Is a Solartron 7081 worth the money? - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 04, 2017, 07:05:32 pm
content: Hi,I have the chance to get a 7081 well below 1000€. But I'm not sure if it is worth the money.Everything I saw from these meters was very noisy and slow. It will take 52s for 8.5 digits. And the other modes are noisier than my 34401A and 3456A. From the datasheet the meter is much better than the 34401A, but how is it in real life? The 34401A will also give 8.5 digits over GPIB and it can take more than 12 measurements until the 7081 will take one. I haven't tried it, but it could be less noisy than the 7081 (average 12 of NPLC100).The main point for me is linearity. My 34401A seems to be able to do a 1:10 Ratio within ~0.1ppm of FS (measured against two 3458A at work). What about real life linearity of the 7081?I found this:https://www.eevblog.com/forum/testgear/8-5-digit-dmm/msg270530/#msg270530It seems to be anyone has measured one (?) unit to 0.1ppm INL. Does anyone have more experience on this topic?The reference isn't a big argument, I think. In both cases a LTZ1000-mod would be possible.So is it worth to spend some money to a Solartron 7081?ThanksPhilipp

author: guenthert
author_rank: Frequent Contributor
timestamp: April 04, 2017, 09:04:44 pm
content: Quote from: e61_phil on April 04, 2017, 07:05:32 pmCan you elaborate on that? I found earlier only that an extra digit (for a total of 7.5digits) can be read from the HP34401a (that unit was meant, right?) GPIB (or 7 digits using its math operations). Either way, that doesn't mean that the maximum discrimination is 1 part in 20 (or even 200) million.

author: ap
author_rank: Frequent Contributor
timestamp: April 04, 2017, 09:43:35 pm
content: IMO, the 7081 is a great meter. It is slow, true, but it has its values. Have a look at the data sheet. And if its properly working, it easily exceeds its spec. And if so, no need to change the reference. Why?E.g. the 7081 has the ohms transfer accuracy specified, not so e.g. the 3458A.If you dont want it, send me a PM.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 04, 2017, 09:53:27 pm
content: Quote from: guenthert on April 04, 2017, 09:04:44 pmI never verified if all of the digits are fully used, but I did it nowI found a drift measurement with the HP 34401A and the Fluke 5440B:14994 Samplesmin. value: 9,9999505max. value: 9,9999637So there should be 132 counts between the values. Therefore, I made a histogram of the data with 132 bins. Only a few counts are missing. It would be interesting to know if these values will never appear..

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 04, 2017, 10:12:52 pm
content: Quote from: ap on April 04, 2017, 09:43:35 pmI hoped one can reduce the noise with a LTZ1000.For example I would like to calibrate my F5450A with the Solartron 7081 in volt mode (a calibrator is used as a current/voltage source). Therefore, most important is the INL I think. The specification of the 7081 is 0,3ppm of Range in 10V mode for transfer. Hopefully it is better. But it seems to me, that the noise is higher than 0,3ppm. Therefore I have to measure many times to reach a good transfer. Isn't it true?

author: mimmus78
author_rank: Unknown
timestamp: April 05, 2017, 12:19:32 am
content: You guys seem you always forget that a multimeter is a "multi" meter non just a 10V ADC.You may be able to stick 4 LTZ reference inside a 34401A and get it very good at 10V, but what is the use of it if you than get the other ranges "going all over the places" exactly as it was before with the LM399?

