Author Topic: DIY 0.1 to 10Hz Noise Amplifier  (Read 20710 times)

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Online Kleinstein

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #75 on: July 14, 2023, 08:22:38 pm »
Much of the reduction in the permeability of Mu metal with higher frequencies is due to eddy currents. This effect is already included in the usual model for the shielding thickness. The relevant permeability is the one wihtout the eddy current effect and thus less drop to higher frequency for actual permeability without the eddy current effect.

Mu metal is relatively expensive and need care handling: bend it too much and the permeability goes down quite a bit from the high ideal values. For a larger system the normal electrical steel (Fe-3%Si) could be an attractive option too, if magnetic shielding is needed.
 

Offline EC8010

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #76 on: July 15, 2023, 12:49:11 pm »
Mumetal is good in theory, until you discover its cost and practical problems. Distance is best for electromagnetic hum. Goes down by between square and cube of distance. Don't forget alignment, either. Worst is when coils are aligned, best when they're at 90 degrees, and that applies to PCB tracks. The attached was obtained by 3ft distance between device under test (breadboard in shortbread tin) and nearest powered mains transformer. The roll-off after 50Hz is due to the 100Hz low-pass LC filter preceding the oscilloscope. The other low frequency peaks are presumed to be aliases between 50Hz and other oscilloscope clocks (60Hz display, etc).
« Last Edit: July 15, 2023, 12:51:31 pm by EC8010 »
 

Offline mawyatt

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #77 on: July 15, 2023, 05:33:26 pm »
Be a little careful when using MumetalTM and other high-permeability nickel alloys for magnetic shielding.
This is a matter of quantitative design.
Permeable metal does not absorb external fields, but re-directs them around the shielded interior.
As a very rough hand-waving calculation, to avoid saturating the magnetic material with the Earth's DC field, consider a rectangular box of finite thickness, oriented for convenience with the direction of the external field perpendicular to the face of the box. 
The total flux that hits that face is the external field multiplied by the area of the face.
Going around the box, that flux is squeezed down into an area given by the perimeter of the face times the thickness of the metal.
That flux density B in the bottleneck formed by the thickness must be below the saturation of the material, which for nickel alloys such as Mumetal is roughly 7,500 to 8,000 Gauss.
Iron alloys have less permeability, but higher saturation, roughly 20,000 Gauss.
Using CGS (Gaussian) units, the magnetic field H just inside the wall is the value (in Oersted) corresponding to that flux density B in the metal, which for the linear approximation is B/mu for non-saturated metal. 
In those units, the flux density in air inside the box is B = H.
For critical applications, it is common to use an iron alloy for an outer layer, with a nickel alloy inside that to achieve extremely low fields.
If the metal saturates on the DC field, it will look like a nonmagnetic layer, with only eddy-current shielding (and relatively high resistance).
In a good design, the magnification of the external field due to the geometry will be a smaller factor than is the reduction in the field due to the high permeability, and the shielding will be useful.

Yes earth's fields can be problematic or useful depending on ones needs!!

We utilized the earths natural magnetic field to detect intruders with a special designed cable and signal processor back in 70s. The long 100M cables had a special core of select metals to map the earths localized magnetic fields into uniform pattern. Windings around the core were arranged and flipped in phase every ~1M to help with far field rejection. The cores were also selected and enhanced for a magnetostrictive effect, so magnetically "clean" intruders would produce a minute stress change in the surrounding ground which induced a minute (nanovolt levels) signal using the earths fields as a "permanent magnet" and the cable magnetostrictive effect would change under the minute stress changes, inducing tiny voltages across the windings. This worked beautifully, and we could detect and characterize various intruders, even "clean" intruders crawling less than 100mm/minute, with the Signal Processor.

Interestingly the cables actually enhanced the effects that MuMetal tries to minimize, that being the change in magnetic properties due to induced stress.

The cables were called MILES, the original signal processor MAID, and later version MSP. Honeywell also produced a commercial version called BLISS 1000 BLS-1000.

