Statistics question on a 11 hour stream of 3458A data - Page 1
Forum section: electronics / general

author: martinr33
author_rank: Frequent Contributor
timestamp: February 27, 2022, 05:40:28 pm
content: Question: what inferences can I draw from the dataset about the 732A voltage source?My thinking is, 732A noise levels are well below the noise levels in the 3458A.Can I conclude anything from the 18% correlation across a short window (100 readings)?The full correl reflects the drift in temperature overnight. One unit is more sensitive to temperature than the other. Neither unit demonstrates material ADC drift."short correl" is just 50 readings.Configuration:732A voltage standard connected in parallel to two 3458A meters on 10V range, NPLC100Both meters nulled to give microvolt level difference readings.11 hours of logging (WinGPIB loses connection for some reason, but this could be a networking problem.)Temperature range approx 18 - 14 C over the dataset3458A 3458A2Average -3.5E-07 -1.0E-06stdev 5.3E-07 4.2E-07p-p 3.46E-06 3.18E-06correl 45%short correl 18%The STDEV over 100 readings is tighter - 200nV and 300nV.The unit with the lower thermal drift therefore reports slightly higher noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2022, 06:14:09 pm
content: I would expect about comparable noise for the 732a and the LTZ1000 based reference in the 3458. The ADC adds a little extra noise. The ADC noise (in AZ mode) should be mainly white noise, while the references should have quite some 1/f or popcorn type noise. For a very short time scale the ADC noise may be a bit higher than the reference noise. Already with the 100 PLC conversions (actually average over 10 x 10 PLC) the 1/f noise if the reference can be a significant part of the noise.In addition there could be a little ADC gain drift (e.g. from temperature variations and random resistor part) - this is very hard to tell appart from the reference noise.

author: TiN
author_rank: Super Contributor
timestamp: February 27, 2022, 07:51:39 pm
content: Data set is not really complete without additional information, such as ambient temperature (or at least TEMP? reading from DMM itself).I'd say there is no way to determine contribution of either DMM or 732A as both are about same.So you either need way longer timeframe (week or more) or having one of devices at constant temperature to evaluate TC of second device.Back when I had no TEC chambers, I used to put a large towel blanket over 3458A back, blocking airflow intake, to cause +10-15°C internal temperature rise to evaluate tempco of 3458A.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 27, 2022, 08:49:00 pm
content: I am not looking for TC in this dataset (although that was my original intent, but my logging crashes - for now).What I find interesting is that there is no evidence of non-random noise between the two meters. Ths observation would suggest that the 732A is significantly quieter than the 3458A.Any noise in the 732A would be recorded in both datasets, but the correlation calculation shows no significant evidence of this. Obviously, my time constant could be:- too short, in which case I could use a longer short window- too long, but less than 100NPLC might be too noisy.(as of right now, both devices are within 1uV of each other - it is warmer. so not seeing significant drift in the meters yet).That said, here's a short snippet of data. I think that there is evidence of some simultaneous shifts, but they are rare enough that the CORREL function rides right over them. I need to tink a bit differently about extracting noise.

author: IanJ
author_rank: Unknown
timestamp: February 27, 2022, 09:20:44 pm
content: Interesting to read you are getting WinGPIB disconnects.If you are using a USB to GPIB gateway adaptor then any power glitches in or around your PC could cause USB disconnects (oh, how I hate USB!). Its one reason I mainly use a E5810 Lan to GPIB gateway for faultless logging.Ian.

author: TiN
author_rank: Super Contributor
timestamp: February 27, 2022, 09:47:30 pm
content: You can correlate different 3458As only if you carefully trigger and synchronize their sampling together. Otherwise you just looking at very low frequency noise of ambient, reference, meters, etc.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 27, 2022, 10:32:58 pm
content: Hello Martin,I really appreciate your measurements, as I always have done them the same way with all of my references.Main goal is always to identify on the D.U.T. (initial) timely drifts, detection of short and medium term instabilities, like these infamous Popcorn downward bumps, single point glitches, arising from external EMI and bad shielding and I also see medium termed instabilities on minutes to hour scale, mostly arising from the 3458A.I investigate on instabilities over temperature (T.C.) in a different manner, because the 3458A has quite a big T.C. of about 0.5ppm/K in total (reference and A/D converter).Therefore, the room temperature (and the 3458A, TEMP?) must always be constant to +/-0.2 °C over the whole time up to 24h.I do not understand what you want tell us about the temperature of '18 - 14 C', is it a typo, or a wrong conversion from Fahrenheit to Celsius?14..18°C would be very cold for a lab, and a 4°C temperature variation is way too high to give such stable measurements, especially with two different 3458A.As the others have said already, it's important to monitor the 3458As temperature. So read TEMP? every 50...100 samples, that's sufficient, and that will also give you the room temperature under certain prerequisites. Mine gives always 12..14°C higher inner temperature values.It's also important to use properly shielded cables with GUARD, at least plugged in at the 3458A side.Then, I always use NPLC 100, as the short term noise value changes with the chosen NPLC value.Then you could also store the absolute values in your sheet, and afterwards calculate the difference in µV or in ppm from the average value.I like such measurements, as one can learn a lot from them, so I open my treasure chest and also provide 3 excel sheets for the different cases I will describe further on.My diagrams have the same scale, your 1µV equals my 0.1ppm, but I have more comfortable graphics (sorry, some German mixed inside)The SZA263/LTFLU and the LTZ1000 based references have about the same noise level.So when doing such absolute measurements like yours and mine, one basically directly compares both references against each other, their noise amplified to 10V or 12 V level.See my first sheet for a Fluke 7000 vs 3458A, you see nearly the same noise level over 17h, and the same midterm bumps (+/- 0.1ppm) probably from the 3458A.It's worthwhile to measure StD on shorter intervals, I do that always on 1h and 10min basis, (selecting less noisy parts), as this gives some hints about the quality of the D.U.T., and is also more adapted to every days measurements.One of your 3458As might show some short spikes or dips, which might arise from its LTZ reference, maybe you zoom in to have a look.Running two 3458A in parallel is not the best option, as the asynchronous 3458As AZ MUX creates large 10nA spikes, which might affect the readings of the other DMM. (See book by Lapuh)And yes, the 3458A adds noise by its signal path and the A/D conversion, as one can see by comparison to differential measurements of two 10V reference sources, 2nd file. It's about 50% less noise, but you also discover some larger bumps or steps, which definitely arise from the D.U.T.s.My third file measures the ADR1000, which definitely has 50% of the short term noise of a LTZ1000 reference @ 7.15Vs, proven by absolute noise measurements.That more than 50% lower level compared to a 10V reference is visible in the absolute measurement versus a 3458A, but due to the mentioned additional effects, not to that extent.The 3458A is not good in judging about the short term noise level. (I think, the absolute noise measurement were about 90nV rms, or so)Ian gave some hints about the lock down of your measurements, maybe you write down some snippets of your code, how you trigger the instruments, and read out the results, in which format, and so on.I don't have any problem with my Turbo Pascal code and N.I. interface card inside an old PC.I think, I covered all your questions, also by the provided diagrams, otherwise pose more questions, please.Frank

