Noise in 7-10.5 kHz Range: Solutions Needed

[nbo10]

Hi all,
I have a high gain current to voltage amp connected to 2-3 meters of coax which is picking up noise in the 7-10.5 kHz range. Does anyone have any expereince in waht could be the source of this noise? Right now I'm using a couple op-amps and a low pass filter to get the noise to a acceptable level but I would really like to get that bandwidth back. Thanks

 

[chroot]

Is the amplifier specifically designed to drive the large capacitance of 2-3 meters of coax? It's entirely possible the amplifier is unstable and oscillating in that region of the spectrum.

- Warren

 

[Averagesupernova]

As long as he has his impedances matched the coax should not appear as capacitive. Tell us more please nbo10.

 

[nbo10]

It's a two stage amp.
The first stage has a 100mV/nA gain.
I can't say much about the first stage beacuse they sanded the part number off the opamp and the circuit is kinda small. But I think it is a AD549(inverting) and 100Mohm feedback resistor. I measured the capactance of the coax I'm using and it's around 500pF. The literature from the Amp manufacturer says that in the worst case I would have 100pF at the input.

The second stage is pretty basic. There is a Instrumentation Ampilfer that provides a gain of, 1, 10, or 100. Followed by a OP27 with adjustable gain 0.75-1.25. Followed by a simple RC lowpass filter. Then another OP27.

I put a 10nF cap across the feedback resistor of the first OP27 to help filter the noise. It does an alright job, but I'm worried that I need the bandwidth.

THe current I'm measuring is about 1nA.

Thanks