Electronics > Metrology

DIY 0.1 to 10Hz Noise Amplifier

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trtr6842:
I'm starting some projects where it will be useful to measure low frequency noise, so I designed and built this 0.1Hz to 10Hz noise amplifier.

(I know my handwriting sucks)


It has a gain of 2,000 V/V, which is plenty to get above the noise floor of my RTB2000 scope.  My main goal was not ultra-low noise performance, as nothing I'm working on requires measuring noise less than 1µV p-p.  Instead I wanted something that settled within a reasonable time, and wasn't picky about finding the perfect low-leakage input capacitor.  To achieve that, I went with a multi-stage design.  I also wanted it to run off a single 9V battery with decent life.




The input stage is just the high-pass RC network with some diode clamps.  While breadboarding the design I found out the hard way that hot-plugging the input to a +10V supply can very easily fry the input to the first opamps.  Also with higher input voltages, the input resistors have to dissipate a significant amount of energy, so I used a total of 4x 1206 sized parts. 

The first amplification stage is 4x parallel OPA2188's with a gain of 200.  OPA2188's are reasonably priced and have pretty good noise performance.  There are less noisy options, but all I've seen consume quite a bit of current, which would kill the 9V battery life.  A gain of 200 is plenty to get the input completely clear of the op amp noise floor, but is low enough to allow typical electrolytic capacitor leakage currents to cause minimal offsets with the 2k input resistance.  Assuming a partially depleted 9V battery, the power rails will be about ±4V, so the input capacitor leakage can be as high as 10µA before the output hits the rails.  This is no problem for most electrolytics, and this strategy worked well as every cap that came on my stuffed boards worked just fine.

Right after the 200x input stage comes a high pass filter.  This removes any DC offsets from input cap leakage and allows more gain to be applied later.  After that is is a 4th order 10Hz high-pass filter, and a final 10x gain stage.  The output is high-passed at 0.1Hz as well, taking into account a 1MΩ scope input impedance.

Overall the performance is good enough for me, but not ultra-low noise.  With a shorted input I get about 261nVp-p.  That's plenty low for what I test, which is more like LM399's in stead of LTZ1000's.  It's also been very consistent, and the FFT of it shows no significant spikes.  If I can make the assumption that my amplifier and the device under test both have truly random noise sources, I could just do the math to subtract my amplifier un-correlated noise from any measurement's I'm taking that are below 1µVp-p, but I honestly don't expect I'll need to do that with what I'm working with.

Intrinsic Noise with Shorted Input



131kPt FFT of shorted input, done with scope data and a python script, since my scope won't do such low frequency FFT's



I tested the gain by using a 1Hz 1Vp-p signal and a 1MΩ / 100Ω resistor divider, and the gain was spot-on.



I tested the frequency response with a 200 second 0.05Hz to 20Hz logrithmic sweep.  The low frequency cutoff is a little high, but not bad.  (The vertical cursors represent 0.1Hz and 10Hz on the sweep)



The inter-stage high pass filter makes the settling time pretty quick, about 30 seconds for a 100µV step.  It can take up to 2 mins when first plugging into a new DUT, but that's not bad at all.  Better than waiting forever for the input cap to re-form!


The current consumption is 5.66mA at 9V, so that should give me a good 50 hours or more on a single battery, which seems pretty reasonable for my use.

Overall I'm really happy with the design, and it has proven very useful so far.  A while back I built a 4-20mA signal calibration source based around an LM399 and some goofy configurations of some NOMCA 8 resistor networks.  It also has a +10V output, and turns out its not too bad at about 6.5Vµp-p.



Another thing I learned is that the cheap 79L12 linear regulators I'm using on a project are about 15x noisier than their 78L12 counterparts! (1mVp-p vs 70µVp-p)  I didn't expect that, but I confirmed it on a sample size of 5 boards, so that's good to know.

I got 5 of these made through JLCPCB's SMT service, so I have 4 extra if anyone is interested in board-only or fully-enclosed one!

mawyatt:
Nice work  :-+

Curious as to why you chose SMD chip caps for use in the Sallen-Key Filters and output DC blocking cap. Are these ceramic because they are physically small size? Also are they C0G/NP0 types, or one of the higher dielectric constant ceramics?

Best,

trtr6842:
I chose ceramics simply for size and convenience.  They are X7R types, nothing special.

My thought process was yes, they will be very imperfect, but their impact should be fairly small since they all come after the initial 200x gain stage.  The most critical ones are the two 10µF ones used in the sallen-key high pass filter since any noise the generate from capacitance variance could show up in the 0.1Hz to 10Hz bandwidth.  However, 10µF film caps are too big to fit in this enclosure.  1µF film caps are a bit more reasonable, but that would push the resistor values to 1.6M and 3.3M.  That's not too bad for thermal noise since it's after the 200x stage, and is actually a really good option.

For the low-pass caps, they are only dominant at 10Hz and above, and their effect is divided by the 200x first stage gain.

Overall I think the 260nVp-p is still dominated by the input resistor and opamps, but exploring film caps could be interesting if I ever need to improve this circuit!

mawyatt:
Yes, the Sallen-Key HP would be the most sensitive with the 10uF X7R.

Agree the 10uF film are huge (used in our LCR DC Bias adapter), so maybe a 2.2uF Film, 10uF Tantalum, Electrolytic, or Poly. All these should be better than a X7R cap, altho larger.

Awhile back we developed a DC Bias adapter for our lab bench LCR meters to test some various caps including SMD ceramic types. This allowed studying DC Bias effects, but we also found that the High K ceramic types not only vary significantly with DC bias & temp, but just about everything, even the quality X7R from Samsung!! Just taping on them creates a huge piezo electric effect, not to mention PCB stress and so on. We decided to only use the High K ceramic caps for supply decoupling and bypassing and nothing else.

Best,

MasterT:
High pass filter is likely to be off, since it is not expecting 3k /( 2-4 ) output impedance from first stage

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