DIY 0.1 to 10Hz Noise Amplifier

[mawyatt]

Since the ceramic high-pass caps seem to be such a concern, I ran this rough test for microphonics.
Board on the left is CH1 on the scope.  The high pass sallen-key parts are 1µF film, 1µF film, 820k, and 3.3M.
Board on the right is CH2 on the scope.  The high pass sallen-key parts are 10µF X5R, 10µF X5R, 82k, and  330k.

Both boards exhibit some microphonics, but from my time using the one I already have in an enclosure once I stop touching the test setup everything stays very consistent.

Whats the gain from the HP to the DSO, checked the first schematic and it shows just 10X, is that correct? Also is the DSO set to it's highest sensitivity?

We grabbed a Samsung 0805 10uF X7R and lightly tapped it with a plastic tuner stick. Scope is 500uV/Div (187).

Then used a noname 0.1uF 50V Ceramic Disc (don't know dielectric tho), same scope settings (188).

Edit: Here's another with the tuning stick (Blue plastic Bourns type) dropped ~ 1/4" onto the Samsung 10uF 0805 case. Same tests with film and electrolytic capacitors show no response.

Anyway, looks like those muRata caps are quite good indeed

Best,

[trtr6842]

Whats the gain from the HP to the DSO, checked the first schematic and it shows just 10X, is that correct? Also is the DSO set to it's highest sensitivity?

The overall gain from input BNC to output BNC of the amplifier board is 2,000 V/V.  My test setup has no attenuation from amplifier board to scope, just a straight BNC cable.  The scope is set to it's highest sensitivity, which is 1mV per division.  With the 0.0005x attenuation factor that displays 500nV/div, which is true to the amplifier board's input.  The scope is also set to high resolution mode, and the high oversampling ratio bring it's intrinsic noise down to <70nVp-p and <7nV RMS, so the scope is well below the noise floor of the amplifier board.

Your test data in interesting, thanks for sharing!  You actually reminded me of something by calling me out on the X7R X5R mixup... I think I picked X5R caps on purpose!  JLCPCB had 4.7µF 1206 X7R caps, but I had a dilemma: Do I pick the larger more stress-prone package for the better dialectric, or do I pick the smaller package with a potentially worse dialectric?  Given my BNC connectors are only supported by the PCB, I went with the smaller package.  I have no idea if that was the right choice, but at least that was my thought process!

I also tried removing the feedback caps from the first stage amplifier: C6, C12, C16, C22.  There were a few comments about auto-zeroing opamps potentially acting odd with input caps, so I gave it a shot.  With an Rin of 1k the effect was negligible (Cfg#8).  With an Rin of 100Ω there was a slight improvement: σ = 19.45nV down to σ = 18.03nV (Cfg#6 vs Cfg#9).

[mawyatt]

Here's an example of slightly blowing across the Samsung 0805 10uF capacitor with scope set to 500uV/Div as shown.

These are some of the reasons we invested in lab bench type LCR meters to investigate the various behaviors of different capacitors, and frankly surprised how bad the High K dielectric types are, at least the ones we've tested.

Way back about 5 decades ago we had a device fail miserably during classic vibration testing, traced to High K ceramic capacitors in the signal path. This was an expensive lesson, as we had to redesign a few PCBs to accommodate larger film capacitors (back then PCB design was really expensive), since we've been shy of utilizing High K ceramics in any sensitive signal paths. Ironically the C0G/NP0 ceramic types are superb in every respect, even utilized in high resolution DMMs integrator ADCs, where one would think the Keysight/HP and Keithley folks would use some exotic expensive types 

BTW gotta love these nice low noise DSOs, we just need more resolution now, (read creating excuses justifications for a 12 bitter)

Best,

[Kleinstein]

Some of the C0G capacitor are really good, but not all C0G are the same. For the small capacitance range up to some 10 nF they out-perform film type capacitors in most aspects, maybe except leakage.
In my test the good COG where about where PTFE capacitors are claimed to be, but with a much smaller form factor to reduce parasitic capacitance.
Even the not so great C0Gs are about on par with PS and PP capacitor when it comes to loss.