Interesting story behind the MILES cable and MSP development if folks are interested.

Best,
« Last Edit: July 16, 2023, 02:00:29 pm by mawyatt »
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Offline MegaVolt

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #78 on: July 16, 2023, 12:06:52 am »
Interesting story behind the MILES cable and MSP development if folks are interested.
It is very interesting.  Tell me, please.
 

Offline mawyatt

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #79 on: July 16, 2023, 02:28:44 am »
Interesting story behind the MILES cable and MSP development if folks are interested.
It is very interesting.  Tell me, please.

Well the early cable development is second hand as we weren't part of this, and likely transpired in the late 60s. The cable was originally developed to detect magnetic type intruders, personnel with weapons, tanks, cars, and vehicles which it did, but the developers noticed that it could detect magnetically "clean" intruders and realized this was due to the cable's magnetostrictive effect. They set about to enhance this feature using proprietary cable cores which became the MILES cable and develop a more sensitive signal processor/detector which became the MAID.

The MAID Signal Processor worked well but did have issues with false alarms, especially during storms, and a replacement development program was started. We got involved in 76~77 as a consultant, later hired and developed the algorithms and circuits, a colleague conceived the unique preamp idea discussed later. The requirements were difficult as the entire signal processor needed to fit in a 150mm cube, have built in batter backup, consume less than 50mw and pass full military requirements including conducted and radiated susceptibility. The program we had was known to Sandia Labs and a group of researchers from major universities working with a new algorithm based around a Adaptive Recursive Filter (ARF), they were also working on a program but we didn't initially know, and they were given all our reports and design details. If you've ever worked on USG programs you'll recognize this as we were supposed to fail and Sandia Labs would be the design of choice for the future production.

Early in the design we realized/discovered to achieve the enormous dynamic range required that by pushing the signal integration right up to the front end was the best possible architecture and this also compensated for the natural rate sensitivity of the magnetic sensor which was proportional to rate of change of the earth's magnetic field which is locally disturbed by a magnetic type intruder, and also by the rate of change of ground pressure due to the magnetostrictive cable effect. Since the cable impedance was very low, pushing an integrator to the input requires a very large feedback capacitance, we used a large wet-slug tantalum selected for low leakage. The amp inside the integrator was a low noise selected dual transistor from National and a selected low power op-amp, and overall called a Rate Compensated Preamp. The transistor was biased for optimum low frequency noise and the base bias current was allowed to flow thru the cable, this created a means to detect a cable disconnect, degradation or cut. BTW this push the integrator right upon the input signal was used decades later at RF/MW in the PolyPhase Mixer which is one reason why it has such good NF & DR. After the integrated signal various frequency bands were selected by active filters with various characteristics, and the energy within these bands and various ratios used to detect and classify target intrusions.

We sent some folks out to collect MILES cable data from various USG facilities around NA to get typical signatures for different terrains, nearby railroads, various aircraft flying end of runway, sonic booms, thunder storms, winds, frozen ground and so on, even setting up a special field site at our facility for long term recordings (false alarms).

Some false alarms did show up and always about 6am. The field had signs to Keep Out, so we set up a camera to be triggered by the alarm and discovered a guy walking his dog across one of the cables. Left him a friendly reminder, and no more alarms!! Local HS kids were hired to try and run, jump, crawl, pole vault, anything they could do to get past the cable, even showed them where the cables were buried and they couldn't defeat the system!!

We were informed about Sandia and the various university efforts based upon the Adaptive Recurrsive Filter just before our 1st field test at Eglan AFB, Florida during "Smoke Week". Sandia Labs showed up with a large Grumman van (like UPS uses), with a diesel generator running a VAX 11780 and a large Igloo size metal box, we had our little analog 150mm cube box, and a couple chart recorders!!!

When asked about the size and power requirements we were imposed with, Sandia folks said they'll shrink everything into a series of custom chips and such...sure!!!

We beat or equalled every test parameter during the test, most importantly the probability of detection and false alarm rates were much better overall and the Sandia/University folks were not happy!!

