3457a DC noise? - Page 1
Forum section: electronics / testgear

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 08, 2015, 07:56:05 pm
content: My 3457a has always been a bit skittish with the last digit on the low ranges and today i noticed it behaves differently to my other meters when shorting the hi lo terminals with a piece of copper.Set to the 30mV scale, DC, NPLC = 10, Autozero=on and with a 1.5mm piece of copper wire between hi and lo front terminals i'm seeing the displayed reading jumping around between -00.00040mV and 00.00045mV. It's much worse with Autozero=off.My other meters (one 6.5 and one 5.5 digit) don't show the same range of readings, they only vary +/- 2 counts on the last digit when shorted.Is this normal behaviour for a 3457a or is mine getting unwell?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 08, 2015, 08:04:57 pm
content: if you can do a short log, you can ascertain for sure by comparing to lukier's reading post#43https://www.eevblog.com/forum/beginners/o-qn-on-funny-nplc-multislopenoise/(= abt 120nV NPLC10)it could also be AC mains noise?mobile RF interference?computers? etcor maybe the PCB is dirty. i got better results after a good "wash". but my DMM is old n used.it could also be a case of capacitors leaking, so most DIY-er would do a "recapping" with new caps

author: lukier
author_rank: Unknown
timestamp: December 08, 2015, 08:31:24 pm
content: I short all 4 terminals with copper wire from ethernet cable. 3457A has wonderful math capabilities, use them to collect short statistics on 10 and 100 NPLC.Also, make sure the meter is warmed up. Just after powering up my offset is something like -4uV and after 30 min it decreases to ~1uV. This drift might appear as large std.dev. when doing statistics.Might be worth to clean the binding posts and apply some deoxit inside and on the thread. Make sure there are no air currents around the binding posts and therefore the shorting wire.In general at uV levels and below things get really challenging. Thermocouples everywhere, triboelectric effects, RF noise etc.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 08, 2015, 08:38:39 pm
content: All the meters have been powered on for 5 hours + and the room temp has stabilised at 20 C.with NPLC=100, NRDGS=10, MATH STAT:std dev= 73nVmean = 112nVI'll give the posts a clean.I hadn't considered the electrolytics could be failing

author: lukier
author_rank: Unknown
timestamp: December 08, 2015, 08:55:32 pm
content: Quote from: rigrunner on December 08, 2015, 08:38:39 pmThat is actually quite impressivestd dev is a tiny bit higher, but man, mean is 112nV! Your meter must be fresh out of calibration or something. On mine I have mean slightly above -1uV.The only mod I did to my meter is to add insulating foam inside, especially around LM399 as it improves stability, see here:https://xdevs.com/fix/hp3456a/#twk_ref.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 08, 2015, 09:02:42 pm
content: MOD IT ! ops ... off topic !

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 08, 2015, 09:14:03 pm
content: Quote from: lukier on December 08, 2015, 08:55:32 pmNot a cal sticker in sight. The meter came from a surplus seller in the US so i have no idea of its history either.I've cleaned the posts and removed and reseated the post <> board connectors.Quote from: 3roomlab on December 08, 2015, 09:02:42 pmI'll put some foam around the reference whilst i have it open. Don't tell 3roomlab...

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 08, 2015, 11:40:37 pm
content: I've wrapped copper around all four top postsAfter cleaning the posts and the internal connections i see:mean = 344.00nV (344.00001 e-9)std dev = 66.69nV (66.699995 e-9)Noise still seems to be there. The last 2 digits on the 30mv range are still jumping about.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 09, 2015, 12:36:56 am
content: i think you should try plotting it, and noting the MAX-MIN (as in p-p noise). it could tell if there is any strange drift. or as kleinstein has ID in my case, "popcorn" noise (see pic, eg @ 17500, entire chunk shifted upwards). so instead of purely numbers, a good look at plots could tell more

