Millivoltmeter better than 3458A - Page 1
Forum section: electronics / testgear

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 08:47:15 pm
content: Is there any Millivoltmeter better than 3458A at low volt range below 1V?I'd like to measure 7mv DC as accurate as possible for the thermal converters.Any info will be appreciated.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 13, 2014, 08:50:14 pm
content: The 3458A is already a $10k instrument! Would you buy one if they made a $50k 9-1/2 digit model?

author: robrenz
author_rank: Super Contributor
timestamp: February 13, 2014, 09:04:59 pm
content: Fluke 8508A is more accurate

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 09:28:52 pm
content: Quote from: dr.diesel on February 13, 2014, 08:50:14 pmThis is for commercial cal shop. 9 1/2 digit doesn't make any difference for mV at ppm level. I like your Hameg meter. I will buy 6 1/2 digit Hameg meter when they make one like your 5 1/2 meter instead of 9 1/2 Keysight meter.Quote from: robrenz on February 13, 2014, 09:04:59 pmYes, you are right. I am looking for used 8508A with reasonable price.But in terms of accuracy at 10mv, not much difference, I think. I will have to review the 8508A's spec for sure.Thank you for your input as always.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 13, 2014, 09:37:47 pm
content: Quote from: nadona on February 13, 2014, 09:28:52 pmAh, so you're not searching for an additional level of resolution, gotcha. Likely the 8508A as robrenz mentioned is your only choice, then again, you're into equipment they don't advertise if you want better!We expect a full teardown if you find one though!

author: robrenz
author_rank: Super Contributor
timestamp: February 13, 2014, 09:51:26 pm
content: Fluke 8508A 200mV scale 1yr specs (2.7ppm+0.5ppm relative) (4.5ppm+0.5ppm abslolute)HP 3458A 100mV scale (9ppm+3ppm relative) (Option 002 5ppm+3ppm relative)

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 09:52:25 pm
content: Quote from: dr.diesel on February 13, 2014, 09:37:47 pmSorry, I don't open the case. I have lots of equipment in the box unassembledThousands if not millions of screws in boxes.Quote from: robrenz on February 13, 2014, 09:04:59 pmAccording to the graph I downloaded from this forum, 8508A is way better than 3458A at sub 10mV range or any ranges but not enough to calibrate my Datron 4920M.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 13, 2014, 10:24:48 pm
content: Quote from: nadona on February 13, 2014, 08:47:15 pmFor that purpose, you need a voltmeter, which is more sensitive, and has less noise.High "accuracy" (however this is defined) as the 8 1/2 digit DMM have, is not needed here.So you better use a nV voltmeter, like the HP 34420A.On its 10mV range, it resolves 1nV, and with much less noise than the 3458A, and the other long scale DMM.There are also other nV voltmeter, the best ones are made by Keithley, that's their specialty, beneath high impedance / ultra low current instruments.Rem.: Regarding the "accuracy" of the 3458A, compared to the Fluke 8508A:Let's better talk about "stability".The 3458A has the best stability on a 10min time scale, used as a transfer standard and also the best linearity of typ. 0.02ppm.No other analogue instrument beats it, yet.The Fluke has a better mid and longterm stability for 90d. and 1yr.That's due only to the mistreatment of the LTZ1000A inside the 3458A @ 95°C, compared to the around 50-60°C of the Fluke.I think, in metrology the 3458A is still preferred due its best linearity and transfer "accuracy".Also, the 3458A is still the only DMM, which is capable of real AUTOCAL, i.e. making a complete artefact calibration of all of its ranges from 10V and 10k references only, which makes it much more versatile and independent from extensive calibration.But both of it's original internal references are lousy, compared to other 8,5 DMMs.But I don't want to start a brand quarrel, anyhow.Frank

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 10:42:29 pm
content: Dr. Frank: Thank you.What Keithley voltmeter do you recommend?

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 13, 2014, 11:03:18 pm
content: Sorry, I don't have any practical experience with those nV voltmeters.Please simply search for datasheet information about resolution and noise, and for actual and terminated instruments from Keithley, if you want to go for the 2nd hand market.But be aware, that those specialty nano-voltmeters may also be extremely expensive.Perhaps you check your needs regarding voltage resolution and noise, and perhaps a much cheaper 6 1/2 digit DMM with 100nV resolution would be optimal regarding your budget vs. requirements.Frank

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 11:15:47 pm
content: Quote from: Dr. Frank on February 13, 2014, 11:03:18 pmActually I have been using a autocaled 5440B, which I believe you have one, to measure 7mV on a 3457A. I offset the difference into the calculation of actual value. But I am not confident at 10ppm level of 5440B. So I am looking for new source of solution.Quote from: Dr. Frank on February 13, 2014, 11:03:18 pmI don't buy brand new TM equipment because technologies are old. And because of my vision, I avoid buying equipments made of smd parts.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 13, 2014, 11:30:42 pm
content: I thought, you want to measure thermal converters at the level of 7mV.In the area, thermo voltages and noise are your enemies, not those ppms.nV DMMs are better designed for such aspects.if you try in the end to measure those 7mV with your 3457A, you will have problems with those two parameters, also not with its absolute uncertainty (20ppm or so). But also, the 3457A is absolutely sufficient to measure the thermo converter with sufficient uncertainty, if you take care for thermo voltages.1µV error for that is realistic, that would give 142ppm error, and those thermo converteres are specified for 200-500pm conversion error, anyhow.If you would like to check your 3457A with 7mV input, the 5440B will also have big thermal voltage offsets, see datasheet, which has absolutely nothing to do with the gain uncertainty of the 100mV output.Remark: to adjust the 100mV and 1V divided range of the 5440B, the 3458A would be first choice, due to its superior transfer accuracy.Rem II: Yep, I own a 5442A.Frank

