Author Topic: DMM linearity comparison  (Read 30419 times)

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Offline Pipelie

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Re: DMM linearity comparison
« Reply #75 on: November 19, 2018, 01:52:29 am »
here are results of Datron 4950 vs 3458,  testing setup: soaping time: 10 seconds, 200NPLC on 3458A, D4950 is using HiAcc mode, Band OFF, CERT ON. Average of 4 reading.

Note: D4950 is 7.5 digit DMM
TEMP?#A1 is the internal temperature of 3458, TEMP?#D1 is the internal temperature of Datron 4950
« Last Edit: November 20, 2018, 02:36:13 pm by Pipelie »
 
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Offline Pipelie

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Re: DMM linearity comparison
« Reply #76 on: November 29, 2018, 04:21:04 am »
I'm done with testing the linearity of DMM now, no more meter for testing.
I hope you guys like it.

the linearity of DMM34401 is impressed. :popcorn:

as usual, each data is average of 4 reading. the delay time for Fluke 5440B is 10 seconds.
 
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Offline niner_007

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Re: DMM linearity comparison
« Reply #77 on: December 09, 2018, 10:22:03 am »
does anyone have one for 34470A and for DMM7510? 34470A is supposed to be 0.5ppm, while DMM7510 is supposed to be 1ppm, would be interesting to see how they really hold
 

Online Kleinstein

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Re: DMM linearity comparison
« Reply #78 on: December 09, 2018, 12:13:48 pm »
The INL error can be different between different units. One of the likely larger error sources is the self heating of the input resistor to the integrator. This mainly results in a 3rd power contribution proportional to the resistors TC. It gets even more complicated with an resistor array with partial, but not perfect thermal coupling and matching.  With good resistors the individual resistors can be quite different and even different sign and the TC and thus the resulting INL can depend on temperature.

So these test only reflect the linearity of individual units, not for a type of meter.
So it would need tests with quite a few units to also judge other units.
 
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Offline TiN

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Re: DMM linearity comparison
« Reply #79 on: January 07, 2019, 02:25:57 pm »
Just having fun during lunch-time break:



Animation shows polynom fit order from 1 to 31, using 8508A sweep from -19.9 to +19.9 V.
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Offline e61_philTopic starter

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Re: DMM linearity comparison
« Reply #80 on: March 23, 2019, 07:55:26 pm »
I measured the Fluke 5730A a some weeks ago, but I think I never posted the results. Here is a measurement against two 3458A and a 8508A.

All line up very well and the 8508A doesn't look as bad as in TiNs measurements. But I measured only from 0 to 10V. Perhaps the 8508A has problems if one is going through zero? I will run some tests about that..
 
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Online Kleinstein

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Re: DMM linearity comparison
« Reply #81 on: March 23, 2019, 10:08:37 pm »
The expected INL is in the same order of magnitude for the 3458, 8508 and the 57xx calibrator. So it can be tricky to tell who is at fault if it's not a straight line.

For going through zero there could be a jump in the calibrator - at least TiN's 5720 seem to have a slight jump there. They tend to use just relays to reverse polarity and thus an offset error would cause a jump near zero.

The better DMMs tend to measure across zero without anything special. Though there are a few exceptions (e.g. the 6581 and similar have a transition in run-up mode very close to 0). The 3458 ADC circuit also shows some odd circuitry (zero glitch jump circuit) to operate near zero crossing - though I somewhat doubt it would be active with normal measurements.

The curve TiN showed has quite a lot of U³ contribution, which could be a thermal effect of resistors or similar. Reducing the range from +-20 to +-10 V would reduce the U³ part by a factor of 8.
 
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Offline TiN

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Re: DMM linearity comparison
« Reply #82 on: March 24, 2019, 06:16:46 am »
Each of the point on my graph is actually 5 or more samples, not single sample. I added that to get idea of point noise graphically as well. But that makes a graph look extra "jumpy".
And conditions/settings matter a lot when trying to do anything below own noise of the DMMs reference.
Wouldn't trust much on anything below 0.2ppm, does not matter what DMM it is, just because in real life at this level you are much limited by reference noise.
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Offline branadic

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Re: DMM linearity comparison
« Reply #83 on: April 22, 2019, 04:45:06 pm »
Measured Prema 5017SC and R6581D against Datron 4000A. Looks like the calibrator needs some calibration. Attached are the measured raw values as well as the mean values with standard deviation.