[trtr6842]

BTW gotta love these nice low noise DSOs, we just need more resolution now, (read creating excuses justifications for a 12 bitter)

Mine's a 10 bitter!  It was the same price as the tek scopes in my range but had a lot of cool extra features.


So inspired by the JFET current limiter idea, but too impatient to wait for JFET's to arrive, I managed to come up with a MOSFET input protection circuit that appears to be working really well!
Here is the basic mod:  A series n-ch and p-ch MOSFET pair, each biased to be on when the input signal is near 0V. 



I was able to rework it onto the board fairly easily:




I did some quick functional testing by feeding the input of the amp board with a 19Vp-p sinewave from a signal generator.  I measured the voltage just after the MOSFET pair, and it was clamped well below the ±4.5V supply rails, which is perfect!  There is actually no current flowing besides opamp bias currents, so there is no risk of blowing up the opamps internal protection diodes.



I then turned off 9V to the amp board, and the voltage simply goes to zero, with no increased current draw.



So with the protection features confirmed, I made sure that it is still accurately conducting signals that should be in range.  I fed in a 100µVp-p test signal, and the output looked perfect.



Noise performance with a shorted input is great!  about 140 - 160nVp-p, or 17.92nV RMS (Cfg#13).  This is by far the best input protection, and best noise performance of any protected variant.  17.92nV RMS corresponds to 10nV√Hz per opamp with a 11.3Hz bandwidth (10Hz with a 4th order LP filter), and the datasheet is 9nV√Hz, so I'd say I'm pretty close to optimized for these opamps. 


I actually have a couple LT1037's on order, so I'll have to test those.  It should be significantly lower noise, even just using one, but the current consumption might go up a bit.  I think with a re-design I could use cheap and low-power 5V chopper amps to do all the filtering.  Any half decent chopper won't have 1/f noise, and broadband noise won't be too important after the 200x gain stage.

[Kleinstein]

The later stages don't need super low noise and with a relatively low gain also don't need high speed. So one could get away with a slower non chopper precision OP-amp (e.g. OP202).

The input protection looks unusual, but seems to work real well

[trtr6842]

The later stages don't need super low noise and with a relatively low gain also don't need high speed. So one could get away with a slower non chopper precision OP-amp (e.g. OP202).

The input protection looks unusual, but seems to work real well

Hahaha the OPA2202 happens to be better and way cheaper than the OPA2188's I'm using, I'll have to pick some up for the input stage!  I'll have to pick some up and give them a try.

[mawyatt]

Clever solution 

Best,

[trtr6842]

So I wasn't exactly right about the input protection circuit limiting current...  It does limit input voltage perfectly!  Here's an XY plot with input voltage on the X azxis and  voltage after the limiter on the Y axis:

The asymmetrical clamp limits are due to the very different threshold voltages between the Si2301 and the 2N7000.  The clamp thresholds also follow the supply rails up and down as I varied the supply voltage.  With modern low-threshold MOSFETs clamping should happen about 1.25 - 1.5V from the rails, which is enough to work with a well depleted 9V battery.


However I found out that inputs to the OPA2188 's are all protected by back-to-back diodes, so the input current goes higher than it should.  Here is an XY plot with input voltage on the X axis and input current on the Y axis.  The gentle slope around 0V is from the 2k input impedance (which I moved to after the limiter BTW).  However as soon as the input voltage gets to ±0.6V the input current magnitude increases.

Luckily my circuit had about 1k on the inverting input, so the overall current didn't damage anything.  For this version, its an OK solution.  For a lower-noise version, 1k gain setting resistance would be too noisy, but going lower would increase the input clamp current too much.

Now the OPA2182 does NOT have input clamp diodes, and has about 1/2 the noise as the OPA2188.  I'm going to try that one next.  Four sections of the OPA2182's should get into LT1037 territory as far as noise performance, cost about the same in total, and draw a little bit more current.  However from what I have found the OPA2182 is one of the only candidates that doesn't have back to back input diodes, so my next revision of the board will include an optional JFET current limiting circuit.

[trtr6842]

I got some OPA2182's in yesterday and swapped them in for the 1st stage amplifier.  I changed the input gain resistors to 120Ω and 30k for a gain of 250x, and I increased the gain of the second stage to 20x for an overall gain of 5,000x.