author: cncjerry
author_rank: Unknown
timestamp: December 09, 2015, 03:38:16 am
content: I ran the test on one of my 3457s for the heck of it. 187e-9 and 228E-9 for mean and std respectively. The statistics were calculated over 993 samples with NPLC = 100 and default NRDGS. I am going to do another test with a better shorting strap as I've seen better numbers from this meter.edit: I ran it again with wrapped copper strapping. 53E-9 and 42E-9 again for mean and std. I doubt it would get much better than that with all the noise around here. I've not run a test with the GPIB attached but my experience is that the noise usually goes up. I have another 3457 that I keep in my lab where the temp is cold (around 60 degrees) but constant.Have you tried doing the test in a dark or unlit room away from all TVs, computers, etc? I have a bunch of those ferrite clamps on the AC cord, don't know if they make a difference on the meter but the do on my moving coil cartridge preamp.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 09, 2015, 04:03:35 am
content: Whilst i was looking around inside the 3457a i spotted that the line filter was a Schaffner made in Switzerland unitIt's in the bin nowEverything back together and powered on for 1 hour gives this plot:Edit:Just ran math quickly before i shut everything down. 286e-9 mean 50-e9 std dev from 58 samples NPLC=100

author: cncjerry
author_rank: Unknown
timestamp: December 09, 2015, 04:30:59 am
content: Rigrunner, how did you get the plot? Sampling it over GPIB?This test has me interested now. I just ran another test with the lights off in the room, fan off in the adjacent bathroom, etc., trying to get all I can control out of the area. After 244 samples the mean is 6.398E-9 with STD at 40.237E-9. I don't remember ever seeing the STD go much below 40E-9. At one point in this test, it reported 544E-15 for the mean. That's as close to zero as I've ever seen. It was reporting xxxE-12 for about the first 100 samples but I think pushing the buttons impact the test. I am going to now add more shielding around it to see if I can get the STD lower. More to come.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 09, 2015, 05:00:32 am
content: Quote from: cncjerry on December 09, 2015, 04:30:59 amYes. I set the options i wanted with the front panel buttons and then issued a read command over gpib and logged to a file.I've just quickly tried lights and other bench gear powered off. Lights make a little difference but other bench gear off drops the noise considerably.I have to give up for this evening. Only a few hours left before i need to be awake again

author: cncjerry
author_rank: Unknown
timestamp: December 09, 2015, 06:10:31 am
content: I'll warm-up my other 3457 tomorrow and run a test. No matter how many times I run this one, the STD comes in around 40E-9 +/- 2 on this one. That's only slightly better than yours at 66, definitely not something to worry about. I'm running NPLC 100 and NRDGS 10. I tried bumping NRDGS up and the STD didn't change.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 09, 2015, 03:30:34 pm
content: Quote from: cncjerry on December 09, 2015, 06:10:31 amcould you try NPLC 1 just bare, to see its p-p noise and then the NPLC 100 and NRDGS 10? i am curious how much peak peak noise does it have(actually i dont quite understand NRDGS, what is it? is it like a NPLC multiplier? an like average to stack multiple NPLC?)

author: lukier
author_rank: Unknown
timestamp: December 09, 2015, 04:09:04 pm
content: Quote from: 3roomlab on December 09, 2015, 03:30:34 pmNo, it is just to limit the number of readings taken after a single shot trigger (normally the meter is running in auto-trigger mode). This way if we know NPLC and the number of samples used in the statistics we are comparing apples to apples I think.

author: cncjerry
author_rank: Unknown
timestamp: December 10, 2015, 03:03:10 am
content: I ran it in NPLC 1 with NRDGS 10. Mean was 11.28E-9 and STD 331.786E-9. Both still really small numbers but much higher than NPLC 100.That STD number is .000000331786.I showed my son the mean at one point was 551E-15 which is .000000000000551I get a kick out of numbers like this in a home lab. My GPSDO regularly reports phase errors in the picoseconds. I realize the length of the antenna isn't quite right, etc, but still our ability to measure at home to microvolts, frequencies above 20Ghz, signals below -150 or even -160dBm is pretty incredible.

author: onlooker
author_rank: Frequent Contributor
timestamp: December 10, 2015, 03:37:56 am
content: QuoteI think the more fair comparison is between "NPLC 1 with NRDGS 10" and NPLC 10 assuming the NRDGS samples are averaged in the calculation.(1 cycle) X (10 sample) ~= (10 cycle) X (1 sample)or if use NPLC 100, then compare it to "NPLC 1 with NRDGS 100".

