Electronics > Metrology

Characterising low frequency noise?

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alanambrose:
Hi,

I thought it would be helpful to gather together a thread with the various pre-amp designs for measuring noise, plus the thinking and characterisation techniques focusing mainly on 0.1-10Hz voltage ref noise, but also bearing in mind ~10-20MHz PSU/regulator noise. This is a new area for me, so I feel only qualified enough to ask some of the dumb questions :) I'm hoping to get people's thoughts on:

+ the pluses and minuses of the various available pre-amp designs
+ techniques for measuring / characterising noise
+ instruments and techniques

As always I'm hoping that we might sometimes wander off topic to relevant sub-topics. Following Rumsfeld, here are some things I think I know, but am pretty sure I don't understand well...

Some of the various amplifiers / filter designs for noise measurement:

Linear (Jim Williams) - http://cds.linear.com/docs/en/application-note/an124f.pdf
TI - http://www.ti.com/tool/TIPD122
Andreas - https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg834013/#msg834013 (see last attachment in message)
Cosylab - http://users.cosylab.com/~msekoranja/tmp/04447683.pdf
Blackdog - https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg684852/#msg684852
c4757p - https://www.eevblog.com/forum/projects/op-amp-for-amplifiying-5uv-of-noise-to-200uv/msg365606/#msg365606 (see png at end of message)
tangentsoft - http://tangentsoft.net/elec/lnmp/misc/schematic3.pdf
Gerhard Hoffmann - http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf
Linear - Measuring 2nV/?Hz Noise and 120dB Supply Rejection on Linear Regulators - http://cds.linear.com/docs/en/application-note/an159fa.pdf

Some noise measurement and low noise design techniques:

Intersil - http://www.intersil.com/content/dam/Intersil/documents/an15/an1560.pdf
Electronic Design - http://electronicdesign.com/analog/build-your-own-capacitor-free-high-pass-filter

Some previous discussions:

Measuring amplifier for 1 / f noise 0.1 - 10 Hz http://translate.google.com/translate?hl=&sl=de&tl=en&u=http%3A%2F%2Fwww.mikrocontroller.net%2Ftopic%2F207061&sandbox=1
Low frequency, very low level, DC biased, noise measurements https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/50/
Op amp for amplifiying 5uV of noise to 200uV? https://www.eevblog.com/forum/projects/op-amp-for-amplifiying-5uv-of-noise-to-200uv/msg364788/#msg364788
Question about preamp for measuring noise. https://www.eevblog.com/forum/beginners/question-about-preamp-for-measuring-noise/msg204478/#msg204478

Filtering voltage ref noise:

TI SBVA010 - "Improved Voltage Reference Filter Has Several Advantages" http://www.ti.com/lit/an/sbva010/sbva010.pdf
Walt Jung - "Build An Ultra Low-Noise Voltage Reference" http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf

Calculating op amp noise:

Nat Semi - "Noise Specs Confusing?" http://www.electro.fisica.unlp.edu.ar/temas/pnolo/p1_AN-104.pdf
AD MT-047 - "Op Amp Noise" http://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdf
AD MT-048 - "Op Amp Noise Relationships: 1/f Noise, RMS Noise, and Equivalent Noise Bandwidth" http://www.analog.com/media/en/training-seminars/tutorials/MT-048.pdf

References:
Noise Reduction Techniques in Electronic Systems, 2nd Edition 2nd Edition by Henry W. Ott http://www.amazon.com/Noise-Reduction-Techniques-Electronic-Systems/dp/0471850683/ref=tmm_hrd_swatch_0?_encoding=UTF8&qid=&sr=
Low Level Measurements Handbook - 7th Edition http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf

As always, cost-effectiveness is helpful. For instance, I read that a 'dynamic signal analyser' (which appears to be a lf spectrum analyser) is great for making 0.1-10Hz spectral noise plots - but this is not easily available technology. Personally, these questions are bugging me atm:

+ what are the pluses and minuses of the various noise amplifier designs?
+ what are the effective lowish-cost measurement techniques?
+ what about <0.1Hz?
+ is there a cost effective technique for spectrum plots?
+ what is the good instrumentation / software for this use - analog scopes?, adcs? adc-style signal acquisition with 3458As? Tektronix 7A22? something else?
+ why do all the commercially available amplifiers and dynamic signal analysers seem to be available only as old boat anchors - does nobody do this stuff these days, or do they use different techniques/instruments?