Later the 2nd test was conducted at Rome AFB in late January for the frozen ground tests, same result. Honestly believe the single most important reason we were successfully was the Rate Compensated Preamp, which was the integrator right at the front end. The system had 146dBV gain available and the ratio of various energy band levels acts as a dynamic AGC giving the system the best possible chance of detecting an intruder while keep the false alarm rate very low. The Sandia & Universities ARF was very clever, but without the advantages of our preamp, they were at a signal disadvantage right from the start.

Anyway, we equaled or beat the ARF in every measurement category with our little analog 150mm cube with the Rate Compensated Preamp, and later this system was selected for production and replaced the MAIDs in the field  :)

Sorry for the long note, maybe the moderators want to move this, unless its OK with other folks.

Best,
« Last Edit: July 16, 2023, 02:08:20 pm by mawyatt »
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Offline mawyatt

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #80 on: July 16, 2023, 02:10:34 pm »
Google doesn't show much, but did find some references:

References Honeywell BLS-1000 (Commercial version of MILES cable and MAID (later MSP) Processor)
https://apps.dtic.mil/sti/pdfs/ADA185001.pdf

MILES/MAID References
https://apps.dtic.mil/sti/citations/ADA056703
https://ieeexplore.ieee.org/abstract/document/6393530

Automated system to characterize the MILES cables we developed
https://apps.dtic.mil/sti/citations/ADA092200

Best,
Curiosity killed the cat, also depleted my wallet!
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Offline svetlov

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #81 on: July 18, 2023, 11:30:51 am »
in this video disassembly of a similar module - see the design
 

Offline trtr6842Topic starter

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #82 on: July 20, 2023, 03:09:34 pm »
I watched this video when it came out, just after I finished the first couple rounds of testing for my LNA!

The DC servo circuit is interesting, very cool that it successfully ignores input cap leakage while maintaining full input range.

One thing stressed in that video is the high pass filter response.  The 1K 1mF input RC gives a 1 second time constant filter with a -3dB point of about 0.16Hz and a first order roll-off.

The high pass filter in my amp is 0.1Hz but 3rd order, plus another 1st order roll off at 0.036Hz.  im thinking I should modify my high pass filter to be 0.16Hz 1st order (simply adjust the RC and remove the sallen key feedback) and see how the noise figure changes.  I can fairly easily decrease the cutoff frequency for the input and output to at least 0.01Hz without increasing resistor noise I think.
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Online Kleinstein

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #83 on: July 20, 2023, 04:10:46 pm »
The 1 second time constant is a bit questionable. The interaction with the DC servo may actually also effect the cross over. So the actual high pass for the input stage is 2 nd order. The transition also looks relatively steep - more than just 2 x 1st order combined, but more like a proper Butterworth or similar filter. The DC servo is a nice idea, but the way it is build is not very good.

A design with a longer time constant at the front and the fine filtering later should have a little advantage noise wise, as there is less noise from the transition region. So the 0.036 Hz from the front and 0.1 Hz later should be the better solution.  Having the high pass filter to cut away more of the single is more like cheating, not improving on the noise performance. Especially with modern instruments there is also the possibilty to do the actual filtering for the intervall of interest only in the digital domain (FFT or DSP based filtering) and have the analog hardware a little wider bandwidth on both ends.
 
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Offline julian1

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #84 on: July 21, 2023, 11:40:03 pm »
Would it make sense to stack/move all gain (200x + 10x) before the high-pass filters to reduce the potential influence of the non-c0g/film 10u caps?.

This could be done by using a sallen-key as DC block to cancel the Vos/offset of the first stage 200x gain, and making it a low-pass where 'good' c0g 100n caps could be used.
Then follows the 10x inverting gain.
And then follow other filtering - the high-pass with 10u caps, and optional/extra low-pass sallen-key filters.

Eg,
200x gain -> sallen-key DC block with c0g 100nF -> 10x gain -> low-pass and other high pass sallen-key with 10u x7r/x5r caps.