author: TiN
author_rank: Super Contributor
timestamp: April 05, 2017, 01:04:21 am
content: Quote from: e61_phil on April 04, 2017, 10:12:52 pmThe most important would be to have ability to measure current output from MFC accurately first, which in term require known resistance standard. Otherwise how you know what current flowing thru the DUT 5450A?I'd say 7081 leaves nothing left from 34401A in case of stability/noise/resolution, but that's based on internet data. I never touched one before.Mickle.Thave done amazing job reviving/modding his 7081's while ago.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 05, 2017, 07:56:57 am
content: Quote from: mimmus78 on April 05, 2017, 12:19:32 amYou're absolutely right, but I only need a good 10VDC voltmeter for calibrations.Quote from: TiN on April 05, 2017, 01:04:21 amIt doesn't matter which currents flows. The 5450A has an "AutoCAl" feature. Therefore, you only need a short time stable current/voltage source and a short time stable meter (even calibration of the meter doesn't matter) but you will need a meter with very good linearity.Starting point of calibration is the transfer from my VHP101 10k "standards" to the F5450-10k. All measurements are only ratios.Quote from: TiN on April 05, 2017, 01:04:21 amI will give it a try. But as already said: stability isn't so important for me, resolution is the same as the 34401A and noise seems to be worse (I will check that). What really matters is the INL and that is better specified, but the specification is worse than the measured INL of the 34401A.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 05, 2017, 09:16:36 pm
content: I did a short noise test with the Solatron 7081 and my Agilent 34401A. Both meters are in parallel on 10V out of the Fluke 5440B.Solartron 7081: 7.5 digit mode ~3,2s sample timeAgilent 34401A: 100NPLC ~4s sample time (with AZ)It isn't much, but it seems that the 7081 is noisier than the 34401A

author: ap
author_rank: Frequent Contributor
timestamp: April 06, 2017, 07:34:33 am
content: It would be interesting to see the noise at 8 digits in your setup (34401 NPLC adjusted). This is where the value of the instrument is, even though its slow.

author: Henrik_V
author_rank: Regular Contributor
timestamp: April 06, 2017, 08:47:24 am
content: e61_phil, thank you for the test and sharing results.How did you wire the guard and ground?

author: tggzzz
author_rank: Super Contributor
timestamp: April 06, 2017, 10:16:52 am
content: My recently acquired Solartron 7081 and 10V "Trancell-II" Zener diode source, which I'm slowly resurrecting as I slowly descent into voltnuttery.The cable is homemade, the 7081 has "baked itself" as described in the manual, and been left on for several weeks. One internal connector has been cleaned and firmly reseated to avoid measurement lockups.Guard connected to the Trancell's "GND", i.e. case. Trancell powered from its internal battery.

author: ap
author_rank: Frequent Contributor
timestamp: April 06, 2017, 02:54:20 pm
content: So looks like your combined noise, including Trancell, is 0.5ppm pp. I have not found anything about the Trancell reference, but would think that it contributes at least 50% of that. Any info?

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 06, 2017, 04:30:36 pm
content: Quote from: Henrik_V on April 06, 2017, 08:47:24 amThe wiring isn't ideal I think. The guard is connected to LO at the front of the Solartron (within the LEMO). The shield isn't connected to anything, but it is connected to PE anyway.Quote from: ap on April 06, 2017, 07:34:33 amI started this measurement now. But 100 samples will take some time

author: tggzzz
author_rank: Super Contributor
timestamp: April 06, 2017, 05:03:43 pm
content: Quote from: ap on April 06, 2017, 02:54:20 pmI'm still "bootstrapping" myself in voltnuttery. Previously I was in the "not sure which is less wrong" territory, but I'm slowly moving to the "two clocks problem" territory.The best info I have ishttps://www.eevblog.com/forum/metrology/standard-reference-labs-trancells-any-information-available/but that is far from comprehensive.Ignoring drift of a few ppm, the standard deviation for 8 nines measurements (calculated in OpenOffice Calc, not the 7081) is of the order of:10V Trancell, 10V range: 1.5uV10V Trancell, 100V range: 10.5uV1.0186V saturated Weston standard cell, guard not connected:0.27uV0V, guard connected to positive and negative: 0.07uVMake of those what you will! They are certainly much better than when I received the devices and started measuring.Edit: added the 100V range reading, which tends to suggest that 1uV of the 1.5uV is due to the meter.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 06, 2017, 06:07:18 pm
content: new data..Solartron: 8,5 digit mode ~52s sample time34401A: 12 times 100NPLC averaged ~48s sample timeThe difference in the noise isn't that big. What is more confusing is this > 1ppm jump from the solartron. I think it is due its auto zero. But it shouldn't generate such a big jump.