The OPA2182's have "MUX Friendly" inputs without the back to back clamping diodes, so now my MOSFET input voltage limiter provides adequate protection, and the input current is only due to the 2kΩ input resistor to ground.  This is the XY plot of input current vs input voltage.  For this test the input 2200F cap was shorted.



Noise with a shorted input is much better, 91nVp-p typical!  Current draw is up to 7.5mA, but that should still be good for ~60 hours on a 9V battery, which should be good given how quickly this design settles on powerup.

[Andreas]

Hello,

I typically measure the noise floor with (new) 8xNiMH cells. (~10V bias)
The leakage current of the input capacitor (the AC part of it) also contributes to the noise floor.
It also needs ~1-2 days until the leakage current is low enough for doing measurements.
So when not used I connect a 9V block to the input to keep some bias voltage on the input capacitor.

with best regards

Andreas

[trtr6842]

From your experience, how noisy are NiMH cells?  I don't have any on hand but might have to get some.

For this design, DC leakage isn't too important due to the two-stage design.  The 1st stage can tolerate ±7µA of leakage and not saturate it's output, and the first stage is immediately followed by a high-pass filter, cancelling out any DC offset before the remaining gain is applied.  Now if the leakage currents have any noise in the 0.1 to 10Hz range, then they'll get multiplied by 2kΩ and the voltage gain of 5,000x, but I have no data on the noise component of electrolytic leakage currents.

[MasterT]

The same rules for electrolytic capacitors's noise as for any semiconductor : higher absolute DC leakage would proportionaly generate higher AC low freq. noise level.
 I get very good results with this 2 caps:

UKL0J102MPD
RNL1C152MDS1

[Kleinstein]

How noisy the leakage current is, is a good question. As a minimum there should be resistor like noise, but the actual noise is likely higher. It is reasonable to expect shot noise, like with semiconductors or tunneling junctions. So current noise from randomly coming electrons and thus proportional to the square root of the current.

There is also the chance to get added current spikes from chemical reactions in the formation of the oxide layer, e.g. if a weak spot appears (weak inhibitor moves) and gets repaired (extra oxide formed).

For the resulting noise voltage it is no only the 2 K resistor to ground that is relevant, but mainly the capacitance itself. In this design the input cut off lower than 0.1 Hz and thus little effect of the resistor.

Batteries are normally considered rather low in noise. They however reactor to mechanical stress and thermal effects. So they should have some time at rest and thermal shielding. NiMH may be attractive because of a relatively stable voltage with discharge state and low internal resistance. 

To some degree electrolytic capacitors can behave similar and also react to stress.

[Andreas]

From your experience, how noisy are NiMH cells?

Depends.
Of course you have to do some pre-cautions.
Keep the cells at constant temperature (so do not use immediately after charging).
Old (defective) cells have some tens of uV.
New (good) cells do not contribute much to my noise floor of my LNA which is < 0.2uVpp.
Part of it is due to leakage current of the input capacitor.

But noise floor can significantly increase if the input capacitor has too much leakage current:
https://www.eevblog.com/forum/metrology/lowest-noise-op-amps-for-low-frequency-low-level-ac-coupled-signals/msg2218644/#msg2218644

with best regards

Andreas

[mawyatt]

From your experience, how noisy are NiMH cells?

Depends.
Of course you have to do some pre-cautions.
Keep the cells at constant temperature (so do not use immediately after charging).
Old (defective) cells have some tens of uV.
New (good) cells do not contribute much to my noise floor of my LNA which is < 0.2uVpp.
Part of it is due to leakage current of the input capacitor.

But noise floor can significantly increase if the input capacitor has too much leakage current:
https://www.eevblog.com/forum/metrology/lowest-noise-op-amps-for-low-frequency-low-level-ac-coupled-signals/msg2218644/#msg2218644

with best regards

Andreas

Wonder if one could use the increased noise level as a quick test of a NiMH cell, rather than having to do a discharge profile? Should be much quicker to evaluate and might even work on cells in a stack, test individual cell within unloaded stack.

Best,

[Gerhard_dk4xp]

From your experience, how noisy are NiMH cells?  I don't have any on hand but might have to get some.

