Statistical arrays

[dietert1]

Thanks. As the VISHAY paper about SPM passivation is dated 2008, we can safely forget NOMCA parts. The datecode of the mouser NOMC parts mentioned above is 2225 and they should be stable against moisture. Those military moisture/temperature tests are rather aggressive and their results describe another regime of operation. For my intended application a 0.1 % resistance change is a failure. I am more interested in 1 ppm resistance changes induced by mild changes in environmental conditions.
The second paper is interesting reading, too. Yet it demonstrates how complicated reality can be and i think sometimes one needs to test parts in their application, in a prototype. That's what i meant above.
Today i ordered some 0.65 mm pitch SMD adapters to get 4-Ohm measurements at the NOMC pins, plus more NOMC parts.

Regards, Dieter

[dietert1]

As i am waiting for parts, i unsoldered the pins of NOMC array 7 and wired it as a 0.7 voltage divider to make a TC test, with seven resistors combined for zero TC according to previous TC results. R1 of the array is not used.
Input voltage of the divider is a 14.1 V reference (10x LM399 array). Output voltage of divider is connected to K2700. A simple setup with a divider output impedance of 1750 Ohm. Accuracy certainly depends on K2700 input leakage, but let's try.
Diagrams show log and TC correlation of three 18..28 °C cycles in incubator. There is some modulation with TC estimated as -0.032 ppm/K - certainly compatible with zero.

The intended application of the divider is inside a small TEC oven. Existing prototypes of that oven show temperature variations of about 10 to 20 mK. So the divider TC effect should be negligible:
-0.032 ppm/K * 0.02 K = -0.000 64 ppm

Regards, Dieter

[miro123]

Great results,
The absence of temperature hysteresis is more appealing for me.

[dietert1]

Meanwhile i tested the divider 7 in a larger temperature range 13 .. 43 °C in order to check again for hysteresis and higher order terms (curvature). Curvature was found, hysteresis not. Arrays 8 and 9 have also been wired as zero TC 0.7 dividers and swept in the incubator.

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Results compared:
Array 7 TC -0,032 ppm/K
    TC(23°C) -0,035 ppm/K (larger temperature span)
Array 8 TC -0,040 ppm/K
Array 9 TC -0,031 ppm/K
Probably the measurement setup causes a systematic shift and the residual TC of these dividers is yet another order of magnitude smaller. One might use this type of divider without oven.

In order to improve my setup i should use a reference with 4-wire output to feed the divider. Currently there is a voltage drop along the cable that feeds about 1.7 mA into the divider inside the incubator, once more with the strong positive TC of copper.

Regards, Dieter

[Ole]

A bit over a month ago branadic posted the comparison in noise for the different resistor networks.

Concerning that I have a question:
In which resistive materials can 1/f noise be expected and in which is it not present?

Thanks in advance,
Ole

[Edwin G. Pettis]

The only resistor technology that has not had 1/f noise detected is wire wound resistors, all other technologies have 1/f noise to one degree or another depending on the resistive element(s) used.  While Evanohm (and derivatives) are used in both wire wound and film/foil type resistors, the wire form does not exhibit any 1/f noise that has been conclusively measured while the film/foil types have mostly been verified to have at least some 1/f noise present due to the way the alloy is worked.  All other resistor types have varying amounts of 1/f noise depending on how and what they are made with, in their final form.

[branadic]

I fear this statement as given is a myth. Also wirewound resistors exhibit 1/f noise as any imperfection in the atomic lattice will lead to 1/f noise. I agree that with the "MIL-STD-202G, METHOD 308 - Current-Noise test for fixed resistors" to measure resistor noise it was literally immeasurably low back then, but with a bridge configuration and with todays test equipment you can show 1/f noise even for wirewound resistors. Been there, done that.

-branadic-

[dietert1]

..
Concerning that I have a question:
In which resistive materials can 1/f noise be expected and in which is it not present
,,

Is this question about producing low noise resistor parts?
The diagrams already posted in this thread are in terms of available resistor parts/families. We can get and characterize those parts. You could try and find out the material of parts that were found to be low noise. If they are all made of nichrome, then this could be an answer.

As mentioned in the ADR1399 thread, i am logging a bootstrap/gain stage based on a NOMC resistor array and results are perfect. The 0.7 divider does not add significant noise at the 0.1 ppm level and the TC of the division ratio is less than 0.1 ppm/K. Characterization of long term stability will take some time, of course.

Another approach to testing long term stability is shown in the photo. I made five of those 4 wire adapters with 5K NOMC parts. The idea is to characterize resistance differences within each array to better precision. With the previous test jig the socket contacts were between the 4-wire conncetion and the DUT.

