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Old 4th September 2008, 10:18 AM   #1
matejS is offline matejS  Slovenia
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Default Measuring low noise

Hi!

I own a "standard" oscilloscope that is capable of measuring down to mV.
But I want to measure uV or even better nV. Who doesn't, right?
... and I do not want to buy an expensive equipment. Typical DIY situation. What to do?
I guess I need to make an low noise, high precision amplifier, i.e. use such Op-amp w/ good PSU (e.g. battery).

My question is how have you solved this problem?

Thanks,
Matej
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Old 4th September 2008, 01:35 PM   #2
AndrewT is offline AndrewT  Scotland
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Here is the link to a 40dB/60dB gain stage that is battery driven (isolated) using low noise opamps and as an alternative the relatively cheap opa228
http://tangentsoft.net/elec/lnmp/

There's also an Audio Express article from 2007 by Dennis Colin doing similar.
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Old 4th September 2008, 02:30 PM   #3
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There is also a mistake in Dennis Colin's article -- in the original the opamp feedback resistor was specified as 1k5 -- the actual value is 51k1.

You want low noise, a.c. coupled and low input capacitance -- the correction appears on page 50 of the June '07 issue of AX.

You should also check out the data sheet for the TL431 from Texas Instruments which has some noise measurement techniques, and the following application notes from Linear Tech:

AN83 -- "Performance Verification of Low Noise, Low Dropout Regulators" and

AN85 -- "Low Noise Varactor Biasing"

If you need to measure really, really low voltages and currents, then there are a bunch of papers in the journal "Superconducting Science and Technology -- which you can access via JSTOR (or at a University Library).

Bob Pease has some interesting stuff on the foibles of teflon amongst his whitepapers at National Semi.

I keep an old analog scope (5223) with a differential amplifier on hand for looking at noise.
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Old 5th September 2008, 03:26 PM   #4
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I might comment a little about low noise measurement. There are 2 important factors:
1. a low noise input gain stage
2. a defined bandwidth, so that only the audible noise is measured and defined.

The 797 is an excellent choice, but be careful of the feedback resistors. They should be as low value as possible. Even 100 ohms is too much.
The reason oscilloscopes generally don't have too much sensitivity, is that they have such high bandwidth. This increases with the square root of the total bandwidth. Therefore a 100MHz scope will be 100 times noisier than a 10,000 Hz bandwidth amp, with the same input stage.
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Old 5th September 2008, 03:45 PM   #5
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The Tektronix 7a22n and 5a22n have adjustable bandwidth for this reason. 5a22's go for less than the cost of a couple Budweisers...problem is finding a mainframe with a decent phosphor.

Another good paper -- from Burr Brown (on TI's website) is "Noise Analysis of FET Transimpedance Amplifiers"
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Old 5th September 2008, 05:38 PM   #6
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Ah yes, the 7a22 my most missed lab instrument.

Current record (I've seen) for low noise .065nV/rt-Hz. I have not read the entire article, but if the input transformer has to be tuned for a constant 3dB noise figure then this is less impressive. In any case this example is built for 3dB noise figure on a .5 Ohm source (SQUID)! Probably nice for ribbon mics too.

http://www.picovolt.com/win/elec/art...-xfmrs_RSI.pdf
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Old 5th September 2008, 05:43 PM   #7
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Quote:
Originally posted by scott wurcer
Ah yes, the 7a22 my most missed lab instrument.

Current record (I've seen) for low noise .065nV/rt-Hz. I have not read the entire article, but if the input transformer has to be tuned for a constant 3dB noise figure then this is less impressive. In any case this example is built for 3dB noise figure on a .5 Ohm source (SQUID)! Probably nice for ribbon mics too.

http://www.picovolt.com/win/elec/art...-xfmrs_RSI.pdf
I worked with somebody on a project for NQR measurement, and we built a pre-amp that delivered about 0.3nV /rtHz, using very large geometry custom JFETs that were left over from somebody else's project. The frontend JFET ran at about 50mA. The current noise was pretty good too.
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Old 5th September 2008, 06:01 PM   #8
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65pV/rt Hz -- were you disappointed that they used an LT1028 instead of an AD797 in figure 3?
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Old 5th September 2008, 07:42 PM   #9
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Quote:
Originally posted by jackinnj
65pV/rt Hz -- were you disappointed that they used an LT1028 instead of an AD797 in figure 3?
It is funny considering the rest of the sockets, but the part was not released until after the publication date. I think they would reconsider now.

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Old 5th September 2008, 11:05 PM   #10
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What about resistor values, Scott?
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