Edit. probably won't work because the only the HP sallen-key not the LP can work as a DC block.
« Last Edit: July 22, 2023, 02:47:04 am by julian1 »
 

Offline trtr6842Topic starter

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #85 on: July 25, 2023, 02:10:07 pm »
No, it wouldn't make sense, because then the tolerance for input capacitor leakage would also go down.  The whole reason why I split the gain into two stages is so that the input cap can be leaky and the amplifier won't saturate.  Also, since the 1st stage gain is significant, any non-ideal effects of those non-C0G/film caps are divided by a factor of the 1st stage gain.  If you go back to the early posts of this thread, you can see that I replaced all the 1µF ceramic caps with film ones and there was no significant change in the noise floor performance.  Also, if you look at the specs of the 1st stage opamps (OPA2182's), you can see that the noise floor is dominated by their input noise, meaning that the rest of the circuit is pretty transparent and quiet.  That means if I wanted to significantly improve the noise performance of this LNA I should focus on finding a quieter opamp, or add more OPA2182's in parallel, but both of those things are tricky.  All quieter opamps I've found have input clamping diodes, which makes input protection very hard.  More OPA2182's in parallel decreases battery life for only marginal performance boosts.
« Last Edit: July 29, 2023, 04:27:03 pm by trtr6842 »
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Offline Hawaka

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #86 on: December 18, 2023, 05:57:28 pm »
Very nice and small design!

Do you plan to release the full documentation so that one could easily order and built a unit?
 

Offline bobuhito

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #87 on: April 17, 2024, 01:09:55 am »
trtr6842, nice work!  Did you ever try LT1037 opamps on the input like you mentioned?

I suspect LT1037's higher input bias current (and higher input current noise) vs OPA2182
might actually cause more noise (even though the voltage noise spec is 50% lower for LT1037)
since input current flows through the 100 ohm resistors and through the hard-to-predict
electrolytic cap...
 

Offline Andreas

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #88 on: April 17, 2024, 06:55:19 pm »
Hello,

I have a (one) LT1037 in my LNA in the first stage (factor 100). (2nd stage is LT1012 also factor 100)
But with input high pass 3300 uF/1kOhm because the LT1037 is optimized for low noise around 1K impedance.
The main problem is the noise coming from leakage currents of the 3300 uF electrolytic capacitor.
(input voltage dependant).
with selected capacitors I have typically 120 - 200 nVpp noise floor (7-10V input voltage).

with best regards

Andreas

« Last Edit: April 17, 2024, 06:57:04 pm by Andreas »
 
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Offline bobuhito

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #89 on: April 17, 2024, 08:17:57 pm »
Hello,

I have a (one) LT1037 in my LNA in the first stage (factor 100). (2nd stage is LT1012 also factor 100)
But with input high pass 3300 uF/1kOhm because the LT1037 is optimized for low noise around 1K impedance.
The main problem is the noise coming from leakage currents of the 3300 uF electrolytic capacitor.
(input voltage dependant).
with selected capacitors I have typically 120 - 200 nVpp noise floor (7-10V input voltage).

with best regards

Andreas



Hello,

LT1037 datasheet says 0.1~10 Hz noise floor is 60 nVpp (130 maximum),
so I guess you're saying half of your noise is from the electrolytic capacitor.

Do you get much less noise when you short your input voltage to zero?
After dielectric absorption settles (which might take a few days!), you
might get much less noise compared to your current 7-10V tests.  I'm
guessing you've tried different capacitor brands and picked the one with
the lowest noise.

In case you've posted such results or your schematic somewhere else,
please let me know of course.  Thanks!
 

Online Kleinstein

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #90 on: April 17, 2024, 09:12:40 pm »
It is not just the capacitor as a noise source. There is also the current noise from the amplifier input in combination with the impedance of the capacitor at low frequency. So the LT1037 needs the rather large capacitors at the input.  AFAIR the simple design used the input coupling capacitor to set the lower frequency limit. This way the noise of the resistor to ground also gets partially effective. Ideally the input coupling has a lower cross over (e.g. larger resistor) and a later filter sets the 0.1 Hz limit.
 

Offline trtr6842Topic starter

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #91 on: April 17, 2024, 09:15:25 pm »
Very nice and small design!

Do you plan to release the full documentation so that one could easily order and built a unit?

You can contact me for ordering a kit!  I can offer bare boards, assembled boards, or full units.


trtr6842, nice work!  Did you ever try LT1037 opamps on the input like you mentioned?

I suspect LT1037's higher input bias current (and higher input current noise) vs OPA2182
might actually cause more noise (even though the voltage noise spec is 50% lower for LT1037)
since input current flows through the 100 ohm resistors and through the hard-to-predict
electrolytic cap...

I never did try the LT1037, or any other opamps that aren't already in the shared results.  Any opamp with input clamp diodes (which is most of them!)  will not be protected.  Since input protection is an integral part of this design, I don't plan on trying any opamps with input clamp diodes. 
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Offline Andreas

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #92 on: April 18, 2024, 09:06:48 pm »
Hello bobuhito,

most answers (including .pdf schematic) are somewhere here in the metrology section. Either in some LNA or LTZ1000 thread.
(I am too lazy to search for it).

I never measure with a short at the input of LNA. This would give a unrealistic low noise floor.
I always use batteries; formerly NiMh now as my NiMh are ageing and getting "loud" LiIon batteries with a voltage near the reference that I want to measure.
(yes the noise floor is input voltage dependant).
And I always have a battery connected to the input of the LNA to keep the input capacitor under bias since I do not want to wait 1 week for a measurement.

with best regards

Andreas
 
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Offline Gerhard_dk4xp

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #93 on: April 19, 2024, 01:11:42 am »
I never measure with a short at the input of LNA. This would give a unrealistic low noise floor.
I always use batteries; formerly NiMh now as my NiMh are ageing and getting "loud" LiIon batteries with a voltage near the reference that I want to measure.
(yes the noise floor is input voltage dependant).
And I always have a battery connected to the input of the LNA to keep the input capacitor under bias since I do not want to wait 1 week for a measurement.

Shorting the amplifier input does not show an unrealistic noise floor. It shows the
floor of the amplifier itself. An amplifier with a noise density of ~220pV/rtHz MUST see
a low source impedance of a few Ohms at most and the noise of the 10K bias network
must be shorted through this source impedance. 60 Ohm is already 1nV/rtHz.

I also would not consider Li-Ion "loud". Show me a different 8V voltage source whose
noise is 13 dB below 1nV/rtHz. This was measured with an ancestor of the preamp
in the picture. It had only 10*10UF Wima polypropylen caps in the input. Clearly
not enough, as you can see by the 30 dB/decade noise rise. This noise rise stops
finally when you get the full broadside of the Bias resistor.
That also shows that you must measure noise density and not some integrated
value where the lowest few bins dictate the result.

Amplifier == 10 * 2 * ADA-4898 in par.

This is NOT 1/f noise; you get the 1/f rise if you short the coupling capacitor on the
OpAmp side to GND. (here approximated by 60 Ohms, black trace)
Red & green are the batteries with & without 47 Ohm DC load.

With the amplifier in the pic, the propylen caps have been replaced by an AVX wet slug tantalum.
It did cost abt. 100€/$ :-(  Vishay wanted twice that.

There was no problem with leakage currents in this low-impedance environment.
In fact, I have made a copy of the ribbon mic amplifier from ArtOfElectronics ed3
with 10*1000uF Panasonic "SEPF" electrolytics. Only single ended with 16 *
Zetex transistors instead of 64 pcs. I get 70 pV/rtHz  as promised.
(single ended instead of differential)

I'm working on yet another FET version to take advantage of the cross correlation
of my Agilent 89441A. The high noise CURRENT of the many BJTs in parallel would
generate a voltage drop across the source impedance that would be common to both
channels and would not average away.


