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Old 6th September 2008, 05:46 PM   #21
matejS is offline matejS  Slovenia
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With Rs<60ohm, I meant R1 (and also consequently Rs). Ideally R1 should equal Rs (output impedance of circuit connected to Vin), to have some noise cancellation. And both should be as low as possible.

Please correct me, if I am wrong.
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Old 6th September 2008, 06:05 PM   #22
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Don't worry too much about matching impedance, BUT keep R1 LOW! 10 ohms is best here, but 30 ohms or so isn't too bad.
Remember, you don't necessarily want to measure 1/f noise below 100 Hz, so you might still use a cap in series with R1 to remove ultra low frequency noise, that will just make your meter flutter. Second, you should put an input resistor on the positive input that connects to ground. This is just good practice. 50K-100K should be OK. It will NOT add any extra noise as it is in PARALLEL, and not in series with the input as long as the source being measured is of low value in output impedance. Third you should keep the 1500pf or so across the main feedback cap, to reduce the bandwidth, or else you will just measure the ultrasonic stuff.
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Old 6th September 2008, 07:10 PM   #23
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Quote:
Originally posted by jackinnj
In case you were wondering:

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Thanks, I'll see how this looks. My actual measurements were very optimistic on 1/f noise. Good duals are very hard to find these days and I wanted to show something in the ballpark of $1-2 in parts. The op-amps are not critical except the headroom issues on the one that sops up the common mode signal.
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Old 7th September 2008, 01:10 AM   #24
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MateJs, don't get confused between matching and low noise design. They are different things. Use the positive input of the IC op amp as the real input. Keep R1 low in value.
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Old 7th September 2008, 08:56 AM   #25
matejS is offline matejS  Slovenia
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Hi!

I have something like this in my mind (PSU bypassing omitted).
First opamp (AD797, LT1028) is gain of 330 w/ low pass filter. I've added second stage so that I could easily make an high pass filter.
For second stage there is no need for an low noise opamp.
I am going to use rotary switches to switch between different LP/gains.

This simple circuit should work nicely. I hope I haven't miss anything.
Thanks for all the tips

Cheers,
Matej
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Old 7th September 2008, 12:37 PM   #26
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That 1 Meg input resistor is problematic if you AC couple the input. You get 330X1000000XIbias offset at the first amp, I don't remember the worst case Ibias on these amps but it might rail the output.
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Old 7th September 2008, 05:39 PM   #27
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Default Measuring 1/F noise

I have an HP35670 -- but can't find the software anywhere -- at any rate here is a nice, jointly authored article on measuring 1/f noise.

http://www.elanix.com/pdf/1overf_Noise.pdf
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Old 7th September 2008, 07:41 PM   #28
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MateJS, you are close, but it might be useful to put a series electrolytic cap in series with R1, maybe 1000 uf, but it not be too large, because it can have a very low voltage rating. This will help the potential offset, and remove any extreme 1/f from the test. Extreme gain may not be useful, because you will still have a measurable noise floor, due to still having a relatively wide bandwidth. Scott is also right about the 1meg. Only fet input stages can get away with that easily. 50K should be OK. This is what I use with the AD797(s) in my Sound Tech analyzer.
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Old 8th September 2008, 04:18 AM   #29
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i just realized im looking right at an excellent noise measuring device..... my PC.
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Old 8th September 2008, 01:37 PM   #30
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Default Re: Measuring 1/F noise

Quote:
Originally posted by jackinnj
I have an HP35670 -- but can't find the software anywhere -- at any rate here is a nice, jointly authored article on measuring 1/f noise.

http://www.elanix.com/pdf/1overf_Noise.pdf
I once went down to 11 micro-Hz with one of those characterizing current noise.
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