Low frequency, very low level, DC biased, noise measurements

[blackdog]

Hi 

Let me show you my design of a measuring set, it is designed for measuring on voltage references and lineair power supply's.
It is not finished yet, I'am building the preamp stage on a circuitboard for testing.
I have 6 different quad opamps ready for testing.
I placed the feedback SMD resistors mounted on the opamp circuitboards, the feedback resistor is 500 Ohms.
Because the signal level is low here, the load for the opamp is is therefore not high and not a problem.
The bandwidth of this stage is just above 100Khz.


The preamp...



Building the test circuit, left the Quad opamp in parallel mode and on the other side the 46dB amp (total 80dB gain)
Next step is de power supply for the quad opamp on the left site from the quad opamp.
This board wil be used to test different opamps for noise lever and bandwith.



20 pieces of WIMA goodness 



The filtersection



I wil use a old Networdk switch box



The wil be also a symetrical input, sorry no schematic yet...
Also there wil be a audio amplifier for lissening to the noise, yes, only in the 10Hz -100Khz mode :-)

Shoot @ it!

Kind regarts,
Blackdog

[Kleinstein]

The precharging not really needs manual control, one might use a FET OP to sense the input voltage, so that no manual adjustment is needed. I think R8 should be larger, to have a lover frequency limit at the input. Input protection might need something like a small (e.g. 20 Ohms) series resistor at the OP inputs, as the voltage from the 1N4007s might be a little to high if power is off.

The OPA140 is not the best choice for the second amplifier stage and other low impedance parts. There are better (e.g. faster, lower power and cost) alternatives for that, and nose is not that critical any more.

There might be to much amplification before the filters - especially 50 Hz, but also wide band noise might already cause clipping at the filter input.  At least some way of detecting clipping at that point is needed. So some of the amplification might be better after the filter, if it is needed at all.
Depending on the source, a 50 Hz notch might be a good idea.

[blackdog]

Hi Kleinstein, :-)

Thanks for your remaks!
The are manny ways to do the precharge, also automatic, but I opted for manual way.

R8 can get a higher value, but I'm going to do the first tests with 5K.

About you remark for extra imput protection 20 Ohm, maybe, i will think about it.

The second stage i tested already, and works fine.
I will certainly test the NE5532a in this position, cost is not a problem, i want quality ;-)

I've thought about it, to change the position of filters, but for the fist test i keep it this way.

There is also a peak level detector, in order to show whether the signal remains within dynamic range.
And the dynamic rage is BIG, almost -+15V

This noise measuring system main input, is for low level use, noise/hum below 1mV RMS, so that i can use my HAMEG HMO3004 to view the noise of references and low noise power supply's.
There wil be a second input, symetrical, with more gain choices.

I also thought of a 50/100 Hz notch filter, but I have a lot of experience in the measurement of small signals and do not need it now.
There is something very wrong when I measure references and the 50Hz is dominant.
That means I can go back to school and then will have to re-learn how to make a measurement setup 

I welcome your comments, they make me think about some points.
The project is not finished yet, this afternoon I'm going to build on the preamp and make some first measurements.
I fixt the gain of the test setup at about 80dB.

I have just received some extra samples quad opamps to test for the first stage.
The first stage wil be dominant for the noise, and afther the first measurements I will decide witch quad opamp i wil use.
OPA4140
ADA4004-4
MAX44252
MAX44243
OPA1664
OPA4188
OPA1654
ADA4077-4  (lower bandwith)
LT1885
LT1679

A lot of testing to do, low and high impedance source (10 Ohm and say 1K source impadance)
It wil keep me of the streets of Amsterdam 

Kind regarts,
Blackodg

[Marco]

"MAX44252"

These are just ridiculous value for money compared to the competition.

[Kleinstein]

The MAX44252 is quite good when it comes to low frequency voltage noise, but it also has current noise and bias, in a range comparable to BJT based OPs. So I don't see an advantage over something like an LT1007 or OP27 in this application. Still an interesting chip.

[Marco]

All the really low noise choppers seem to be BJT based unfortunately (except discrete designs like Jim Williams's 40nVpp design idea).

So yeah, for this purpose the OPA140 is still the best bet unless you want to go discrete ... but in general the MAX4425x is ridiculously good value.

[Kleinstein]

The MAX44252 is still MOS based. However due to charge injection and leakage in the rather low impedance choppers, there is a fair amount of current flowing in and out of the input. It's just coincidence to end at a current noise comparable to the LT1007. Most copper-stabilized  don't have such detailed data in the DS, but some noise current is still there.

The discrete chopper stabilized circuits like Jim Williams's 40nVpp design idea will also have input currents and current noise. pC charge injection and kHz chopper frequencies end up in the nAs current range, even if much if this compensates this will be accompanied with noise. So a rather low chopping frequency might help.

A better alternative with lower noise current and good voltage noise would be an ADA4528. Still strange why there is no 1/f part in the noise current visible.  So the max44252 is not that exceptionally low noise - it's a rather high speed AZ amplifier in first place.

With very careful trimming of switch controlling voltages and possibly with JFET choppers there may be a chance to get a little better, but there will be still a significant higher current noise than with a pure JFET amplifier.

[Andreas]

Shoot @ it!

I am missing the battery compartment for the power supply.
And where is the cake box where you put all in (including DUT)?
The 1K resistor at the input seems to be too low when measuring a unbuffered LTZ1000.
And why do you use sockets (with thermal noise) and thick film resistors for the input stage?

With best regards

Andreas

[Marco]

However due to charge injection and leakage in the rather low impedance choppers, there is a fair amount of current flowing in and out of the input.

The rather ancient TLC2654 says it has fA input noise current (much higher bias current of course). Dunno if it's true though.

