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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Merry Christmas folks!
I am trying to do some neuroscience here and I have a multichannel amplifier system that has way too much noise going on. It's half commercial system (a little preamp with gain 10 link), half hacked together by various postdocs and technicians over the years (gain 500 main amplifier / filter box), so even I don't really know what I am dealing with. I just need some very generic advice where I could start looking. The figure shows three of the channels, blue is across a 1MOhm resistor which is the nominal impedance that we would be recording from. Scale is in Volts after the 5k gain, so the RMS noise is about 30 microvolts. Green is a short circuited channel from the preamp, and red is short circuited directly into the main amp. The time scale is 30kHz, i.e. ~30ms full scale, so there is no strong mains component but just a lot of high frequency stuff going on.
With so little information on the system, what am I looking for here? I think it comes down to two things. Firstly, what is a good grounding scheme? I have separate signal and power grounds and if there were any loops I would be picking up a big mains component, so I seem to be doing alright in that respect? Secondly, maybe there are some random things I should be aware of, like switching off fluorescent light, laptop power supplies, anything that has a switch mode power supply? I am sure someone here has been working with a similar system and has some anecdotes on what works and what doesn't...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Two things to be aware of when dealing woth noise.
a) Use a filter to se[erate npoise in channel from noise out of channel (hinr, noise put of channel becomes irrelevant) - genrally this can be done spectrally, but any orthoganl basis will do, sequency for instance
(have you notied ,y post has some in-channel noise? Ot's late, and I've been at the Port, and I'm staying at my Brother's, whose keyboard has all the characters worn off)
And any spetrcal peaks of noise kock down with a filter until they;re equal to the general noise level (called "whitening")
b) once the noise os spectra;y flat, use a matched filter - which is the optimum way of detecting signal in white noise
bit before you do those, make sure your grounding and other injections of noise you don't *real'y* need to suffer, are at a minimum.
OK, need to get back to the Poer, with my weather forecast whic is why I wenr to the PC in first place!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, compare the measured noise voltage to the theoretical minimum: sqrt(4kTBR) or whatever it is. That's RMS, peak-to-peak is about 6 times it. And referred to the input, so you have to multiply by the gain.
If you have about that much noise on your screen, then the DAQ is working as well as can be expected. The problem is in the experiment, and you need to make the signal stronger, or lower the source impedance (R in the above equation- that's why the noise gets better when the input is shorted) or use some filtering to decrease the measurement bandwidth (the B in the above equation)
If you have a lot more noise than the theory predicts, the amplifier is dodgy and it would be worth spending time to repair or improve it.
"B" is half the sampling rate, if your ADC has a proper anti-alias filter. If it doesn't, noise could alias from any frequency whatsoever, B could be practically infinite, so check the anti-alias filters.
One final possibility is, maybe the amp is working fine, but not optimised for the source impedance of the experiment. (Which may not even be 1M as I hinted above.) An amp that gives its best noise figure into 1M ohm is very different from one that performs best with a 200 ohm source.
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Denoising my data acquisition system
whic is why I wenr to the PC in first place!
