lt1367 amplifier

Hi,

I am seeing 1Meg resistor in your feedback loop, I think what you have here is a filter.
Integrator uses Capacitor as feedback.

Can you confirm the schematic?

Thanks.

Thank you for  your answer, it is really a LP filter (it is known as "not ideal integrator"), but at 50 Hz its behaviour is as integrator.  And it is working as integrator, but the gain is not stable (>2% ) as I have stated  in the previous message.

Hi,

It is possible if you can capture the whole circuit you have used in your simulation?
This is for us to further understand the concern.

Thanks and regards.

Thanks and sorry for being so late, I have had some problem.  I attach the whole circuit, but the problem is that I have not seen problems in the simulation but in the real circuit.  I have made it on a bread board and on a pcb but the gain shows a 5% of variability.

Is it possible that C1 is a ceramic capacitor with a large voltage coefficient?

thank you for the answer, C1 is x7r1206334k050dfct10000   rated 50v, and vin range is always < 2 volt rms with very little offset

X7R capacitors have a large voltage coefficient. 2VRMS is approximately 4.8Vpp. That is enough to cause a 5% change in the capacitor value. Try changing the capacitor value by 5% in your simulation and see what happens to your gain.

thank you for the answer, I have found :

vin->1v----->442mv  vout-------330n cf  @50Hz
vin->1v---->466mv  vout-------313n cf
vin-1v---->486mv  vout-------300n cf

so it might be that the shift of the pole can cause this gain change at 50 Hz.   Can you suggest me a best capacitor to use?

Really thanks

Use a NP0, C0G ceramic or film capacitor such as mylar or polypropylene. These do not have the voltage coefficient issue.

thank you very much I'll try soon