallen key with gain, single supply problem

Question asked by NK_- on Feb 27, 2017
Latest reply on Feb 27, 2017 by harryh

Hello guys,

 

i'm experiencing some kind of problems with my filter design. I've tried to design sallen-key filter with some gain, but i didn't wanted to use symmetric supply. So i've modified the circuit with adding voltage reference and i've succesfully simulated it in ADIsimPE but with another similar OpAmp to MCP619. When my board arrived i've soldered all components and i tried to test it, but it's not working.

 

This is way i've tested that circuit:

 

Voltage at the point REF_OUT was trimmed to the 2,5 volts, confirmed by measurement
Generator of 1kHz, 20mv sine wave was connected between the VST_I or VST_Q and GNDA and confirmed by measurement

 

and i measured with oscilloscope those voltages which are all related to GNDA:

 

VST_I : sine wave 20 mV, 1kHz
Point A : V = 2,5 V + sine wave

Point B : almost same like A
STUP1_+ : same like A
STUP1_- : same like A
STUP1_O: manually created oscillograme on the picture with scheme.

 

at the second stage it is at all point 2,5volts + trace on the picture.


But at the output of the stage2 i've measured constant 20 mV.

 

I expected that at the ouput of stage two opamp will be amplified sine wave  level shifted by 2,5 volts, but unfortunatelly there is nothing. I've already four times checked the board and it should be same like scheme. I've also tried few modifications like:

 

remove R25 and connect the middle of 10k / 10k voltage divider to the point A
tried to add or remove all capacitors C29, C30 and C31
change U1 for another MCP619
change IC2 for another AD8031

 

But circuit is still not working and i've no idea why. I'm not experienced like you and  it's very  possible that i've made mistake and this circuit is not able to work neither theoretically. So guys i wanna ask, is this circuit able to work at least theoretically? And if yes, where could be mistake? I'am really out of any more ideas.

 

Thank you for your help.

 

Scheme of my circuit.

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