author: mimmus78
author_rank: Unknown
timestamp: April 06, 2017, 06:30:33 pm
content: This 1ppm jump is not new to me ... I read about it some year ago on this forum somewhere. If I remember someone speculated it can be a firmware bug.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 06, 2017, 07:06:27 pm
content: Quote from: mimmus78 on April 06, 2017, 06:30:33 pmInteresting! I can try the software (whole board with EPROMS) from my broken 7071.Does anyone know how many NPLC the 34401A can do? Somewhere in the manuals of the HP 3456A I read the 3456A is capable doing 10NPLC at maximum. If you set the instrument to 100NPLC it will take 10 samples with 10NPLC and average them. Does anyone know if the 34401A will do the same?edit: First results with 1000 samples 10x10NPLC

author: Kleinstein
author_rank: Super Contributor
timestamp: April 06, 2017, 07:49:40 pm
content: Due to internal 1/f noise, there is a limit to extending the time for a single ADC conversion. It is well possible that a 100 PLC time is already past the optimum for the 34401. So it does not really matter how long the meter internal integration time can be, one can always use later averaging - this is what many modern DMMs do internally when using longer NPLC setting. I am not sure the 34401 actually does 100 PLC as a single integration or as averaging. Comparing the noise (with shorted input to exclude reference noise) could give a hint.The 7081 (like the Datron 1281) is different in this respect, as it uses an chopper stabilized input amplifier and AZ OP in the ADCs integrator. So here the longer integration time is still a real improvement as there is very little 1/f noise. This is also why it works quite well without autozero mode and does AZ only at long intervals.There is a good chance that reference noise of the 7081 shows significant scattering. So not all the 7081 are equally good or bad. To some extend this is also true for other meters.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 06, 2017, 07:59:38 pm
content: Thanks, Kleinstein!At the moment I'm running some measurements on the 34401A with 10x10NPLC. By now, I think the ADC resolution of the 34401A is only 7.5 digits. The rest ist averaging. This could also explain why the 34401 will cut one digit above 10V, but you can get it back if you set an offset value (thanks Martin!). And that can also explain the missing data points in my histogram abvoe. If the 34401A is only capable of doing 10NPLC it is properly better to set up the instrument for measuring 10 times 10NPLC and do the averaging on the computer.Next part in testing the Solartron 7081 should be a linearity test. But unfortunately this is much harder than the other tests

author: Andreas
author_rank: Super Contributor
timestamp: April 06, 2017, 09:51:41 pm
content: Hello,one question is also what floating point or math resolution is used within the instrument.A single float (4 byte in "C") has only 24 bit mantissa resolution. (~7 digits)So subtraction of a small number (zero offset) near full range can be a problem.Double float has 6 Bytes mantissa. But I do not know if it was used when having no Co-Processor.But at that time also programming languages (non "C") with 5 byte floats where popular.Early Turbo-Pascal versions in CP/M had 6 byte floats.Double Floats where used later with the 8087 Co-processor.So what resolution is really usable (without math artefacts) within the instruments?With best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 06, 2017, 10:02:28 pm
content: Quote from: Andreas on April 06, 2017, 09:51:41 pmIt is also possible that the 34401A is using fixed point math. It will drop to 7.5 digit resolution if you apply voltages above 10V.As shown inhttps://www.eevblog.com/forum/metrology/is-a-solartron-7081-worth-the-money/msg1178755/#msg11787558.5 digits seems to lead to some small artefacts. I would say that doesn't matter, if it is only 10 times averaged 10NPLC. You will need a computer with an interface to the 34401 to get more than 6.5 digits, anyway. Therefore, one can easily run the 34401A with 10 samples of 10NPLC and do all the math on the computer.Andreas, I read somewhere you are using a resistor chain for measuring the linearity of your LTC2400 setup. Is it possible to measure linearity down to 0.1ppm FS with such a setup?

author: guenthert
author_rank: Frequent Contributor
timestamp: April 06, 2017, 11:16:31 pm
content: It might be worth clarifying how exactly you're reading the data from your HP 34401A. Those extraneous digits seem to be an artifact of the programming interface. From the remote interface reference (part of the User's Guide, p.114):--8<--Use the resolution parameter to specify the desired resolution forthe measurement. Specify the resolution in the same units as themeasurement function, not in number of digits. For example, fordc volts, specify the resolution in volts. For frequency, specify theresolution in hertz.-->8--Which I read, as that you can specify any resolution you like. The instrument will then scale the measured value to the desired resolution. Try to specify nV for the 10V range, perhaps you get even 9.5 digits!