-branadic-
« Last Edit: April 22, 2019, 09:28:36 pm by branadic »
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Offline e61_philTopic starter

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Re: DMM linearity comparison
« Reply #84 on: April 23, 2019, 06:23:57 pm »
The graphs looked like the shape of both instruments is somewhat similar. Therefore, I would use the Advantest as reference. I also assume that is the most linear instrument of the three.

The standard deviation of 0.3ppm at 10V for the 5017 seems to be really high. That is almost 2ppm peak peak. Is that a normal value for the Prema?
 

Offline branadic

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Re: DMM linearity comparison
« Reply #85 on: April 23, 2019, 06:42:42 pm »
I think I found the cause for that. I have the automatic filter activated (refering to the manual all specs given are valid only with this filter actived), which calculates the moving average over 10 values and additionally caluclates the difference between the latest two values and compares it with a difference stored in the meter. This filter is only restarted if range, function or integration time is changed. So obviously the measured values are worse due to the filter settings and I should have manually restarted the filter after changing the values on the calibrator or wait long enough for the filter to settle.
I will perfom the measurements for the Prema 5017 SC again.

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Offline e61_philTopic starter

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Re: DMM linearity comparison
« Reply #86 on: April 23, 2019, 06:44:52 pm »
Isn't it possible to switch the filter off and do the averaging later?
 

Offline TonyStewart

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Re: DMM linearity comparison
« Reply #87 on: April 23, 2019, 06:50:17 pm »
I find the best ADC INL test is to use a perfect DAC (tested with a hardware counter for every value)

- Then compare analog input vs analog output on an AC coupled scope in A-B differential mode  ( DC coupled may saturate DSO input) or use a Diff Amp. DC coupled and use DAC as X axis and Diff err. output as Y axis.  (Feeding ADC digital to DAC)
 
- By sweeping the input with a triangle and find exactly why the ADC has errors from noise, linearity , missing codes from noisy Vref, gain and offset errors all can be explicitly measured. 

 That is how I discovered a flaw in Burr Brown's early best performing ADC  hybrids that were built to Mil-Std 883 quality level.   

- They had internal noise on Vref from internal digital noise at some thresholds like xx0111xxxxxx to xx1000xxxxxx 

Then error for rising and falling slope triangle waves at high speed ,
    may indicate either a  filter group delay error or a Nyquist aliasing error or  S&H "memory error" from previous value and  not using a  proper plastic dielectric for the S&H.  (Ceramics have memory)

- That seems to be the problem on the PREMA results.
« Last Edit: April 23, 2019, 06:54:59 pm by TonyStewart »
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Offline maginnovision

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Re: DMM linearity comparison
« Reply #88 on: April 23, 2019, 07:14:56 pm »
I think I found the cause for that. I have the automatic filter activated (refering to the manual all specs given are valid only with this filter actived), which calculates the moving average over 10 values and additionally caluclates the difference between the latest two values and compares it with a difference stored in the meter. This filter is only restarted if range, function or integration time is changed. So obviously the measured values are worse due to the filter settings and I should have manually restarted the filter after changing the values on the calibrator or wait long enough for the filter to settle.
I will perfom the measurements for the Prema 5017 SC again.

-branadic-

The fast filter is auto filter but settles quicker. I've tried average filter also which you can manually reset but I'm not sure what it's really doing because it seems less stable.  :-// Last time I calibrated the datron I ended up just doing a quick range switch and back to reset it. I think you can manually 'trig' it but the range switch up/down was easier.
 

Offline branadic

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Re: DMM linearity comparison
« Reply #89 on: April 23, 2019, 08:36:59 pm »
Did a quick check with filter off, integration time 2s, 100 values per voltage step, range 0V ... 10V. The pictures looks much better now.
Will check different integration times the next days over the full range. Prema 6048 uses the same ADC and is specified to have 0,1ppm linearity, refering to Mickle T. table:
https://www.eevblog.com/forum/testgear/8-5-digit-dmm/msg270530/#msg270530
I'm not sure if they are selected, since there is some marking on top of it.

