Effect on the + input of op amp with large cap

[glias]

Hello all,
In a simple non inverting stage, I would want to filter the noise which come from a reference placed on the + input of the op amp. My question is can I place a RC filter with a big capacitoron the + input (1000µF) to filter my "noise" without risk of instability of the op amp ?

 

[FvM]

Is the +ve input part of a feedback network? If not, how should it affect stability?

 

[checkmate]

Of course you can.
And since the +ve input of an opamp configured as a non-inverting amplifier is the input, it would also mean that your input driver must be able to drive the 1mF cap as well.

 

[glias]

Thanks for your help.
I'm wondering the effect of the integration of this large cap because the impedance of the op amp input is not infinite, it doesn't matter for 'medium' cap, but I never tried with a big one like a 1mF.

 

[vimalkhanna]

You cannot put a large 1mF Elco cap at the input as the leakage current for this is likely to be much higher than the bias current of the OPAmp.
as such the results are indeterminate .
In case you are wanting to use this as a capacitance multiplier by the Gain of the OPAmp . there is a limit to the special type of OPAmps that you can use ...Kindly go for FET input types which have picoamps as the input bias currents.The caps have to be polyester foil wound or low leakage types too.

 

[FvM]

You cannot put a large 1mF Elco cap at the input as the leakage current for this is likely to be much higher than the bias current of the OPAmp.

You surely can. You mean to say, it's not reasonable. Right. Most people would use an active filter with e.g. uF range capacitors.

But the original poster neither gave a specification of the signal and observed noise nor asked for suggestion how to solve it. He asked, if the said 1000 uF involves stability issues. The question has been answered.

 

[glias]

Thanks for your help.

Ok, the leakage of the cap is higher than the input bias current of the op amp, so what is the effect ? since my RC filter is placed after a voltage reference (that I would want to filter), what can be the problem with the leakage current of the cap and the input current of the op amp ? this may produces an offset, but I don't care about it... is there any other effects that I could forget ? if there is a current which is sink by the op amp and the cap (leakage) the voltage reference could provide it ... isn't it ?