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 07, 2017, 06:04:12 am
content: Quote from: guenthert on April 06, 2017, 11:16:31 pmYou can't increase the resolution above 7.5 (8.5) Digits with this function. This is only usefull if you want reduce the resolution. Perhaps, measure 10V with only 1mV resolution. This will increase the measurement speed.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2017, 06:36:59 am
content: Especially with the Solartron there is also a Quantization limit of the ADC. They use just a kind of PWM ratio with a relative low frequency. The ADCs resolution is relatively limited and thus scaling for the gain and than conversion to a decimal number can introduce some kind of rounding errors in the decimal format. AFIK there is a way to get the result in a floating point format with plenty of digits, however only a limited number of values that actually occur. This might be an issue in the 8.5 digit mode as the integration time is not that much longer there and the internal resolution of the ADC should only increase linear in time (if done the simple way).A similar effect might happen with the 34401 too. There is also a limited internal (binary) resolution - though things could be a little more tricky with counting time slots for integration and the additional use of an extra ADC for residual charge. So there will be an extra scaling factor involved and thus an slightly more complicated situation of possible raw results from the ADC. Using averaging of several readings (internal of at the PC) could be a good idea here.The math part can be important if one wants to check DNL from histogram data.A possible partial test of the INL is by measuring two voltages and the sum of the two voltages. In the simplest form this is reversing the voltage source, knowing that the sum in 0, but it can be done with other test points too. In principle this should work down to the limits set by thermal EMF (and similar) when switching between the voltage. Noise may require quite a few repeats. Usually this needs a well isolated (e.g. battery powered) low noise source for two test voltages in series. A single source and a resistive divider is the most convenient setup.

author: tggzzz
author_rank: Super Contributor
timestamp: April 07, 2017, 08:17:00 am
content: Quote from: Kleinstein on April 07, 2017, 06:36:59 amI DUmp the values using the RS232 interface, and they have 9 digits in the mantissa.I've repeated myearliermeasurementswith the 7081 on the 100V range rather than the 10V range. On the front panel display there is one digit less, as you would expect. Dumping the values gives, for example, 9.99972406E+00, 9.99972592E+00, 9.99972406E+00, 9.99972902E+00, 9.99972034E+00. On the graph below it can be seen that the resolution is effectively 1uV, but the datapoints are rarely exactly quantised; again that is what you would expect with "spurious resolution" in the floating point numbers.The standard deviation is around 10.5uV whereas on the 10V range it was around 1.5uV. Thistendsto suggest that 1uV of the 1.5uV is due to the 7081, with the remainder down to the Zener source - but I haven't thought about the maths and physical processes in details.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2017, 08:59:23 am
content: There are 3 main noise sources: the Input amplifier, the ADC itself and the zener Reference.To check for the noise contribution of the reference, one compares measuring with a shorted input and with an external low noise reference. The difference is due to the references noise.Changing to the 100 V range inserts an 1:100 divider and put the input amplifier to x 10 mode. The reference contribution stays the same (as it is proportional to the actual input signal), but the noise of the input amplifier is scaled with a factor of 100 and there is additional noise from the 100 K output impedance of the divider. Noise of the ADC itself is scaled with a factor of 10.So there is not that much to learn from just comparing the 10 V and 100 V range. Usually the noise in the 100 V range has contributions from the ADC, the amplifier and the divider, that can all be important. So this is the most difficult range.So all we can say from comparing the 100 V and 10 V readings is that the 1.5 V noise in the 10 V range is at most 1 µV from the amplifier and ADC. So the reference noise must be between about 1.1 µV and 1.5 µV (RMS).To get the noise contributions, usually the 100 mV range with shored input will give mainly noise from the input amplifier. The 10 V range shorted gives mainly the ADC - a small amplifier contribution could be corrected. The 10 V Range with an external voltage gives additional noise from the two references.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 07, 2017, 05:48:00 pm
content: Quote from: Kleinstein on April 07, 2017, 06:36:59 amIf I understand it correct one could build a chain of similiar resistors. 10 times 1k for example. I would use resistors with small tolerances and (more important) low TC (some S102 perhaps). In the next step I would measure the voltage on every resistor in the 1V range. The linearity of the meter shouldn't matter because of the very close values. Even with a 6.5 digit voltmeter I'm able to measure the 1V steps to 0.1ppm of FS (10V).In the next step I would use the DUT in 10V range an measure across some resistor combination (easiest way 0 to R1, 0 to R2 and so on). In the end I throw all the data in a table. One column contains the sum of the resistor voltages (R1, R1+R2 and so on) and the next column contains the DUT measurements. Afterwards, I apply a linear regression to the data to get rid of offset and gain errors. The deviation from this linear regression should show the INL. Is this correct?Is a battery powered reference for the resistor chain really that important? Or is it possible to use the floating output of the calibrator (Fluke 5440B)?