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Offline jaromir

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Re: DMM linearity comparison
« Reply #90 on: September 09, 2020, 08:58:18 pm »
Apologies for resurrecting this old thread, but I thought it's good idea to ask about DMM linearity in DMM linearity thread.

I wonder if anyone measured linearity of old 'brown-shoebox' series of Keithley multimeters? Perhaps it's scattered somewhere here on forum, but I can't find it.
I know it's not typical volt-nut gear, but types 196 and 192 or similar 193 should be able to output 3M and 2M counts, being borderline there. They do have some differences in internal design (and step to K2000 series was quite a bit of rework again) so I wonder how and if it resulted in linearity differences.
 

Offline antintedo

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Re: DMM linearity comparison
« Reply #91 on: September 10, 2020, 03:22:32 am »
Results for K192 and some newer DMMs can be found in a thesis linked here: https://www.eevblog.com/forum/testgear/dmm-linearity/msg698554/#msg698554
 
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Online Kleinstein

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Re: DMM linearity comparison
« Reply #92 on: September 10, 2020, 07:25:38 am »
The ADC part in the keithley 192-199 look quite similar. There is not much in common with the K2000. There is a little similarity in the K2001. In some aspects it's similar to the HP3456, using CMOS logic chips for reference switching and switching only one side.

One general contribution to INL is resistor self heating - this can vary between units, as there are good and not so good resistors. 
 

Offline krasimir.k

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Re: DMM linearity comparison
« Reply #93 on: April 08, 2021, 06:52:38 am »
Hello,
I got yesterday my linearity test report for mine 3458a and 34401a against Josephson Voltage Standard.
The measurement was performed in Bulgarian Institute of Metrology.
The report includes data for 0.1/1/10V ranges.
I will try to send my Advantest R6581T till end of this year, but first I have to replace the dim screen.
Regards,
Krasi
 

Offline guenthert

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Re: DMM linearity comparison
« Reply #94 on: April 09, 2021, 04:37:58 pm »
Hello,
I got yesterday my linearity test report for mine 3458a and 34401a against Josephson Voltage Standard.
The measurement was performed in Bulgarian Institute of Metrology.
The report includes data for 0.1/1/10V ranges.
I will try to send my Advantest R6581T till end of this year, but first I have to replace the dim screen.
Regards,
Krasi
    Do I read the measurements correctly?  The linearity looks outstanding, pity about the gain error.  There's a 100ppm error in the 10V range for the 34401A.  That is out of specification.  Has that (old) instrument been recently calibrated?


EDIT:  I didn't.  The graphs are in [µV] not µV/V.  Don't mind me then.
« Last Edit: April 09, 2021, 04:42:33 pm by guenthert »
 

Online Kleinstein

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Re: DMM linearity comparison
« Reply #95 on: April 09, 2021, 04:49:11 pm »
With the 34401 is does not really make that much sense to use a JJA for a INL test. The test would be limited by the LM399 meter internal reference. This would already limit a test with a more common Fluke 5700 or similar. As a countermeasure it would take mutlitple runs of the votlage range, not just some 20 readings in a row at each votlage.
So much of the deviations seen for the 10 V range may very well be explained by reference noise.

The INL in the 100 mV and 1 V ranges look like mainly a U³ component, like what is expected from resistor self heating (e.g. die gain stage).  This part can vary between units  - so another meter may have different errors.
 

Offline Mickle T.

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Re: DMM linearity comparison
« Reply #96 on: April 09, 2021, 06:02:37 pm »
The INL in the 100 mV and 1 V ranges look like mainly a U³ component, like what is expected from resistor self heating (e.g. die gain stage).  This part can vary between units  - so another meter may have different errors.
Single resistor self heating (with linear R(T) approximation) gives only a U2 component of INL.
 

Online Kleinstein

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Re: DMM linearity comparison
« Reply #97 on: April 09, 2021, 07:15:47 pm »
The first component from resistor self heating is usually an error proportional to U³:  the power is proportional to U² and the voltage error than resistor change times voltage.
In some circuits there is a DC offset added at parts of the circuit and this may than give a U² term. Also heating from amplifiers may be more proportional to current.

The 34401 seems to correct the turn over error (mainly the square part) numerically based on calibration. The service calls for a special cal point for the -10 V/10 V.
So the U² term if present for any reason (e.g. nonlinerity at the ADC's 4053  switches, which can result in a small U² term) but also thermal effect in the resistor array (temperature effecting the offset) is likely corrected. As only the lower ranges show the pronounced U³ term, I would suspect the resistors setting the gain.
 