Regards, Dieter

[Kleinstein]

From the data shown, it is not just the resistive material. One can use the same resistive material with a thicker layer and finer etched pattern to get a longer trace instead of a very thin layer and less pattern to get the same resistance. Expect less noise from thicker film and as one step up the bulk metal foil, that usually is a bit thicker. So not all thin film resistors of the same size are the same. Things can also change with resistor value. Chances are the lower resistor values (e.g. < 1 K) are just a simple layer of suitable thickness with no pattern. The high values (like >100 K) would hardly get away without a pattern.

AFAIK the details of the 1/f excess noise are not fully understood. My understanding is that the bulk metal should not cause much excess noise, but the problem is more with the surface and possible cracks / weak links. With the relative thick material wire wound resistors are naturally low noise, though not per se noise free.  For tunneling contacts it is known that it only takes minmal changes to jump from some 13 K to near infinite or 6.5 K. For me the surprising point is more that some resistors (carbon, but also some thin film) are that noisy. How critical the resistor noise is also depends a lot on the application - often the normal thin film resistors (or even thick film) are good enough and no need to worry. It is rare to require really low noise resistors (like better than -40dB Ni).

[branadic]

The answer to this question is: "It highly depends...". Many different aspects have to be taken into account. There is high evidence that e.g. surface roughness of the substrate in thin film resistors has an influence using the very same resistive material and process to apply the resistive layer to the substrate, but also by which process the resistive layer is applied to the substrate. You can read through the paper "Measurement of Excess Noise in Thin Film and Metal Foil Resistor Networks" by Nikolai Beev and associated papers referenced to in there.
It's a complex story and while some process and material combinations exhibit lower noise, others show very large excess noise. While wirewound and metal foil resistors in principle can show very low excess noise compared to thickfilm or carbon composit resistors, this isn't true in general and highly depends on how the wire and foil is processed and treated during manufacturing. To some extend you might reach the limit of your measurement setup, but that doesn't mean there is no excess noise at all and with an improved setup you might spot excess noise even in the best resistor available.

You can think of excess noise as a fluidic channel running water through it. The smaller the amount of water (similar to the amount of current through a resistor), the lower the noise due to imperfections (such as stones representing imperfections in the lattic of the resistive material) in the channel. Since we are not living in an ideal world, no resistor technology is perfect. Beside the resistive material also the leads and how they are attached to the resistive material contribute to the overall noise performance of a resistor.

-branadic-

[Ole]

Thanks for the informative answers,
Nikolai Beevs paper was quite enlightening.
The question was originally due to my own interest in noise and my plans to build a low noise amplifier.
Since most of the noise is voltage dependent it wont be that bad for a low noise amp.

Cheers
Ole

[RandallMcRee]

Since most of the noise is voltage dependent it wont be that bad for a low noise amp.

No, most of the noise in a low-noise amplifier comes from the temperature noise of the resistors and amplifier in the front end. Doesn't matter what type of resistor.

Consult, for example Motchenbacher, "Low Noise Electronic Design".

Even an online noise calculator like http://dicks-website.eu/noisecalculator/index.html will show the major factors in a noise calculation and resistor type is not one of them.

There are many, many, good low noise amplifier designs out there, hopefully you are well acquainted with them?

[dietert1]

For the time being i finished measurements of the five NOMC arrays with 32-pin adapters mentioned above (for improved 4-wire ohms). I measured each array four or five times. After the first rounds i cleaned the devices once more in order to reduce leakage currents. Still results are mixed.
TCs are measured to about 0.01 ppm, better than expected.
The relative resistance within each array - that is important for divider stability - appears more difficult. Since the setup is measuring the eight resistors interleaved i was hoping for a similar accuracy, but it seems to be more like 0.3 ppm or so.
I inserted a delay to help offset compensation, yet without obvious improvement.
Will try and repeat those measurements after some months.

Regards, Dieter

[dietert1]

Meanwhile i found that i forgot to clean flux from the 32-pin socket wiring. Little patience when checking the new 4-ohms wiring to the Keithley multiplexer.
The first tests after cleaning the socket look better. Repeatability on the relative resistances improved to about 0.05 ppm. With a 5 KOhm resistor as DUT this corresponds to a leakage "resistance" of about 100 GOhm.

Regards, Dieter

[branadic]

After Nikolai has published his paper on Measurement of Excess Noise in Thin Film and Metal Foil Resistor Networks with a version of it accessible via the Arxiv I'm happy to let you know that our paper Review on Excess Noise Measurements of Resistors was today published too. It is OpenAccess.

-branadic-

[Dr. Frank]

Hello branadic,

thank you very much for these very interesting papers & measurements.