author: nadona
author_rank: Regular Contributor
timestamp: February 14, 2014, 01:06:30 am
content: Dr. Frank:Thank you very much for your inputs

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 15, 2014, 05:40:19 am
content: $10:http://r.ebay.com/zWuqpM

author: acbern
author_rank: Frequent Contributor
timestamp: March 29, 2014, 12:41:57 pm
content: When you measure thermal converters you normally generate a DC voltage that you then compare to an AC voltage and you adjust its level until you get the same reading for that ac (at least at low frequencies). As a consequence ablsoute accuracy is not that important. If you need that, it is hard to beat a precise voltage source (5440B, 4808) and a nullmeter like the Keithley 155. A 3458A certainly cannot. What you need is resolution and low internal noise.The statement that these converetrs are only accurate to some 200ppm in a post earler is not true, they are much more acurate (=measurement repeatable), but historically one could not define the mesarement uncertainties better, especially at higher frequencies, this has changed, and good cal lab can go down to a few ten ppm in the 100kHz+ region, up to some MHz, at lower frequencies even better. The old Ballantine 1605a transfer standard could already do that, if you had it well calibrated.A good meter for measureing low voltages and at the same time relatively cheap is the Keithley 182 or better the 182-M with its better input noise level. Its resolution is a factor of 10 better than the 3458A, but absolute measurements are not that accurate.

author: quarks
author_rank: Frequent Contributor
timestamp: March 29, 2014, 01:33:00 pm
content: here is a chart with DCV comparison of some 8.5 Digit gear and the very best calibrators Wavetek 4808 and Fluke 5730

author: free_electron
author_rank: Super Contributor
timestamp: March 29, 2014, 01:57:37 pm
content: Agilents nanovoltmeter or a keithley electrometer

author: acbern
author_rank: Frequent Contributor
timestamp: May 04, 2014, 01:56:13 pm
content: So back to the 34420, which is very acurate measuring low voltages (much more accurate than e.g. the Keithley 182):When doing TVC calibration one would use a characterized TVC and compare it against the DUT. One would do that at DC, record both output voltages (average with pos and neg. dc voltage), apply the various frequencies and adjust the ac output voltage of the source to generate the default voltage of the standard TVC and then vary the amplitude until the DC default voltage of the DUT is reached. The ac voltage deviation would then indicate the inaccuracy of the DUT TVC.In doing so, one needs to know the transfer accuracy/stability of the nanovoltmeter. While this is indicated for the Keithley 182 (3+2ppm in the 30mv range), it is not for the 334420, which makes it hard to certify a certain accuracy in these measurements.Of course one could use one's best guess (in the absence of any data), but does anybody have a better answer?

author: acbern
author_rank: Frequent Contributor
timestamp: May 05, 2014, 08:46:04 pm
content: Here are some more thoughts re. the transfer stability of the Agilent 34420A:First of, again, due to this not being specified, in its absence one needs to come up with a value when doing error propagation calculations during calibration. I am mostly interested in the 10mV range, as I am using it for thermal converter calibration. I want to rather come up with a conservative value first, that should be validated by measurements (they should actually be much better). measurements allone, as some e.g. have done it with the 3458a resistance transfer accuracy is not what i would like to do. it is typical only, and not based on any manufacturer specs. but again, should be done, just to vaildate the calculation (could e.g. be done with a stbale and specified fluke 5440 or the like).The 34420A manual gives some core values that can contribute. these are (+/-: ppm of value + ppm of range, all related to 10mV range listed in brackets below):thermal drift (4 + 2)/Klinearity (0,8 + 0,5)general measurement uncertainty (2ppm of range, equals 20nV basic uncertainty)For details see 34420A data sheet/manualThis would lead to a transfer uncertainty of a conservative and rounded +/- 3ppm + 4.5 ppm (all added, rss method not applied to be conservative although some values are not correlated) over +/-half a K (half a K +/- is well doable for a 10 minute period while doing the transfer measurement in a somewhat controlled environment). this is actually not a very exciting result for a transfer accuracy if one has the 3458A in mind, especially if the voltage applied is not full scale. but at the same time, we are talking <10mV of measurements/nV resolution, that should not be forgotten. At 10mV that equals only +/- 75nV. the keithley 182 has a specified 5+9 ppm in the 3mV and 3+2 ppm in the 30mV range, so the calculation above is pretty conservative I would think. question is if there is any justification based on the 34420A data sheet values to derive less conservative values. The only that comes in mind is applying the rss method.comments welcome