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

in short: the bigger the better.

Cheers, Gerhard

[trtr6842]

I ran a test using two 18650 LiPo cells.  I discharged them down to 4V each to get closer to the flat portion of their discharge curve.  I probed the output of the first amplifier stage so I could see how large the leakage current through the cap is.  This cap is prettymuch new and I did only a few minutes of aging before this test.

The basic setup:  A small cardboard box to limit airflow on the batteries, but this whole setup was just on my desk as I was still using the computer.



Here are the results.  I'm paying attention to the low values between the large disturbances.  The first stable time was starting at t = 3 hours where I left my desk and everything was still in the room.


Overall I don't think this design is too picky about cap leakage.  The amplifier was able to settle down even with as much as 75nA and only 3 hours under bias, and useable for noisier measurements like bandgap and some buried zener references with way less time under bias.

[Andreas]

Hmm,

I am missing a cookies box (see LT AN124) or
a steel paint can (see TI avt_081307)

with best regards

Andreas

[Gerhard_dk4xp]

I built some amplifiers in the usual way: CS stage with many/large JFETs, cascode
against Miller effect, OpAmp and feedback to the 1 Ohm source resistor.
Unless you are happy with audio bandwidth or have a GHz opamp, that thing
develops negative input resistance as soon as the feedback loop is closed.
An inductive signal source with the right L, and you have an oscillator.
I also checked designs from others who swore that would be stable. It was not.

I finally gave in and dismissed the feedback. I enforced the source current by
using a current mirror. Then the source had to be decoupled by LARGE caps
to get common source, ac-wise.

It turned out that every electron that defects through the electrolytic produces
a noise pulse at the amplifier output.  OSCONS were worst, 1/f corner in the KHz
and levels at a few Hz that the 89441A FFT analyzer had dynamic range problems.

Nippon Chem. alu electrolytics were much better but still not usable.
The only thing that borderline worked was an AVX wet slug tantalum 4700uF/25V,
€/$ 100 a pop. Vishay even wants 3 dB more money.

It seems, that circuit is the optimum cap noise current detector.

JFETs have a pos and a neg TC that may cancel.
At least in simulation, 16 pcs. CPH3910 have their thermal sweet spot at 45 mA total.
That allows simply grounding the sources.
The new TI FETs seem to be best at IDss, too much current to be practical
if you want 16 pcs in par. for noise reasons.

Gerhard
 

[trtr6842]

Hmm,

I am missing a cookies box (see LT AN124) or
a steel paint can (see TI avt_081307)

with best regards

Andreas

Do Hammond diecast enclosures count?  Or are only cookie tins or paint cans good at scaring away the noise?



I couldn't easily probe the output of the first stage with the board in the enclosure, and that's how I measured my leakage current.

[MegaVolt]

Or are only cookie tins or paint cans good at scaring away the noise?

Somewhere I came across an explanation of why tin cans are so good. They have 3 layers of tin-iron-tin. The result is better shielding than a material of the same thickness but made from the same material.

[trtr6842]

Or are only cookie tins or paint cans good at scaring away the noise?

Somewhere I came across an explanation of why tin cans are so good. They have 3 layers of tin-iron-tin. The result is better shielding than a material of the same thickness but made from the same material.

Hahaha I was joking about that... I doubt the material really matters much this close to DC, thermal and air current effects are probably way worse than any sort of EMI issues here.

[Andreas]

Do Hammond diecast enclosures count?  Or are only cookie tins or paint cans good at scaring away the noise?

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

[MasterT]

Or are only cookie tins or paint cans good at scaring away the noise?

Somewhere I came across an explanation of why tin cans are so good. They have 3 layers of tin-iron-tin. The result is better shielding than a material of the same thickness but made from the same material.

Hahaha I was joking about that... I doubt the material really matters much this close to DC, thermal and air current effects are probably way worse than any sort of EMI issues here.

 RF is interfering with my noise test ,  Wi-Fi - easily getting into since packets transmission about 10 Hz - demodulated  on any P-N junction with 2.5 cm piece of wire attached to.  Resistors on a breadboard about same size. Worst OPA is LM4562: