DIY 0.1 to 10Hz Noise Amplifier

[trtr6842]

The enclosure is not bad. But how do you get your DUT (voltage reference) and Power supply within your (shielded) Hammond case?
By the way: If my 34401A is too close to the amplifier (< 0.5m). I get heavy mains hum on my output signal. (good visible with a FFT)

A tin can has advantages against magnetic fields at low frequencies. (but of course also a thick walled case gives some shielding).

with best regards

Andreas

That's why I kept my amplifier size as small as possible, it can be put in whatever larger enclosure makes sense for the DUT.  Also, for a low-impedance DUT output I don't know how important it is for both amp and DUT to be in the same enclosure.  As long as DUT and amp are each stable I think their noise will dominate noise added by a halfway decent connection.

For the 60Hz interference, what order low-pass filter do you have on your amplifier?
But even if these amps don't reject 60Hz, as long as it doesn't inter-modulate down to the 0.1to 10Hz range it would not be hard to remove the 60Hz spike from the FFT and then run an IFFT to correct the time domain signal, if it even contributes significantly to the overall noise.  Or just applying a digital notch filter would achieve the same effect.

[Andreas]

Hello,

I have a 4th order low pass.
In my case I have 50 Hz mains frequency.
Before doing some IFFT I rather put my cookies box to a quiet place.

with best regards

Andreas

[trtr6842]

I Just got Rev X2 of this LNA built up:




The performance is better than the modified X1 revision a typical 78nVp-p and 10.5nV RMS shorted input noise floor.



The updates schematic is attached.  It looks different to fit on US Letter sheets, but is pretty similar to the modified X1 configuration.  I added a BNC connector for the DC output of the first stage to monitor input capacitor leakage.  There is also a status LED the lights up when that signal is at least ~V away from the both supply rails, indicating that the first stage is not railed.

I'm very happy with the electrical performance, but the mechanical form factor isn't great.  I had to over-drill the holes for the BNC connectors because otherwise the board would not fit into the box.  I also put the switch on the wrong side of the board, requiring longer battery wires.  An extruded enclosure with PCB slot would be nicer, but I already had these hammond diecast boxes on hand, so I figured I'd work around those.

[iMo]

Why not U1B (stage 2) with 40x gain such you'll get 10000 in total?

[Kleinstein]

There is little need for even more gain and with more gain one may run into clipping earlier, e.g. with a more noisy source. Already the gain of 5000 often higher than needed.

[Andreas]

Already the gain of 5000 often higher than needed.

Oh come on.
A modern wideband oscilloscope has ~1mVpp input noise.
To measure a serious reference like ADR1000 you really want a higher amplification to get below 200uVpp noise floor.

@trtr6842
I have seen that you are using "hires mode" with a relatively low (10kS/s) sample rate.

what exactly does this software filter do? (I am not familiar with R&S)
I hope it does a sliding average over neighbouring measurement points and not average complete traces.
where is the Edge frequency of this filter?
Have you done a FFT?
when averaging 256 neighboured points the sample rate would lead to around 40 Hz which is too close to the 10 Hz Bandwidth.

with best regards

Andreas

[trtr6842]

Already the gain of 5000 often higher than needed.

Oh come on.
A modern wideband oscilloscope has ~1mVpp input noise.
To measure a serious reference like ADR1000 you really want a higher amplification to get below 200uVpp noise floor.

@trtr6842
I have seen that you are using "hires mode" with a relatively low (10kS/s) sample rate.

what exactly does this software filter do? (I am not familiar with R&S)
I hope it does a sliding average over neighbouring measurement points and not average complete traces.
where is the Edge frequency of this filter?
Have you done a FFT?
when averaging 256 neighboured points the sample rate would lead to around 40 Hz which is too close to the 10 Hz Bandwidth.

with best regards

Andreas

Here are some specs for my RTB2004:
I left the probe settings according to a gain of 5000, the same as all my tests with the LNA.

