Author Topic: Noise in resistance measurements  (Read 4156 times)

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Offline maxwell3e10Topic starter

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Noise in resistance measurements
« on: July 30, 2018, 09:29:34 pm »
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/#msg1439757

Feel free to prove me wrong and show better data with another precision DMM :)
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #1 on: July 30, 2018, 11:07:16 pm »
With AZ:ON and OCOMP:OFF?

how was the variation of room temperature at the time ?
 

Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #2 on: July 30, 2018, 11:17:01 pm »
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.
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #3 on: July 30, 2018, 11:57:29 pm »
I will try to log some data with 10k or 1k WW over night , with my 3457
 

Offline CalMachine

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Re: Noise in resistance measurements
« Reply #4 on: July 31, 2018, 12:11:22 am »
Feel free to prove me wrong and show better data with another precision DMM :)

Here is 0.03 ppm sdev with a sample size of 64  :P

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Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #5 on: July 31, 2018, 02:23:20 am »
Its a cool machine! But you couldn't measure an 8.6 kOhm resistor with it  :P
 
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Offline dl1640

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Re: Noise in resistance measurements
« Reply #6 on: July 31, 2018, 05:19:01 am »
set up my 3457 as :
NPLC 100
AZ ON,
OCOMP OFF,
DELAY .05,
OHMF,
NRDGS 100,
MATH STAT,
TRIG SIGGLE

MEAN is 10.000358e+3
SDEV is 3.0709574e-3


about 0.3ppm against the mean.

CalMachine you have so good machine :D
 

Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #7 on: July 31, 2018, 06:18:56 am »
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 here
https://www.eevblog.com/forum/testgear/$5-usb-gpib-adapter-for-ezgpib/msg1650155/#msg1650155
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #8 on: July 31, 2018, 07:05:28 am »
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... :palm:
 

Offline CalMachine

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Re: Noise in resistance measurements
« Reply #9 on: July 31, 2018, 12:27:29 pm »
Its a cool machine! But you couldn't measure an 8.6 kOhm resistor with it  :P

 :popcorn:

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Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #10 on: July 31, 2018, 12:54:26 pm »
 :) 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.
 

Offline CalMachine

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Re: Noise in resistance measurements
« Reply #11 on: July 31, 2018, 12:57:27 pm »
:) 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.

Nope,  I'm on 100% band on 10 kOhm.  I just turned off the bandwidth limitations   8)
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Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #12 on: July 31, 2018, 01:11:56 pm »
Interesting. So are the measurement bands just a firmware feature or is the hardware actually built around taking differences from a few discrete values?
 

Offline TiN

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Re: Noise in resistance measurements
« Reply #13 on: July 31, 2018, 01:21:50 pm »
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.
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Offline CalMachine

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Re: Noise in resistance measurements
« Reply #14 on: July 31, 2018, 10:24:12 pm »
Interesting. So are the measurement bands just a firmware feature or is the hardware actually built around taking differences from a few discrete values?

I believe it is mostly a firmware feature.  I also believe the stated specifications are only valid within the specified bandwidth limits.
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Offline Conrad Hoffman

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Re: Noise in resistance measurements
« Reply #15 on: July 31, 2018, 11:57:33 pm »
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.
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #16 on: August 01, 2018, 01:28:18 am »
the setup was allowed to sit over night.
I just repeat the measurement from front panel.

took 100 samples.

MEAN: 10.000380E+3
SDEV: 0.0000000E+0


I also believe Conrad's comment, 3457 has limitations if we measure the noise only from the resistor.
 

Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #17 on: August 01, 2018, 01:46:29 am »
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.
« Last Edit: August 01, 2018, 03:42:13 am by maxwell3e10 »
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #18 on: August 01, 2018, 02:09:53 am »
 :popcorn:
I even did it again, same results.
also use RMEM to recall 100 readings, all the same 10.000380  \$\Omega\$
 

Offline maxwell3e10Topic starter

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Re: Noise in resistance measurements
« Reply #19 on: August 01, 2018, 02:27:11 am »
That's impressive stability! Try it again tomorrow :)
 

Offline quarks

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Re: Noise in resistance measurements
« Reply #20 on: August 01, 2018, 04:24:22 am »
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.

I remember the JCan project from Joe Geller.
He closed Geller Labs, but details are still available.
For anyone interested in Johnson noise see

http://www.gellerlabs.com/JCan%20Parts%20and%20Kits.htm
« Last Edit: August 01, 2018, 04:27:27 am by quarks »
 

Offline Conrad Hoffman

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Re: Noise in resistance measurements
« Reply #21 on: August 01, 2018, 02:13:01 pm »
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.
 

Online Kleinstein

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Re: Noise in resistance measurements
« Reply #22 on: August 01, 2018, 07:58:59 pm »
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.

A 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.

:popcorn:
I even did it again, same results.
also use RMEM to recall 100 readings, all the same 10.000380  \$\Omega\$

If the noise is low, that can be rounding artifacts.
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #23 on: August 02, 2018, 01:35:39 am »
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 continuously :palm:
data 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.
 

Offline dl1640

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Re: Noise in resistance measurements
« Reply #24 on: August 02, 2018, 05:37:48 am »
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.
 


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