 On page 17 of your paper, why is the technology of Alpha Electronics resistors not documented?
I always thought they are also BMF like the VHx and similar, directly from Vishay?

Frank

[branadic]

Hi Frank,

it's not directly included in the datasheet, but if you search through the ultra precision resistor cataloge it is somewhat indirectly given that they are BMF resistors.

-branadic-

[miro123]

 Thanks for sharing. I value the incredible job done to test and document all this.
I have few questions
1. Do you belief that  1/F^a noise increases to infinity? - question is related to  page 4 "However, in[15], Horowitz and Hill argue that excess noise increases forever. "
2. How do you remove the resistor TC in equation? I mean every oven introduces some waves /temperature frequency/
3.  Does HP3458 U180 and  W7000 use TaN resistor networks? If yes, why this technology scores so bad in such short term(24h/50uHz)tests

feedback from other forum members are welcome too.
I think that answer to this questions will give us also better understanding of LTZ1000 vs ADR1000 behavior. Mid term vs long term noise
BR
Miro
===========
Here is my input
1. No it does not  goes to infinity. It is based to on my mathematical understanding of Fourier transform and observing the nature. e.g. we have not seen in long term 1/f low for earthquakes?
2. My question was toward TaN resistors. Generally they have higher TC.
3. No idea. I know that many automotive application uses TaN due to tight long term stability requirements.
On the other side I belief that mass production wins in long term. So at certain point the advances in mainstream thin film Nichrome will reach and surpass bulk foil and TaN

[branadic]

There were several discussions in the past, with opposite arguements, However, there was not final conclusion as far as I know.
Ignoring all effects such as influence of temperature, humidity, pressure, chemical processes, just to name a few of them, we still have to keep in mind that for the age of the universe the spectrum would only go down to about 2.3 aHz, while each decade adds the same amount of power to the spectrum, one year is about 32 nHz.
We simply haven't observed anything long and stable enough to tell and there are too many effects that are way larger than what we want to observe I guess.

The measurements were performed under controlled environmental conditions and we waited a while for the resistors to thermally stabilize after the bias voltage was applied, hence the temperature was stable during the measurement and t.c. of the resistors could simply be ignored. Proper shielding, cables and thermal insulation is required. We had a cookie box inside a styrofoam box plus the stabilized ambient temperature.

As far as I'm aware 3458A and W/F7000 uses NiCr resistors, not TaN. On the other hand TaN or NiCr networks with TaN passivation have the advantage of growing a self-controlled oxid that surpresses humidity influences.

-branadic-

[Kleinstein]

The resistor excess noise is not only a function of the material used, but also a question of how much. A combination of 4 resistors as 2 in series and 2 in parallel has 1/2 the excess noise of one the resistors alone. The network in the HP3458 is quite large in size. In addition they call it fine line, which would imply a relatively long track and thus thicker film compared to resistors that us a coarse etched structure.

[dietert1]

As far as i remember this thread had some studies about NOMCA arrays until the Beev et al. paper appeared, with about -30 dB excess noise for those arrays, while NOMC arrays measured better, at about -60 dB, that is similar to metal foil resistors. Later we learned that NOMC arrays have a "new" proprietary passivation (Vishay).
Meanwhile there are some results on long term stability, also Andreas had a long term log for a 7-10 V gain stage with a similar array (TDP). Yet there was some doubt concerning humidity. I used a NOMC array for a 7-10 V gain stage with an ADR1399, with sub-ppm stability.
I did not find the NOMC arrays in the new paper.

Regards, Dieter

[xerbo]

Throwing my 2 cents here, an exact 7/10 divider for lower voltage references (i.e. LM399)

[Edwin G. Pettis]

The 'myth' is not a myth, if you had read my published article in EDN, it explains the source of wire wound woes.  In this case, any 1/f noise is going to be generated in the flawed, non-welded joints, not in the wire.  You very well may see a bit of 1/f noise but the odds are highly in favor of the 'weld' joint causing the noise.  In high quality resistance alloys, the metal mix is as near perfect as modern technology can produce.

As I have pointed out in the past, the cold-rolled alloy film/foil Vishay uses has imperfections in the matrix from not only the cold-rolling but also by the etching/laser tweaking process leaving very rough microscopic edges which generates 1/f noise in pretty good quantities.  Precision wire alloys are made with a process that leaves little to no 'rough' edges and the alloy mix is highly homogeneous, again leaving little imperfections to generate 1/f noise.

The 'art' of measuring 1/f noise is far from easy especially when there is little to be seen.  There are no amplifiers, sans those in liquid Nitrogen, that have no 1/f noise of their own, separating out which one sourced the 1/f spikes is nigh onto impossible.