Shorted input, 20Mpts capture, 10.9kS/s, Hi-res mode:
23nVp-p, 2.54nVrms (115µVp-p, 12.7µVrms 1x gain equivalent)

Shorted input, 20Mpts capture, 1.67MS/s, Sample-mode:
136nVp-p 16.1nVrms (680µVp-p, 80.5µVrms 1x gain equivalent)

So based on my last test where the LNA showed 13.2nVrms of noise, if you remove the 2.54nVrms of scope noise, the LNA actually has 12.95nVrms of input-referred noise.  Increasing the gain to 10kx would give a shorted input noise of 13.01nVrms, which is not a significant improvement for me.  Other scopes may vary though, but since my scope has such low intrinsic noise I'd rather have a wider measurement range.  With an old 9V battery (drained to 6V) I can measure signals up to about 1mVp-p.  Since I can't do that with scope naturally, that's pretty useful.  (Although this particular version has the 250x output as well, drastically increasing the range).

Essentially I get no benefits from a gain of 10,000x, but that's just my setup, others may vary.


As for sample mode vs hi-res, there is absolutely no concern for bandwidth.  The hi-res mode performs a moving average, and is reflected in the sample rate.  You can see from the two measurements above the oversampling ratio is about 153 (1.67MS/s vs 10.9kS/s).  In both cases the ADC is running at 1.67MS/s, but the decimated datarate of the hi-res capture is 10.9kS/s, 1000x higher than the 10Hz range we need.

Here is a basic amplitude test with the scope in sample mode:


Here is Hi-res mode, with an even lower sample rate than my tests.



5.0185V Mean Amplitude in Sample,
5.0144V Mean Amplitude in Hi-res
Only a -0.0071dB drop, perfectly acceptable given the benefits of lower input noise.


Back to the LNA, Here is a 131kpt FFT of a 60-second capture with a shorted LNA input:

You can clearly see the 10Hz LNA high-frequency cutoff, and then below that the noise floor of the scope beyond that.

Here is a 20Mpt FFT of a 240 second capture with shorted LNA input:

[2N3055]

Already the gain of 5000 often higher than needed.

Oh come on.
A modern wideband oscilloscope has ~1mVpp input noise.
To measure a serious reference like ADR1000 you really want a higher amplification to get below 200uVpp noise floor.

@trtr6842
I have seen that you are using "hires mode" with a relatively low (10kS/s) sample rate.

what exactly does this software filter do? (I am not familiar with R&S)
I hope it does a sliding average over neighbouring measurement points and not average complete traces.
where is the Edge frequency of this filter?
Have you done a FFT?
when averaging 256 neighboured points the sample rate would lead to around 40 Hz which is too close to the 10 Hz Bandwidth.

with best regards

Andreas

Even a 1mV P-P scope noise gets to be attenuated to 1µV P-P effective with 1000X preamp.

For my SDS2000X HD  with setup for 1000X amp I get  140-150 nV P-P  and 14-15 nV RMS effective scope noise. 2MS/s
Picoscope 4262 with setup for 1000X amp I get  23 nV P-P  and 3 nV RMS effective scope noise. 1MS/s

+-8µV full screen for Siglent and +-10µV full screen for Picoscope. Siglent can go down to 500 nV/div (+-2uV full screen), but noise stays the same.

That is on time base that captures 10s worth of data making sure it captures low frequency noise of scopes. On Siglent ERES 3 is on (lowpass filter in effect, i cannot be bothered to calculate frequency now sorry, but some kHz..) and so is on Picoscope (200Hz).

With good modern scopes with low noise inputs 1000X amp is more that enough.

[Andreas]

  The hi-res mode performs a moving average, and is reflected in the sample rate.  You can see from the two measurements above the oversampling ratio is about 153 (1.67MS/s vs 10.9kS/s).  In both cases the ADC is running at 1.67MS/s, but the decimated datarate of the hi-res capture is 10.9kS/s, 1000x higher than the 10Hz range we need.

Hello,

I was not aware that the instrument shows the "effective" sample rate instead of the raw value.
So your FFT pictures show that the LNA determines the frequency response.

with best regards

Andreas

[trtr6842]

Hello,

I was not aware that the instrument shows the "effective" sample rate instead of the raw value.
So your FFT pictures show that the LNA determines the frequency response.

with best regards

Andreas

To be fair, I wasn't aware of that either until I did those tests! 
Also the hi-res mode doesn't perform a moving average, it averages N samples and then treats the average value as a single data point.  I guess that is essentially equivalent to an N moving average followed by a decimation by a factor of N though... Either way, just wanted to clear that up.

For the FFT's, the 131kPt one uses the "display data", so it has the hi-res mode factored in, but with an even lower sample rate than shown by the capture.  It does show that the scope still has plenty of bandwidth for 10Hz though.