author: Andreas
author_rank: Super Contributor
timestamp: April 07, 2017, 05:51:30 pm
content: Quote from: e61_phil on April 06, 2017, 10:02:28 pmHello,that depends mostly on noise and short term stability of the setup.For the LTC2400 I get down to around 1 ppm FS.With best regardsAndreas

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 07, 2017, 07:01:38 pm
content: Quote from: Andreas on April 07, 2017, 05:51:30 pmHi,I would like to measure 1:10 ratios within 1ppm. Therefore, I think 0,1ppm INL of FS is needed.I have an old ESI SR1010-10 transfer standard. Unfortunately I have not the shorting bars. Therefore, it is a bit useless. My idea is to salvage the very nice 10R resistors for other things and use the housing to install some other resistors. I have some russian hermetic sealed 1k14 specified with 5ppm/K but they seem to be better. I think 1k is a good value. High enough to have an acceptable current at 10V and low enough to avoid loading errors. Or is that a bad idea?Is there anything I should have in mind before soldering in a SR1010? Any toxic solder or something like that?BestPhilipp

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2017, 07:21:25 pm
content: Quote from: e61_phil on April 07, 2017, 05:48:00 pmThe way described here is different from what I thought, but it might be a very viable way offering quite a few test points in a single run. If the measurement is reasonable fast and repeated, there is no need for very good stability of the resistors and stability of the references is less critical too.Manually changing cables could be rather slow or introduce extra thermal EMF. Something like a board with lots of places for two jumpers might be an intermediate level solution - still manual switch over, but simple and low thermal EMF and reasonable fast to operate. With a set of 8 resistors something like 30 measurements for one run. This won't be practical in the 8 Digit mode of the Solartron, but could be in the 7 digit mode with external averaging (to reduce the time lost during switching).If the floating output of the calibrator is good enough, depends on the degree of isolation the source and the meter offer. It might be well good enough, especially if the resistor values are not that high (e.g. 1 K each).

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 07, 2017, 08:24:02 pm
content: I can imagine, one could use a HP 3488A togehter with two multiplexer modules (to control HI and LO separately). With such a setup it is possible to switch HI and LO independent from each other to every node of the resistor chain. If the calibrator is also controlled via GPIB it should be possible to cancel out most of the thermal EMF errors. But first I will try it manually.