<        http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf      >

There are also battery measurements performed by Walls at NIST, time-frequency group.
I think the filename is 1111.pdf, but it is a moving target on their server.



regards, Gerhard
« Last Edit: April 19, 2024, 07:29:10 am by Gerhard_dk4xp »
 
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Offline Gerhard_dk4xp

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #94 on: April 19, 2024, 01:17:02 am »
2 pics were gone:
 

Offline miro123

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #95 on: April 19, 2024, 12:22:00 pm »
Hello,
I have followed this interesting thread. I have learned some new design tricks.
I have one question - why measuring of DC voltage sources are performed on this way?
Why not use differential measurements between 4 or more different DC sources and use cross corelation info to extract the behaviour.

I apologies in advance if I highjacked this interesting thread, or should I start a separate thread?
I asking this question because I find that all my six ADR1399 e pretty close to each other. many application like Gerhard battery noise measurements are also possible in DC differential mode. Why is all those application are doing AC decupled way? It should be a reason. The sub-100mV DC differential measurement is area where the modern integrated ADCs shines.
BR,
Miro
« Last Edit: April 19, 2024, 12:40:21 pm by miro123 »
 

Offline Andreas

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #96 on: April 19, 2024, 09:53:15 pm »
@Miro,
I think most people read application notes like AN83 AN124 DN6 from LT. And of course also other manufacturers.

@Gerhard,
perhaps I was somewhat unclear: what I wanted to say:
My NiMH cells are ageing and thus getting noisy.
That is why I use LiIon cells (Mine are low noise).
Of course a battery is a low impedant input to the amplifier.
But the input capacitor is a part of the amplifier (which has leakage current dependant noise i.e. dependant from input voltage bias).

with best regards

Andreas
 

Offline trtr6842Topic starter

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #97 on: April 21, 2024, 05:04:39 pm »
Hello,
I have followed this interesting thread. I have learned some new design tricks.
I have one question - why measuring of DC voltage sources are performed on this way?
Why not use differential measurements between 4 or more different DC sources and use cross corelation info to extract the behaviour.

I apologies in advance if I highjacked this interesting thread, or should I start a separate thread?
I asking this question because I find that all my six ADR1399 e pretty close to each other. many application like Gerhard battery noise measurements are also possible in DC differential mode. Why is all those application are doing AC decupled way? It should be a reason. The sub-100mV DC differential measurement is area where the modern integrated ADCs shines.
BR,
Miro

Often times the initial DC offset between references of the same type is much greater than the 0.1Hz to 10Hz noise they each have.  For example, the ADR1399 has a listed initial tolerance of +250mV / -300mV (550mVpp), but the noise is only 1.44µVpp.  The noise is 38 thousand times smaller than the DC offset, so  whatever ADC you feed that into would need over 112dB of dynamic range, which is very high.  Now I know it's possible to hand pick your references with better DC matching, but even if you match them to 55mV, 1/10th of the initial spread, you still need an ADC with very high dynamic range.  So even with a differential measurement, it makes sense to AC couple the signal and amplify it.  If you are going to do that, you end up pretty close to the common AC coupled LNA design you see around here, and then you might as well just measure one device at a time for simplicity.
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Offline Alex Nikitin

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #98 on: April 21, 2024, 06:22:28 pm »
I’m getting about 500nV p-p noise with a passive 0.1-10Hz filter and Hioki DM7275 voltmeter, no amplifier, and 1.5uV p-p noise from 10V Fluke 731B reference is measurable in this setup.

Cheers

Alex
 

Offline trtr6842Topic starter

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Re: DIY 0.1 to 10Hz Noise Amplifier
« Reply #99 on: April 22, 2024, 02:09:48 am »
I’m getting about 500nV p-p noise with a passive 0.1-10Hz filter and Hioki DM7275 voltmeter, no amplifier, and 1.5uV p-p noise from 10V Fluke 731B reference is measurable in this setup.

Cheers

Alex

What sample rates are you using with each DMM?
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