Noise in resistance measurements - Page 1
Forum section: electronics / general

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 30, 2018, 09:29:34 pm
content: I made measurements of the noise in measuring a ~10kOhm resistance with HP3457A. The noise looks very low, 0.15 ppm standard deviation for 100 NPLC (which takes 4.15 sec each). The resistor itself is a 10 ppm, 0.05 W Vishay 8.6 kOhm, the cheapest approximation to a "precision" resistor I could find on ebay.This looks even better than measurements with HP3458A:https://www.eevblog.com/forum/metrology/optimal-configuration-of-hp-3458a-to-measure-10k/msg1439757/#msg1439757Feel free to prove me wrong and show better data with another precision DMM

author: dl1640
author_rank: Regular Contributor
timestamp: July 30, 2018, 11:07:16 pm
content: With AZ:ON and OCOMP:OFF?how was the variation of room temperature at the time ?

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 30, 2018, 11:17:01 pm
content: AZ is always on, the meter has pretty crappy performance without it. OCOMP is not used on HP3457 for resistance ranges greater than 3 kOhm.Temperature I am not sure, just sitting on dinning room table overnight. I am planning to setup a temperature logging system at some point.

author: dl1640
author_rank: Regular Contributor
timestamp: July 30, 2018, 11:57:29 pm
content: I will try to log some data with 10k or 1k WW over night , with my 3457

author: CalMachine
author_rank: Frequent Contributor
timestamp: July 31, 2018, 12:11:22 am
content: Quote from: maxwell3e10 on July 30, 2018, 09:29:34 pmHere is 0.03 ppm sdev with a sample size of 64

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 31, 2018, 02:23:20 am
content: Its a cool machine! But you couldn't measure an 8.6 kOhm resistor with it

author: dl1640
author_rank: Regular Contributor
timestamp: July 31, 2018, 05:19:01 am
content: set up my 3457 as :NPLC 100AZ ON,OCOMP OFF,DELAY .05,OHMF,NRDGS 100,MATH STAT,TRIG SIGGLEMEAN is 10.000358e+3SDEV is 3.0709574e-3about 0.3ppm against the mean.CalMachine you have so good machine

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 31, 2018, 06:18:56 am
content: To get full performance, one has to use the hires register, which is not included for built-in statistics. I read the data over GPIB and add the hires register. The code is herehttps://www.eevblog.com/forum/testgear/$5-usb-gpib-adapter-for-ezgpib/msg1650155/#msg1650155

author: dl1640
author_rank: Regular Contributor
timestamp: July 31, 2018, 07:05:28 am
content: thank you Maxwell,but my 3457 gpib has intermittent issue on sending and receiving.especially on receiving, it would report a "bad header" by ERROR key, and sometimes send message also reports an error.so for now I would only use front panel key operations...

author: CalMachine
author_rank: Frequent Contributor
timestamp: July 31, 2018, 12:27:29 pm
content: Quote from: maxwell3e10 on July 31, 2018, 02:23:20 am

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 31, 2018, 12:54:26 pm
content: I thought 4950 can only measure around round values. But I guess if you select 100 kOhm range and 0% band, it still works pretty well.

author: CalMachine
author_rank: Frequent Contributor
timestamp: July 31, 2018, 12:57:27 pm
content: Quote from: maxwell3e10 on July 31, 2018, 12:54:26 pmNope, I'm on 100% band on 10 kOhm. I just turned off the bandwidth limitations

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 31, 2018, 01:11:56 pm
content: Interesting. So are the measurement bands just a firmware feature or is the hardware actually built around taking differences from a few discrete values?

author: TiN
author_rank: Super Contributor
timestamp: July 31, 2018, 01:21:50 pm
content: 4950 is like a mutant baby of 1281 DMM and 4920 AVMS. But that's offtopic here.I will have something to say about this thread, but not now.

author: CalMachine
author_rank: Frequent Contributor
timestamp: July 31, 2018, 10:24:12 pm
content: Quote from: maxwell3e10 on July 31, 2018, 01:11:56 pmI believe it is mostly a firmware feature. I also believe the stated specifications are only valid within the specified bandwidth limits.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 31, 2018, 11:57:33 pm
content: Not sure what's being measured here. We know the Johnson noise of a wire wound resistor with high accuracy for any given value, and excess noise is near non-existent. If you're not getting that number, something else is being measured.

author: dl1640
author_rank: Regular Contributor
timestamp: August 01, 2018, 01:28:18 am
content: the setup was allowed to sit over night.I just repeat the measurement from front panel.took 100 samples.MEAN: 10.000380E+3SDEV: 0.0000000E+0I also believe Conrad's comment, 3457 has limitations if we measure the noise only from the resistor.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 01, 2018, 01:46:29 am
content: You got lucky that fluctuations were entirely within the 7th digit.The noise in resistance measurements is just voltage measurement noise plus current noise of the current source. The Johnson noise of the resistor is usually smaller, unless the resistor is approaching 1 MOhm.

author: dl1640
author_rank: Regular Contributor
timestamp: August 01, 2018, 02:09:53 am
content: I even did it again, same results.also use RMEM to recall 100 readings, all the same 10.000380

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 01, 2018, 02:27:11 am
content: That's impressive stability! Try it again tomorrow

author: quarks
author_rank: Frequent Contributor
timestamp: August 01, 2018, 04:24:22 am
content: Quote from: Conrad Hoffman on July 31, 2018, 11:57:33 pmI remember the JCan project from Joe Geller.He closed Geller Labs, but details are still available.For anyone interested in Johnson noise seehttp://www.gellerlabs.com/JCan%20Parts%20and%20Kits.htm

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 01, 2018, 02:13:01 pm
content: I built that and suspect it's actually better than he and others are willing to claim. The problem is you need FETs that aren't available anymore.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2018, 07:58:59 pm
content: Quote from: maxwell3e10 on August 01, 2018, 01:46:29 amA well made resistance measurement can eliminate the voltage reference noise to a large part. So it would be the voltage noise for a measurement near zero and the current noise without the contribution from the voltage reference that would matter. With some meters there can also be a significant current noise from the voltage reading input, that can get important for high resistors.Quote from: dl1640 on August 01, 2018, 02:09:53 amIf the noise is low, that can be rounding artifacts.

author: dl1640
author_rank: Regular Contributor
timestamp: August 02, 2018, 01:35:39 am
content: leave the setup to get some data last night.I overcome my 3457 GPIB read issue by re-initialize the bus (IFC), if an issue is encountered.so after IFC the 3457 would be *happy* again to talk to the PC which makes it be able to read back the data continuouslydata was taken each 7s or about 12s if a read issue occurred, 3457 setup is 100PLC, manual range, DISP OFF.I forgot to get extra 7 digit in HIRES register, however the 10 hours run seem to be stable with 6.5 digits, a few outliers observed.

author: dl1640
author_rank: Regular Contributor
timestamp: August 02, 2018, 05:37:48 am
content: by utilizing RMATH HIRES, one can even get *8.5* digits.took 100 samples,SDEV is about 0.13ppm against MEAN.then we can see some instability.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 02, 2018, 10:10:31 am
content: Yes, HP3457 is about the only meter with honest 6.5 or 7.5 digits on the log scale. Also on 100 NPLC the HIRES register gives really 2 digits, because its internally averaged from 10 NPLC. So at the end one can get real 8.5 digits.