Opamp current noise.
I got a call the other day from someone seeing =91too much=92 warm up drift in the Johnson noise voltage from our Noise fundamentals instrument.
I had them send it back to me. (A few hundred dollar learning experience) After much head banging, it turns out that the voltage noise of the first stage opamp increases by ~3-5% in an exponential way with a ~10 =96 20 minute time constant. (I haven=92t had time for a carful measurement.)
Anyway, along the way I became worried about the current noise of the opamp (opa134). I=92m wondering if any one has an idea on how to measure this. I think Phil H. mumbled something about a correlation technique when I mentioned this a year or so ago. But now I can=92t see how to make that work.
My current idea (NPI) is to just switch in a second similar opamp. Like this,
switch | V |\ +-/.---|+\ | | >-+-->(output to nowhere) | +-|-/ | | | |/ | | +------+ |\ +---------------|+\ | | >-+-->(output to more gain, R +-|-/ | filter, multiplier) R | |/ | R +------+ | GND
And measure the increase in noise for different R=92s. Will this work? Any other ideas?