I hope this is useful / interesting. As always, I also hope that some of our more experienced members will give us the benefit of their knowledge & thinking. Does anyone care to set the context by describing a 'map' of where we are now?

Regards, Alan

Andreas:

Hello Alan,

thanks for collecting all that info onto one site.

The design depends very much on the purpose for which you want to use the pre-amplifier.
There is no easy way which fits all.
So what application do you want to do with this amplifer.
Which noise levels and which DC-offsets do you want to cover?

Most of the designs are only valid if you have no DC-bias into your cirquit.
The input impedance is a design criteria.
Most OP-amps with low voltage noise have a large current noise.
So a low voltage noise needs a low input impedance.
With a low input impedance you cannot measure directly a unbuffered LTZ1000.
The charging pulse of the input capacitor+input impedance will degrade the LTZ1000 for many months,
since the setpoint of the temperature controller goes to infinity.

So my design is done with the constraint of being able to measure references with DC-Bias offset.
A maximum reference differential resistance of up to about 10 ohms without much affect on the measurement.
A noise << 1.2uVpp (< 0.2 uVpp) for a LTZ.
This is possible when selecting the input resistance of around 1K.
But during charging the input capacitor you will need a extra resistor in order not to kill the LTZ.


--- Quote from: alanambrose on January 31, 2016, 04:25:06 pm ---+ what about <0.1Hz?

--- End quote ---

For 0.1 Hz you will need a minimum observation time of 10 sec.
Below 0.1 Hz you have to be very patient.
And how do you distinct thermal drift from noise below 0.1 Hz?

with best regards

Andreas

T3sl4co1l:
If you've got AoE3, there's a lot in there about noise and measurement, too. :)

Tim

Kleinstein:
The instruments one could use these days would be a reasonably quality bench DMM, likely a low noise amplifier and software on the PC. The software could be just something like excel or a package to do FFT. There should also be some free versions available.  The main part of interest for the DMM would be a mode to get continuous 1PLC readings with more than about 3-4 digit resolution - so nothing really that demanding. Higher resolution might be helpful if the amplifier does not have fine adjustment.  Alternatively something like a 12 -16 bit ADC card for the PC should work too. Sound-cards are usually missing the DC and LF  part - otherwise they would be an option.
Digital scopes can be tricky and have too much low frequency-noise, and 8 Bit resolution might need a limited frequency-band. Some USB scopes with higher resolution converters might be a very good option.

50 Hz readings should be just enough to get the noise up to 10 Hz - the lower limit is possibly set by the amplifier or the time used to take date. Also thermal fluctuations get more imports at very low frequencies, especially with amplifiers based on discrete transistors.

For very low values the classical 2 channel method with 2 FET based amplifiers , ADCs and than correlation in software is also worth looking at. It may just take some time to find suitable software. The nice thing is that one could trade in longer measurement times for lower noise relatively easy. It's somewhat an advantage as thermal fluctuations are not that reproducible to allow accurate subtraction of single channel measurements - as a first approximation this still works.

For amplifier one has the choice of BJT based (high current noise), low noise AZ OPs (possibly high current noise, essentially no 1/f noise) and JFET based amplifiers (often relatively high 1/f noise and thermal effects). As far as I know, there is no clear winner here. The low current noise of JFET based versions can be essential even at low frequencies. Its just expensive to make them work well with low impedance. Usually modern amplifiers are good enough for all but extreme cases (e.g. resistors at low temperature).

Very low frequencies (e.g. < 0.1 Hz) might need DC compensation instead of AC coupling at the input.

TiN:
Nice idea, Alan, subbed to thread.  :-+

I don't have anything to fill in this thread as of this moment, but I think Keithley's 7th Edition Low Level Measurements Handbook is relevant.

Also while ago one of EEVBlog members started thread about Keithley 1801 nV preamp for 2001/2002 DMMs. I had finish design of preamp supply plugin board for it with isolated PP SMPS and ultralow noise LDO's, but did not had much time to play with that setup and EM A10 preamp head yet. Thread owner and few other volt-nuts have that board too  :-DMM.

Hopefully I'll have something worthy to show here later on.

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