Offline krasimir.k

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Re: DMM linearity comparison
« Reply #98 on: April 10, 2021, 05:16:03 pm »
Here is more info about the gain error of the 3458a in the INL report. Sorry for the long story ...

Lets starts with the drift of my LTZ1000. Between 19.06.2019 and 11.06.2020 I made two calibrations against Fluke 7009N in Bulgarian Institute of Metrology (BIM). The absolute difference was 2uV (0.2ppm), but the measurement uncertainty was 21uV in 2019 and 34uV in 2020. The Fluke 7009N itself was calibrated in BIPM France in 20.12.2017. Based on these numbers, I think that my LTZ1000 is stable enough.

When I bought the 3458a at the end of 2020, there was -5.3ppm difference against my LTZ1000 based voltage reference.
Not bad for a DMM calibrated before 8 years with 8ppm year spec :-)

The 3458a and 34401a were adjusted with same LTZ1000 based 10V reference in period of around 6 months.
The 34401a was adjusted in June 2020 by me with the LTZ1000 ref. and the 3458a was adjusted by me again in 29 Dec 2020 after adding 100K resistor parallel to the 15K in A9 voltage reference board. Because of this modification, during the whole Jan 2021, I saw 25uV down drift with 3458a measurements against my 10V reference, because of the new temp set point which is normal.
In middle of the February I got an unexpected call from Bulgarian Metrology Institute. They ask me if I would give them my 34401 and 3458 for INL test, because they need equipment to test the Josephson Voltage Standard INL test procedures with as many DMM they can found. The price which I had to paid was very low so I agreed without any doubts :-) If I know that before, I had to skip the 100K resistor modification before the INL measurements in this short 1.5 month period.
So lets return back to the gain error:
I do not have statistics about the drift of the 3458a during Feb 2021, but I expect to be around 10uV. So if I sum the Jan+Feb drift + LTZ1000 calibration uncertainty this will make around 70uV possible gain error. Not near the 115uV, but big enough to explain this big gain error.
When the 3458a returned back from BIM in February, I adjust the 3458a once more time at the end of February and during March I measure about 7uV drift against my LTZ1000. The first 10 days of April, the drift is down to 2uV, so seems the 3458's voltage reference is starting stabilizing around 3 months after 100K resistor modification. But the full stabilization can take few more months.

My LTZ1000 will be calibrated in May 2021 again against the Fluke 7009N. But this time the Fluke 7009N itself will be calibrated against Josephson Voltage Standard during April 2021. So the uncertainty will be very low.

Regards,
Krasi
 

Online Kleinstein

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Re: DMM linearity comparison
« Reply #99 on: April 10, 2021, 06:33:23 pm »
I remember reports that having multiple DMMs in parallel can cause some unexpected interactions - especially the R6581T (and similar) can have quite some spikes in the input current from auto zero switching, that may effect the votlage source and thus the readings on the other meters.

One would normally not adjust the 3458 meter that often and better just note the difference how much it has drifted. This makes it easier following drift over a longer time.

For the INL tests it may be nice to also note the mode the DMMs were operated in. From the RMS noise my guess is 10 PLC for the 34401 (which makes sense).

The 3458 noise data are not so clear:  The noise is a bit on the low side for 1PLC mode, especially at higher voltage where the reference noise would also add. For 10 PLC mode the noise is a bit high, at least for the 10 V case.  The reference voltage should be OK, as the noise in 100 mV and 1 V range looks quite a bit lower. The INL may be slightly different for the 1 PLC mode compared to the 10 PLC mode, normally used for precision readings.

For completeness one should also note the number of readings used, as this would show the uncertainty due to noise. The number of repeats does not seem to be that high, as the noise numbers are scattering quite a bit. For the 3458 the noise could be quite a significant part of the observed deviations.

The plots would make more sense after subtracting the gain error.
The error bars for the 34401 seem to correspond to the Stddev. data, which are likely for single readings, but the final data should be more like the average over some 10-100 such readings. So the actual noise may be lower. However epseically with the 34401 there can be added reference drift and low frequency noise.
 


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