author: guenthert
author_rank: Frequent Contributor
timestamp: April 07, 2017, 09:36:22 pm
content: I still don't quite grok how you get those 8.5 digits (or where they are coming from). Reading my HP 34401A while measuring an AD588 reference using default configuration (10NPLC) the last two digits are always 0 (hence 7.5 digits, not necessarily 20 million discrete values):+1.00014400E+01,+1.00014500E+01,+1.00014400E+01,+1.00014500E+01,+1.00014500E+01,+1.00014500E+01,+1.00014400E+01,+1.00014400E+01,+1.00014500E+01,+1.00014400E+01,+1.00014400E+01,+1.00014500E+01,+1.00014500E+01,+1.00014500E+01,+1.00014400E+01,+1.00014400E+01,+1.00014400E+01,+1.00014500E+01,+1.00014400E+01,+1.00014500E+01,+1.00014400E+01,+1.00014500E+01,+1.00014400E+01,+1.00014400E+01,+1.00014400E+01For comparison, I measured the same reference at the same time (*) using a Datron-Wavetek 1271:HP34401A 10 NPLC mean of 25 rdgs: 10.0014444 variance: 2.46399999981e-11DW1271 fast mode mean of 25 rdgs: 10.001369164 variance: 2.50304000168e-13Using NPLC of 100, still the last two digits are 0 [edit: this might have been due to an error in programming the DUT]:+1.00013300E+01,+1.00013200E+01,+1.00013300E+01,+1.00013200E+01,+1.00013400E+01,+1.00013400E+01,+1.00013400E+01,+1.00013300E+01,+1.00013300E+01,+1.00013400E+01,+1.00013300E+01,+1.00013200E+01,+1.00013400E+01,+1.00013300E+01,+1.00013200E+01,+1.00013500E+01,+1.00013300E+01,+1.00013400E+01,+1.00013300E+01,+1.00013300E+01,+1.00013300E+01,+1.00013300E+01,+1.00013300E+01,+1.00013400E+01,+1.00013300E+01HP34401A 100NPLC mean of 25 rdgs: 10.001332 variance: 5.60000000063e-11DW1271 normal mode mean of 25 rdgs: 10.001369256 variance: 1.54464000023e-13*) Even in 'fast' mode, the 1271 is much slower than the 34401A. The test series only started at about the same time (block read to memory on both).

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 07, 2017, 09:54:07 pm
content: Quote from: guenthert on April 07, 2017, 09:36:22 pmYou will only get 8.5Digits with 100NPLC and if the voltage is below 10V. If your voltage is above 10V you can add an offset:HP34401A.write("CALCulate:NULL:OFFSet 1")I think (not know) the 34401A ADC is only capable of 10NPLC with 7.5 digits. If you switch the unit to 100NPLC it will take 10 times a measurement with 10NPLC (like the 3456A) and average them. This leads to an extra digit. I think this digit is cut above 10V due to the internal math capabilities. Further, I think these internal math capabilities are the reason for some missing codes at 8.5digit.I made a histogram with 10NPLC with 100nV spacing like in the histogram before (see attachment). I can't see any missing code at 7.5digits. I think you will get real 7.5 digits from the 34401A and it is possible to average 10 of these measurements to increase the resolution. I did another measurement with 10x 10NPLC averaged (averaging is done on my pc) and there is no missing code in 8.5 digits (that's clear)The difference in the standard deviation between the 34401A and the 1271 is perhaps due to the better reference (and ADC)?

author: splin
author_rank: Frequent Contributor
timestamp: April 08, 2017, 12:36:35 am
content: Quote from: e61_phil on April 07, 2017, 05:48:00 pmThis is what HP had to say about linearity testing of the 3458A, from page 14 of the April 1989 HP Journal:QuoteI'm not sure that's very helpful though unless you know a lot about the relative levels of odd and even order non-linearities in the meter.Since you want to do 10:1 transfers, then one idea I had is to connect 20 reasonably well matched resistors in a ring with pins at every connection. Connect one point to +Vref and the opposite connection to ground such that you have two parallel strings of 10 resistors. Measure the voltage at the top of the two grounded resistors.Now move the Vref and ground connections one step around the ring and repeat until you have 40 measurements of 1/10 of Vref; averaging them should give a pretty good result - both by averaging out the resistor value mismatches and by reversing many of the thermal EMFs arising at the connections which may all have small temperature variations. Each resistor is connected +/- when 'travelling up' the divider chain, then reverses when coming down the other side.As to how close the resistors need to match, and how well all the EMFs can be cancelled to achieve .1ppm is left as an excercise for the readerMany other arrangements are possible including using 40 resistors, or re-ordering/reversing sections of the ring and repeating. The ring should allow a complete set of tests, rotating every resistor into the low and high sides, to be done quickly enough to minimise thermal EMF problems due to temperature changes.To that end, the 20 resistor connections could all be brought to an in-line 2x22 way header. A 2x3 pin socket would connect Vref, ground and the two adjacent resistors to the meter (via a switch) The socket would be advanced one pin at a time along the header to make all 40 readings quickly and easily. Rotary or electronic switches could also be used. Thermal EMFs arising at each pin header/switch contact might not all cancel though, so it would be good to ensure that are all kept at the same temperature as much as possible.You can of course take readings at other points to measure linearity at any or all 1/10 intervals.Having establised the linearity at exactly 10% full scale, you can then use a low level triangle/sawtooth/sine wave with a 10% FS offset to measure the differential linearity of the steps near to the 10% point and thus calculate the INL over that range. The linearity of the waveform has to be better than your required accuracy, hence the necessity for a small amplitude signal. It also has to be small to allow the total measurent to be done in a reasonable time.I hope that made sense. Do let us know how you get on.

