Noise in 7-10.5 kHz Range: Solutions Needed

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Discussion Overview

The discussion revolves around identifying and mitigating noise in the 7-10.5 kHz range encountered in a high gain current to voltage amplifier setup. Participants explore potential sources of noise, amplifier design considerations, and filtering techniques, focusing on both theoretical and practical aspects of the problem.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant describes their setup involving a high gain current to voltage amplifier connected to coaxial cable, experiencing noise in a specific frequency range.
  • Another participant questions whether the amplifier is designed to handle the capacitance of the coax, suggesting it may be unstable and oscillating.
  • A different participant asserts that if impedances are matched, the coax should not present as capacitive, requesting further details about the setup.
  • The original poster provides details about the amplifier stages, including gains and components, and mentions efforts to filter noise with a capacitor across a feedback resistor.
  • Concerns are raised about the need for bandwidth despite the filtering efforts, given the low current being measured.

Areas of Agreement / Disagreement

Participants express differing views on the potential causes of the noise and the effectiveness of the current setup. There is no consensus on the source of the noise or the adequacy of the filtering methods employed.

Contextual Notes

Some assumptions about impedance matching and amplifier stability are not fully explored, and the specific characteristics of the op-amps and their configurations may influence the discussion but remain unresolved.

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

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