The 20MPt one uses the acquisition data, and that is the raw acquisition data with no hi-res averaging or decimation applied.

[Sariel]

Hey,

This is a very nice low noise amplifier.
Well done!

Could you please share the P/N of the enclosure?
And also what is the connector type of the 9V battery on the board (I am not familiar with it).

Thanks,

[AnalogTodd]

For those interested as to why a cookie tin or paint can is so often used for shielding, I would recommend picking up a copy of "Noise Reduction Techniques in Electronic Systems" by Henry W. Ott. Chapter six is 'Shielding Effectiveness of Metallic Sheets' and covers why one would use a particular material for shielding.

Basic idea has to do with frequency you are trying to shield against will determine the type of material you want. For high frequencies, copper and aluminum enclosures work very well. At low frequencies, you want to provide a low reluctance magnetic shunt path to divert fields around the circuit being protected. A high permeability material is optimal (MuMetal is a good example), steel (or tin) will work better at low frequencies as it does well magnetically. Using multiple enclosures spaced a distance apart can provide better results than a single thicker enclosure.

[thermistor-guy]

...A high permeability material is optimal (MuMetal is a good example), steel (or tin) will work better at low frequencies as it does well magnetically...

Is there any benefit in wrapping a steel can with copper tape?

[Andreas]

Hello,

shielding is mainly related to skin effect.
the skin  depth is proportional to square root of the product of conductivity times permeability.
So copper has a factor 10 better conductivity than a tin plated steel.
But permeability is only 1 against 50 - 1000 of steel.

So as end effect: you need a factor 3-10 thicker copper tape than your steel can to get the same shielding effect at the same frequency.

with best regards

Andreas

[iMo]

This is the "portable" box I designed long time back (1/f noise measurements).. Inside we had a second shielding layer made of copper foil.
https://www.eevblog.com/forum/metrology/a-portable-box-for-1f-and-low-noise-measurements/msg4652194/#msg4652194

[svetlov]

if possible, try an opamp in this circuit OPA209 series (OPA2209)
 or more quiet OPA2211
interesting to see what will happen

[AnalogTodd]

This is the "portable" box I designed long time back (1/f noise measurements).. Inside we had a second shielding layer made of copper foil.
https://www.eevblog.com/forum/metrology/a-portable-box-for-1f-and-low-noise-measurements/msg4652194/#msg4652194

That sort of box was something I was looking at when doing low noise measurements. Thick steel to really try and squash the low frequency stuff and inside some copper to help against high frequency stuff. Fortunately, I was able to get MuMetal in 50mil thickness to create a box for shielding. That was set inside a box made from double-sided copper, all nestled in a cookie tin. A lot better in terms of weight and 'portability'. Even with it all, you still get some line frequency stuff getting in the measurement, but it's a huge improvement over no shielding.

Page eight shows what my box looks like: https://www.analog.com/media/en/technical-documentation/app-notes/an-159.pdf.

[thermistor-guy]

Fortunately, I was able to get MuMetal in 50mil thickness to create a box for shielding. That was set inside a box made from double-sided copper, all nestled in a cookie tin. A lot better in terms of weight and 'portability'. Even with it all, you still get some line frequency stuff getting in the measurement, but it's a huge improvement over no shielding.

Page eight shows what my box looks like: https://www.analog.com/media/en/technical-documentation/app-notes/an-159.pdf.

Presumably, the tin's lid is a loose fit, with some gaps, so there will be EMI leaking through the tin-lid interface. OTOH, the lid is quick to remove, which is handy.

Can any simple modification reduce the EMI leakage, while keeping the convenience of quick release? Apply conductive tape, for example, to make the lid fit a little tighter?
(Reminds me of copper gaskets on GTEM cell access doors).

[Kleinstein]

Some of the cooky boxes have a relatively tight seal. Mainly the paint on the outside that is not ideal and may not give an electric contact for the lid.
If really critical one could likely solder spingy wires to the inside to make a few extra contacts.

For testing with a bread board or similar it is hand to have the bock upside down, so that one has easy acess from all sides and can add the cover if needed. The more normal way around is more suitable for mounting connectors, though a bit wobbely with the thin metal.

[EC8010]

Biscuit and baccy tins have a good seal but often have an insulating coating. That caught me out when I made an electrometer in a baccy tin and had buckets of hum. Soldering a 50mm multi-strand wire from lid to tin solved that (electrostatic) problem.