author: guenthert
author_rank: Frequent Contributor
timestamp: April 08, 2017, 02:26:22 am
content: Quote from: e61_phil on April 07, 2017, 09:54:07 pmI tried that (together with "CALC:FUNC NULL; STATE ON") which set the offset, but doesn't give me any more digits:--8<--+9.00132200E+00,+9.00133400E+00,+9.00134300E+00,+9.00132600E+00,+9.00134500E+00,+9.00133400E+00,+9.00132600E+00,+9.00133400E+00,+9.00133600E+00,+9.00133600E+00,+9.00135100E+00,+9.00133100E+00,+9.00132500E+00,+9.00133400E+00,+9.00134500E+00,+9.00133100E+00,+9.00133500E+00,+9.00135300E+00,+9.00133800E+00,+9.00133900E+00,+9.00133000E+00,+9.00133400E+00,+9.00134000E+00,+9.00133000E+00,+9.00132600E+00-->8--Perhaps there is a difference in firmware? Mine reports itself as--8<--HEWLETT-PACKARD,34401A,0,5-1-11991.0-->8--(*IDN? followed by SYSTEM:VERSION)

author: guenthert
author_rank: Frequent Contributor
timestamp: April 08, 2017, 02:50:39 am
content: Quote from: e61_phil on April 07, 2017, 09:54:07 pmHeaven knows. I don't think they are easily comparable (one being a fine bench meter, the other meant for ATE with left-over parts of the metrology-grade 1281).The 1271 is actually not sooo much slower in normal mode using 8.5 digits (~10s for one rdg) than the 34401A in 100 NPLC, nor in fast-mode using 7.5 digits, than the 34401A using 10 NPLC. It (at least mine) is still more precise then:--8<--HP34401A 10NPLC avg. of 25 rdgs: 9.0013308 8.22720000002e-10DW1271 'fast' mode, 7.5 digits avg. of 25 rdgs: 10.00137676 var: 2.34240000072e-12-->8--Given that both instruments are more than 25 years old, here's a big thank you to the engineers and other folks who made them!

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 08, 2017, 09:51:46 am
content: Quote from: guenthert on April 08, 2017, 02:26:22 amI have two 34401A. One with Agilent Brand (but still the old buttons) and the other one is HP branded.I run this script to compare with your readings:Code:print("======== Agilent 34401A ========")A34401A.write("*RST")A34401A.write("SENSe:VOLTage:DC:RANGe 10")A34401A.write("SENSe:VOLTage:DC:NPLC 100")A34401A.write("INPut:IMPedance:AUTO ON")print( A34401A.query("*IDN?") )print( A34401A.query("SYSTEM:VERSION?") )print( A34401A.query("READ?") )print( A34401A.query("READ?") )print( A34401A.query("READ?") )print("======== HP 34401A ========")HP34401A.write("*RST")HP34401A.write("SENSe:VOLTage:DC:RANGe 10")HP34401A.write("SENSe:VOLTage:DC:NPLC 100")HP34401A.write("INPut:IMPedance:AUTO ON")print( HP34401A.query("*IDN?") )print( HP34401A.query("SYSTEM:VERSION?") )print( HP34401A.query("READ?") )print( HP34401A.query("READ?") )print( HP34401A.query("READ?") )The result is:Code:======== Agilent 34401A ========HEWLETT-PACKARD,34401A,0,10-5-21991.0+8.99989590E+00+8.99989670E+00+8.99989640E+00======== HP 34401A ========HEWLETT-PACKARD,34401A,0,10-5-21991.0+9.00005570E+00+9.00005600E+00+9.00005650E+00Both units show 8.5 digits (but it seems to be the same firmware)

