High gain current amplifier has excess noise
- From: alan <no-longer-valid@xxxxxxxxx>
- Date: Tue, 07 Jun 2005 04:34:39 -0700
Hello people,
I'm building a very simple current amplifier consisting of a LT1793 and a 10G resistor. I took reasonable precautions to reduce stray capacitance and leakage currents (e.g. the - pin is not inserted into the op-amp socket). It works ok, except at frequencies above a few tens of Hz there is excess noise over the expected johnson noise of the resistor. It seems to level off to about 25ish uV/rtHz by ~100Hz. I have nothing hooked up to the input of the amp, so the input voltage noise should not be a problem. Also, the input current noise is only supposed to be .8fA/rtHz or so, so that shouldn't be a problem either. Right now, the amp is powered by 7V batteries, so power supply shouldn't be a problem either. I suspect it might have something to do with "something is coupling to something else", or that there are some output pathologies with running the amp at such a high gain. Due to the self capacitance of the resistor, the amp has a rolloff near 400Hz or so.
Any ideas what the problem is? .
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