Tins are usually 0.1mm thick steel, so they provide electromagnetic screening >200kHz (penetration depth as mentioned earlier). You have to keep them away from 50Hz stuff. My bench has a steel frame, so I put a few neodymium magnets on the front girder and stick amplifiers to that, effectively putting 2mm of steel in the way of hum. And (more importantly) distance.

A gain of 5000 is about right to overcome oscilloscope self-noise. Oscilloscopes are designed for bandwidth and that means low capacitance FETs, but low 1/f noise requires paralleled devices. In short, oscilloscopes produce significant 1/f noise. You need the amplified noise of your LNA to be more than three times oscilloscope noise at the frequency of interest for oscilloscope noise to be negligible, and that's what leads to needing a gain of 5000. But with that much gain, you have lots of high frequency noise to overload the oscilloscope, so you also need an LC low-pass filter (RM cored inductor and polypropylene capacitor work well, plus damping resistor to prevent the filter peaking at its cut-off). I use 1kHz for general stuff with a x100 amplifier, but 100Hz for really low noise stuff with x5000. 100Hz is still high enough to be able to measure white noise and 1/f noise on the same FFT. If the 'scope can implement a low-pass filter as part of its decimation, that helps in taming the record length. I export the FFT to a spreadsheet for analysis.

[iMo]

Getting rid of 50Hz hum is extremely difficult, and we had a lot of troubles with it in my 1/f days.
Mind the "penetration depth" is the depth where the 50Hz is attenuated only to 37% (1/e) of its outside value.
For copper the 50Hz penetration depth is ~10mm and for steel/iron ~60-80mm.

[mawyatt]

This is where MuMetal can help, recall we had various thickness sheets of this in the labs way back when and made kludge type boxes for sensitive electronics & measurements.

Also, for sensitive measurements we moved everything away from the test bench and only had the absolute necessary equipment at hand, often powering things with batteries. This wasn't only for very low 1/f measurements, as very low phase noise measurements at higher frequencies required the same attention.

Best,

[mawyatt]

This is the "portable" box I designed long time back (1/f noise measurements).. Inside we had a second shielding layer made of copper foil.
https://www.eevblog.com/forum/metrology/a-portable-box-for-1f-and-low-noise-measurements/msg4652194/#msg4652194

Nice, portable as in get out the forklift as it looks like a tank

Best,

[AnalogTodd]

Getting rid of 50Hz hum is extremely difficult, and we had a lot of troubles with it in my 1/f days.
Mind the "penetration depth" is the depth where the 50Hz is attenuated only to 37% (1/e) of its outside value.
For copper the 50Hz penetration depth is ~10mm and for steel/iron ~60-80mm.

Yes, but there's a lot more to look at than just "penetration depth" when it comes to attenuating fields. Realistically, for a lot of noise measurements you are worried about AC fields as DC fields don't add to your measured signal. Now you need to think about absorption loss and reflected loss and how those change as a function of frequency. Add in there that you can use low reluctance magnetic materials to divert magnetic fields around a region instead of attenuating or reflecting, and you get a myriad of things to review depending on what you are trying to shield against. Because of their "penetration depth" copper and aluminum are great for absorption loss at high frequencies, but are less effective against low frequencies. For low frequencies, you can get good low reluctance materials like MuMetal but the permeability starts dropping at ~1kHz and matches copper/aluminum at ~100kHz.

This is where MuMetal can help, recall we had various thickness sheets of this in the labs way back when and made kludge type boxes for sensitive electronics & measurements.

Also, for sensitive measurements we moved everything away from the test bench and only had the absolute necessary equipment at hand, often powering things with batteries. This wasn't only for very low 1/f measurements, as very low phase noise measurements at higher frequencies required the same attention.

Best,

MuMetal is definitely good for low frequency shielding. Set up a transmitter and receiver 0.1in apart and put a shielding metal in between. At 1kHz, 30mils of MuMetal gives ~18dB of attenuation where the same thickness of copper is ~4dB. At 10kHz, the MuMetal is ~25dB compared to copper at ~20dB. But when you get to 100kHz the 30mils of MuMetal is ~29dB attenuation and it only takes 10mils of copper to give the same attenuation. Hence the multiple layers of different materials that can be used for a wide frequency range shield setup.

This doesn't even touch on how additional distance also attenuates or concerns about material saturation.

[TimFox]

Be a little careful when using Mumetal^TM 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.