author: guenthert
author_rank: Frequent Contributor
timestamp: April 08, 2017, 07:05:50 pm
content: Hey, thanks for your patience there with me. I tried that (plus the 1V offset) and it still yields only 7.5digits onmyHP 34401A. As if there were different firmware versions claiming to be 1991.0+9.00141400E+00+9.00141300E+00+9.00141500E+00+9.00141500E+00+9.00141400E+00+9.00141400E+00+9.00141500E+00+9.00141600E+00Ah, hmmh, GPIB programming ... I just noticed that the order of the statements matters, e.g. "CONF:VOLT:DC" will reset any math operation and "SENSE:VOLT:RESOLUTION" will implicitly set NPLC.I can now also confirm that the HP 34401A doesn't use fixed-point or BCD arithmetic (as it arguably should), but some sort of float. Specifying a low resolution as a test, I found:--8<--+9.00089900E+00+9.00190000E+00+9.00140000E+00+9.00089900E+00+9.00140000E+00+9.00140000E+00+9.00140000E+00+9.00250000E+00-->8--And 10uV in the 10V range seems to be the best it can do, "SENSE:VOLT:DC:RESOLUTION 1e-6" yields '532,"Cannot achieve requested resolution"'Earlier there was an issue with setting integration time. Now I seem to have got to work and I get:+1.00014240E+01,+1.00014230E+01,+1.00014220E+01,+1.00014220E+01,+1.00014240E+01,+1.00014230E+01,+1.00014230E+01,+1.00014220E+01,+1.00014230E+01,+1.00014220E+01,+1.00014230E+01,+1.00014230E+01,+1.00014220E+01,+1.00014230E+01,+1.00014220E+01,+1.00014230E+01,+1.00014230E+01,+1.00014230E+01,+1.00014230E+01,+1.00014230E+01,+1.00014230E+01,+1.00014240E+01,+1.00014230E+01,+1.00014240E+01,+1.00014240E+01HP34401A 100NPLC, avg. of 25: 10.00142296 var: 4.38400000162e-13DW1271 'normal' mode, 8.5digits, avg. of 25: 10.001364592 var: 2.51136000008e-13So, some progress

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 08, 2017, 08:33:53 pm
content: @guenthert: Thanks for sharing the data. It is very intersting to see almost the same variance on the 34401A and the 1271.In the meantime I tried some linearity measurements. Unfortunately, my only resistor chain is a SR1010-10 with 10Ohm/step. Not quite the best values for such a test, but I gave it a try.I used my Knick JS 3010 calibrator to source 100mA through 10 of the SR1010 resistors (not very nice due to selfheating and so on). After half an hour of settling time I measured the voltages on the 10 resistors 3 times with my Agilent 34401A. The second and third run were very well within 1ppm (avg below 0,5ppm).After that I measured the sum of the resistor voltages in the 10V Range of the Agilent 34401A and also with my HP34401A.The picture 34401A_linearity.png shows a measurement against the 3458A at work.The picture 34401A_lin2.png shows the result of my little experiment.I don't care about the sign, because I'm not sure in which way I calculated the deviation with the 3458A. It looks quite good I think.I also made a spot check and compared (offset compensated) the ratio between 1V and 10V of the SR1010 chain with the measured 1V and 10V ratios:Agilent 34401A 1,14ppm deviation from the SR1010HP 34401A 3,17ppm deviation from the SR1010So the Agilent is quite close to the 0,1ppm FS. In the next days I will test the Solartron 7081 (wasn't warmed up today and I'm running out of time).