So the max44252 is not that exceptionally low noise

It's exceptionally cheap for a relatively high voltage chopper.

[blackdog]

Hi Andreas, :-)

The power supply is in a another enclosure, and this includes a good double-insulated transformer.
Comming from a precision measuring instrument.
With this I hope to suppress the common mode as much as possible.
If this does not work well enough, then there is a battery supply consisting of AA batteries.

The amplifier wil be dubble shielded.

The 1K resistor wil be shorted if i start to measure a LTZ1000. (the d.u.t. will se then, 440uF in series en 5 or 10K to grond, if the 440uF is pre charged)
First i measure the LTZ1000, I precharge the 440uF capacitor to te LTZ1000 voltage.
Put the mute switch to te "ON" setting, the right side of the capacitor wil be at ground level,
the left site wil be via the 1K resistor connected to the d.u.t. (reference) to charge the last view tens of mV.
Maybe 1K is still to smal, not mutch experience with the LTZ1000, i have two parts on stock en i will test them :-)
Normal reference's (d.u.t.) wil be no problem, the 1K wil be a light load for the last part of charging the 440uF capacitor.
If the charching is ready i wil flip the mute switch and start measuring the noise.

There will be 2x4 LED with 8 comparators to sense the output of the amplifier to detect level of the output   (Clipping enz)

I use sockets because this is a test setup circuit board, to test all the low noise quad opamps i have ready for this project.

I hope its more clear now.

Kind regarts,
Blackdog

[blackdog]

Hi,

A little update of buildig the test board, not yet finished...

On the left site you can see the + and - 6V Low Noise Power Supply for the quad input opamp, the noise level of this power supply? about 1uV at 20Khz bandwith 


There is now already one part of the shielding on the board, ofcource there will be more shielding.
The red switches is voor testing the noise input impedance dependency, ik can switch R8 in the schematic from: 10K, 1K, 60 Ohm and "0"'


It takes a lot more time to think en build the board :-)
Tomorrow maybe more about this project.

Kind regarts,
Blackdog

[Marco]

There is still no real reason to do this with film capacitors by the way ... 0.002CV leakage electrolytics can do it just fine in the feedback path. A 22000uF capacitor in the feedback path with a 100k/1k feedback divider adds just adds just 4.4V on top with worst case leakage (after it stabilizes) which is no problem with the OPA140's 36V supply range.

[splin]

There is still no real reason to do this with film capacitors by the way ... 0.002CV leakage electrolytics can do it just fine in the feedback path. A 22000uF capacitor in the feedback path with a 100k/1k feedback divider adds just adds just 4.4V on top with worst case leakage (after it stabilizes) which is no problem with the OPA140's 36V supply range.

But its not just the leakage current that is a problem - the open circuit voltage of an electrolytic is very temperature sensitive. Eg. I just connected a 10mm x 25mm 100V 220uF to a 34401A DVM set to 10G ohm input resistance, and left it to stabilize for 30 mins. It showed approximately 89mV, decreasing by approx 2uV over 100s. Then holding my fingertip approx 10mm above the body of the capacitor the voltage rose by 65us over 100s. Touching it with a fingertip caused the voltage to increase at around 3uV/s.

Not the most scientifically controlled test but it illustrates the point. Very careful thermal control, together with long stabilization times may be needed to avoid thermal effects swamping the noise measurements. Larger capacitors having larger thermal capacity will respond more slowly to temperature changes but require longer to gain equilibrium.

A 10uF 63V filmcap seemed to be much better but was harder to measure due to the increased sensitivity to mains pickup induced by approaching the capacitor/test lead. Using warm air from my laptop exhaust from a distance showed little change in the rate of voltage drift.

[Kleinstein]

Using the OPA140 for the input, one may not need the large input capacitance, at least for the 0.1 Hz -10 Hz range. This may be different if 0.01 Hz lower limit is needed.

Getting something like 3 µV/s drift from touching the cap is not that bad. Thermoelectric effects on IC pins and drift of the chips are of similar size. So a stable thermal design is needed anyway. But it's true electrolytic caps can cause some trouble and at least need to be tested.

[splin]

Getting something like 3 µV/s drift from touching the cap is not that bad. Thermoelectric effects on IC pins and drift of the chips are of similar size. So a stable thermal design is needed anyway. But it's true electrolytic caps can cause some trouble and at least need to be tested.

True, but thermal EMFs arise from temperature differences between connections which can be kept quite low by good layout and thermal design, whereas the electrolytic is sensitive to absolute temperature which can only be closely controlled by using an oven.

3uV/s has to be kept in context with the requirement to be able to measure noise levels down to 100nV or better over .1 to 10Hz, so drift needs to be less than 10nV/s - and preferrably rather better than that to allow for other noise and error sources in the instrument such that in total it is at least 4x better than the noise level precision required.

Touching the capacitor with a finger 15 degrees C warmer than ambient is a fairly extreme thermal event however, so as you say some testing would be needed.

[Marco]

The MAX44252 is still MOS based. However due to charge injection and leakage in the rather low impedance choppers, there is a fair amount of current flowing in and out of the input. It's just coincidence to end at a current noise comparable to the LT1007. Most copper-stabilized  don't have such detailed data in the DS, but some noise current is still there.

The MAX4208/4209 have negligible input bias current period. Thinking about it, with some auto-zero architectures the opamp's inputs can simply be switched between the two transistors of a differential pair. This should create near zero average charge injection when both inputs are at the same voltage, due to the symmetry (assuming for a moment the transistors/switches are CMOS, charge can only be transported from one input to the other ... with symmetry why would it go one way and not the other, a couple nV combined with a couple of pF isn't going to transport enough coulombs to matter).

Of course there are a lot of different architectures, so MAX44252 might still be MOS.