author: martinr33
author_rank: Frequent Contributor
timestamp: February 28, 2022, 02:45:03 am
content: Thanks, Dr. Frank. I appreciate the feedback.These instruments are in my garage. I started the measurements at 18C (it is winter here...) and the temperature dropped to 14C overnight. This environment makes me very interested in the temperature sensitivity of various devices.I was really interested in the TC of the two meters, but my logging drops before I can complete. I need to troubleshoot that, per IanJ's post my E8510 should be perfect!The units are connected in a Y configuration, with both meters terminating at the 732A.Cable is shielded twisted 16GA PTFE with crimped gold over silver over copper.Guards are connected at all three devices, with one of the 3458As also providing ground.The meters are as synchronized as they can be with IanJs program. Given that I am running NPLC100, I am not sure that they will be too far off.These two units also show relatively low TCs, but I need to fix my logging first. I do have an ambient temperature log for these data.Previous logs have shown us that 732As are remarkably insensitive to ambient temperature.The F7000 is a bit more sensitive to temperature, at least for me. Makes sense, though, as its oven is about 1/1000 the size of the 732A's unit, and it has a lot less insulation.I use the offset simply because it makes it easier to monitor the chart on IanJ's program.On the 3458A input spikes - it would take quite a lot to cause a drop on the 732A output voltage - especially with NPLC100.I anm going to dig through the files you provided, thanks for that.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 28, 2022, 02:49:56 am
content: Ian: Picture of my error attached. Apologies for not digging deeper, but I thought you might have an instant answer.This error happens on both channels, but not simultaneously. A stop / restart of logging fixes it.Configuration is an E5810 Agilent Ethernet to IEEE gateway, but the logging PC is on a wireless network. I'll put it on wired.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 28, 2022, 05:39:00 am
content: Update after reviewing Dr. Frank's files.1) My units under test (732A) appear to be a bit noisier in the short term than Dr. Frank's (7000) setup.Over the same 10 minute window I seeUnit 1: 416nV STDEVUnit 2: 343nV STDEVThis is on 10V, not 7V so compare values would be 300nV and 250nV.Dr. Frank's average of all 10 minute windows is 170nV.(that's thousands of windows. doing it this way eliminates deviations of a longer timescale).2)However, when I look at my F7000 readings from a different test time (and done with a nanovoltmeter), I find an STDEV of 200nV. Under these same conditions, my 732As were reporting 290 - 450nV STDEV. The F7000 and 732As were measured against a very good 732B. Measuring two 732Bs against each other produced an SD of 130nV.(The nanovoltmeter we used is known to be a little noisy, so these numbers could be 30% smaller).Therefore, there is some evidence that the F7000 is a bit quieter than a 732A, and that a 732B is quieter than both.3) Hidden in all of this is the true noise characteristic of a 732a/b/7000 reference voltage.The tools we have all require relatively long integration times. I suppose we could sum a stack of 3458As all on short integration times, as long as they were properly synchronized.

author: Andreas
author_rank: Super Contributor
timestamp: February 28, 2022, 06:28:08 am
content: Hello,dont know if it plays a role:when looking at the timestamps of Martins readings there are exact 3 Seconds between the stamps.Even at 60 Hz line frequency I would expect 3.3 seconds for 100 NPLC.So every 3 time stamps a total difference of 10 seconds and not 9.with best regardsAndreas

author: IanJ
author_rank: Unknown
timestamp: February 28, 2022, 10:52:18 am
content: Quote from: martinr33 on February 28, 2022, 02:49:56 amThe full description of that error is "ViWrite CONNECTION LOST" i believe, so at this point i'd take a guess its something to do with your wireless connection.Ian