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 10, 2015, 04:21:06 am
content: Quote from: 3roomlab on December 09, 2015, 03:30:34 pmFor 3roomlab's curiosityNPLCC = 1 NRDGS = 10Upper 430.99996E-09Lower -1.1299999E-06Mean 57.450991E-09SDev 385.30428E-09NPLCC = 10 NRDGS = 10Upper 269.99998E-09Lower -320.00001E-09Mean 57.999992E-09SDev 214.51537E-09NPLCC = 100 NRDGS = 10Upper 110.00000E-09Lower -100.00000E-09Mean 32.999999E-09SDev 63.953109E-09

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 10, 2015, 01:30:50 pm
content: wow the 1NPLC p-p is so low. compared to my K2015using 3600samples = 3.1uV (using 1NPLC log on 3rd Dec) 100mV rangea segment of 100 samples, the p-p is 2.3uVa segment of 10 samples, the p-p is 1.0uVo gawd! keithley, why so noisy !NRDGS 10 is meaning 10 samples? i want to see if i can extract same number of samples@NPLC10 (using 1dec log)4500 samples = 2uV100samples = 1uV10samples = 0.43uVnow im curious @ agilent/HP, they seem to pay alot more attention to make less p-p noise. i wonder if it is true also of 34401a

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 10, 2015, 05:20:54 pm
content: Quote from: 3roomlab on December 10, 2015, 01:30:50 pmYes, NRDGS 10 = 10 samples.Does the noise on your K2015 affect the stability of your low microvolt readings?

author: onlooker
author_rank: Frequent Contributor
timestamp: December 10, 2015, 06:15:20 pm
content: I see NRDGS is set for varies measurements listed here, but I am not so sure whether they are rightly or wrongly used in the noise calculation. Just to continue my last post, here is my understanding:In essence, NRDGS provides an alternative to the "TRIG" command to trigger the readings (sampling). It is a sort of "burst" mode. But, regardless what NRDGS is set to, how a reading is done is always defined by "NPLC". In other words, if each reading is a data point in the statistics, NRDGS setting is irrelevant.To compare the noise figures between 2 machines, the (total) integration time per data point (not necessarily per reading/sample) for the calculation should be the same. To a good approximation the total integration time per data_point should be"total_integration_time_per_data_point = NPLC x samples_per_data_point", assuming the value of a data point is taken as the average over samples per data_point.Then, It will be interest to know, but, I did not test, as to what will be the actual difference on a 3457a betweendata_point= "NPLC=10 and average over 10 samples"anddata_point= "NPLC=100 and 1 sample"

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 10, 2015, 06:38:53 pm
content: Quote from: onlooker on December 10, 2015, 06:15:20 pmAn alternative to TRIG? I see it as a companion to it. I've been setting TRIG SGL and NRDGS = nn so that when i clear the math values and trigger the readings i'm always taking nn readings before i read the math registers.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: December 10, 2015, 07:04:33 pm
content: Quote from: 3roomlab on December 10, 2015, 01:30:50 pmhmmm... so many samples and the noise is going up... could it be thermal "noise" from air movement in the lab?i tested a short-circuit (covered with styrofoam for thermal isolation) and easily got approx. 0.5µV p-p for the 0.2V range on my KEI2001 across 4096 readings (NPLC=10, AVG=10, moving, 2 sec between readings), but only after leaving the lab and _not_ coming back before measurements had finished. plotted the data in excel and applied a trendline mx+b over the data. the trendline's slope was just 2*1e-12, i.e., "flat", so, basically, no long term (thermal) drift that could have screwed the p-p noise voltage. however, seems to me that using SDEV on the data expressed as ppm of full scale range would make more sense.

