How to get rid of a sine noise from AC mains?

[SE]

noise?

Hi all, I have designed a photodiode sensor and pass it through a op-amp
the output signal have a sine noise.

a friend of mine stated that it might be the AC mains. Any idea how to vertify that and get rid of it??

thanks.

 

[IanP]

Re: noise?

I would start with checking the supply voltage quality: do you have de-coupling caps (100nF) and storage caps (10-100µF) next to op-amp?

 

[SE]

Re: noise?

I would start with checking the supply voltage quality: do you have de-coupling caps (100nF) and storage caps (10-100µF) next to op-amp?

Hi, thanks for the quick reply,
I have a 17nf cap in parallel to the cap to reduce the noise,
I do not have a cap next to the op amp as I am detecting a dc signal, the dc signal which I think would be filtered off if I placed a cap right beside the op amp

Thanksy
SE

 

[Okkam]

Re: noise?

Hi SE!

IanP ask you absolutely right question about cap on the OPAMP output.
I suppose that negative feedback in your circuit became positive on some frequency.
OPAMP with a positive feedback will become a generator.
Is OPAMP in your circuit implemented as follower with capacitive load?
If so you have unstable work of OPAMP.
In case of follower you can include low value resistor ( 1-10 Ohm ) on output of OPAMP before cap. This RC-circuit will be passive LOW-PASS RC-filter and work of follower will become stable.

If your circuit not a follower maybe you have other cause of unstability.
There exists very good book:
Ron Manchini - OPAMPs for everyone.
In this book included chapter about stability of OPAMP circuits with deep analisys.

This book is free for download through internet.
I can give you link on it.

Best regards.

Added after 4 minutes:

I'm terribly sorry. Miastaki in author surname

Mancini - OPAMPs for everyone

Added after 1 minutes:

**broken link removed**

 

[markty]

noise?

Sine noise. Sounds like your opamp is oscillated.

As you said, the signal you detected is dc, so put more capacitor will steady the signal. U may try it.

In my opinion, u should check yr opamp's phase margin. Make sure oscillation won't happen.

 

[mrm]

Re: noise?

can we have some info on the frequency of this sinewave...