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
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.