author: onlooker
author_rank: Frequent Contributor
timestamp: December 10, 2015, 07:25:01 pm
content: QuoteI did not mean "alternative to" to be a "replacement of". Maybe, "a secondary TRIG" is more understandable wording.QuoteI think you also set STATS some where before all these. Now, I guess I understand what you were doing: You were taking statistics (mean, sdev) for every nn readings. That is, each reading is one data point for statistics calculation. In this case, the statistics is "independent" of nn, if nn is large enough. Then, nn=10 samples may be too few for stable statistics.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 10, 2015, 08:24:41 pm
content: Quote from: onlooker on December 10, 2015, 07:25:01 pmWhat would be considered the minimum number of samples to give stable values?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 10, 2015, 08:38:08 pm
content: @post#20 @ rigrunner100mV range? it would appear to be stable @ slow setting (=NPLC10 + moving average 10 = modified NPLC100, real NPLC 100 = NPLC10 + repeating average 10). in NPLC 1, i guess last 2 digits are useless. however if i reference from alex's K2015 logs to compare with my original logs, i have to conclude it seems, this is normal = K2015 have somewhat high noise (ref :https://www.eevblog.com/forum/testgear/dmm-adc-noise-comparison-testing-project/150/... post#191)@post21 @ onlookeri tested this approach see pic chart @ the same DMM noise thread , see the rows with NPLC 10 vs 2x5 (https://www.eevblog.com/forum/testgear/dmm-adc-noise-comparison-testing-project/?action=dlattach;attach=184637;image)also note that NPLC 2x5 is taken at 4500 samples/hr. see the averaged p-p noise is also quite similar. with this, i assumed that opening up a larger time window of data allows probability of collecting larger p-p noise. (= i have noisy environment)p-p noise become more of a probability game, of when the circuit will experience a spike due to external or semiconductor "mini pop-corn" spike? again this is my assumption/observationboth types of NPLC give very similar result imo. maybe better test = measure same source, twice in diff NPLC way, but even then, it is still a different 2 samples. maybe i need to use a DMM in alternate universebecause at same instance it would be impossible to log 2 kinds of samples. or otherwise, mathematically, this will always give same result?@ post 23 @ Le_bassisteat a later log, i soldered jump wire inside the DMM input_HI to input_LO before it reaches the mechanical switch (short of cutting the PCB tracks lol). the noise does not seem to show major change. before this, with banana jack. the sockets are wrap with cotton. so with this i deviate back to conclusion, circuit noise > thermal noise.this is where i got unsatisfied @ 10v range p-p noise, always exceeding 3uV.imo you should use repeating average, so that 10 new readings is use to produce exactly 1 result. and then any of the 10 readings are not repeated, but fresh 10 readings are taken. moving average mix 1 new reading into previous 9 readings to produce 1 result. so each new result is not independantly averaged from last result.@ post #25in the resistor thread, there was a PDF about statistics. 100 samples is what i conclude ... but i end up using more than 1000(i didnt know then that 100 could be a minimum)***edit : haha today should be exciting, spare parts are coming in to re-energize the K2015. i be "noisy" with noise logs again

author: onlooker
author_rank: Frequent Contributor
timestamp: December 10, 2015, 10:41:28 pm
content: QuoteThanks for pointing me to this thread. I did see that topic before, but did not go through all the pages. I guess it is because 3457a was not one of its lineup.QuoteI think the thread "www.eevblog.com/forum/testgear/dmm-adc-noise-comparison-testing-project" that 3roomlab referenced had discussions on the number of samples they use to get stable statistics.For my take, 1st, one collects, say, 1000 of the nn-samples-averages. Then find the sdev (call it sigma2) of these 1000 nn-averages. Now compare this sdev with the average of the nn-samples sdev (call it sigma1). If sigma2 << sigma1, the nn-samples can be considered enough.For random noise, the theory would say, for nn=10, sigma2 ~= 0.32 x sigma1. Whether a relative 32% jumpiness in the nn-samples-averages is stable or not is up to interpretation and, yes, is subjective just as many other things in statistics and probability.Then, all the discussions are assuming one is not tracking systematic fluctuations or "long" term drifting and all the measurements are done under the same environmental conditions.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 11, 2015, 04:16:19 am
content: Quote from: onlooker on December 10, 2015, 10:41:28 pmI've bookmarked the thread. i'll read through it and perhaps add my readings to it.Quote from: onlooker on December 10, 2015, 10:41:28 pmThanks for that. I'll see if i can automate that via gpib.