What is the minimum voltage change I can measure with a 6.5-digit multimeter? - Page 1
Forum section: electronics / testgear

author: strahd_von_zarovich
author_rank: Contributor
timestamp: January 14, 2025, 09:29:33 am
content: Hi everyone,I am testing a very sensitive analog circuit, and I am unsure if I am measuring the values correctly.The resolution of the multimeter is crucial for me. I need to measure changes of 100nV.Can I do this with a 6.5-digit multimeter? My multimeter model is the Agilent 34411A. According to the datasheet, the resolution is given as 0.03 ppm in the 10V range and 100 nplc. So, I concluded that I can measure a voltage change of 30nV in 1V range. Is this correct?Here is the relevant part of the datasheet:

author: Momchilo
author_rank: Regular Contributor
timestamp: January 14, 2025, 10:32:25 am
content: Hi,not sure if you want to measure AC or DC. I guess DC. The last digit in the 1V range is 1µV. Nanovolts are not shown on the display. In the 100mV range the last digit is 100nV.For DC the attached part of the datasheet is important. Those specifications are already with 100 NLPC. Your table is for additional error margins (RMS noise) with lower NLPCs.If you want to measure 0.900003V, the multimeter is set to the 1V range.With the 1 year calibration specification the tolerance would be: 0.900003V*0.000035+1V*0.000007=+/-38.500105µV0.900003V-38.500105µV=0.899964499895V0.900003V+38.500105µV=0.900041500105VThat means everything between 0.899964V and 0.900042V would be in specification if you want to measure 0.900003V without taking the temperature coefficient into account.In conclusion you cannot measure 10nV in the 1V range. It is not even displayed. The last digit is 1µV but you have also a tolerance of +/-39µV.

author: nctnico
author_rank: Super Contributor
timestamp: January 14, 2025, 10:59:57 am
content: The first question is whether the measurement is relative or absolute (resolution versus accuracy). Over a short time period, it should be possible to measure small -relative- changes using a high NPLC setting.

author: strahd_von_zarovich
author_rank: Contributor
timestamp: January 14, 2025, 11:09:04 am
content: Thank you very much for your answer.Yes, nanovolts are not displayed on the screen, but the data collected via PC shows much higher precision. I'm not sure how accurate these readings are.My main goal is to measure changes rather than absolute accuracy.For example, if I measure 0.100,000,000 V and then increase the source voltage by 50 nV, I want to measure 0.100,000,050 V. Is this possible?As long as I can measure the change, I don't really care about the actual values.

author: Momchilo
author_rank: Regular Contributor
timestamp: January 14, 2025, 11:15:55 am
content: You cannot measure the increase of 50nV exactly. With luck, you will see that there is a rise.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 14, 2025, 11:25:46 am
content: thks i learned ...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2025, 11:47:35 am
content: The 34411 may just resolve the 50 nV change. Of possible one would repeat the 2 readings a few times. So alternate between the higher and lower value a few times. Because of drift and low frequency noise from thermal effects just a single pair of readings may not be good enough.Another point could be the limited linearity - though usually still monotonic it is no longer guaranteed.Another point can be the reference noise. The 34411 and most 6 digit meters use a LM399 reference. These usually show relative frequent jumps up and down of some 0.5 ppm. This would be 50 nV if a 100 mV voltage is read and would make it hard (need many repeats) to detect such small steps. It would be less of an issue if the voltage to start with is smaller, like 10 mV.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 14, 2025, 01:29:40 pm
content: you're going to need an 34420a 7.5 digit nano voltmeter ?? no ?or at least 7.5 digit dmm ?

author: NE666
author_rank: Frequent Contributor
timestamp: January 14, 2025, 04:52:39 pm
content: Quote from: strahd_von_zarovich on January 14, 2025, 11:09:04 amNothing more than a random number generator in all probability. You're asking for almost two orders of magnitude below the published 6.5 digits. If it could really do anything meaningful (reliable and consistently) in that range, it would be marketed as a much higher resolution device and have a commensurately higher price tag. HP/Agilent/Keysight aren't ones to leave money on the table.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2025, 05:39:40 pm
content: The 0.03ppm resolution or noise spec you are looking at is for the 10VDC range, your 100mV range will be noisier in relative terms. The last digit represents 100nV and if you get one more digit over the serial connection then that is 10nV. The result will be that your 100nV change will likely be buried in the noise. You could use mathematical filtering to dig it back out of the noise but I'm not very confident that this would be practical or reliable. What proportion of the total signal does your 100nV change represent? IOW, is this going from 0.0000005 to 0.0000006V or from 10.0000000 to 10.0000001V? Provide some more details and perhaps we can suggest a better way of measuring your signal.

author: strahd_von_zarovich
author_rank: Contributor
timestamp: January 15, 2025, 09:24:14 pm
content: Thanks for the answers.I will be measuring in the range of 100mV to 130mV.It seems I need to buy at least a 7.5-digit multimeter to verify the accuracy of my measurements.What do you think about the 7.5-digit Keithley 2001 and 7510? It seems like the Keithley 7510 is better than the 2001 but much cheaper. Why is that?Should I consider the 8.5-digit Keithley 2002 or the Keysight 3458A instead?

author: Momchilo
author_rank: Regular Contributor
timestamp: January 15, 2025, 11:41:47 pm
content: You should consider a Nanovoltmeter. They can be cheaper than a Keithley 7510 or 3458A.Maybe a Keithley 181 is already enough. The 30nV measurement won't be perfect but at least you would have the resolution.This would be by far the cheapest option.With a DMM7510 or 3458A you wouldn't get a better resolution but of course a greater accuracy.If you want to measure 100.000,30mV precise you would need something like a Keithley 2182A (1nV res.) or Keysight 34420A (100pV res.). Way better than a 3458A for those measurements.It is worth to compare the datasheets before buying such an expensive device. There are probably different models as well, I am not an expert for those low voltage measurements.

author: egonotto
author_rank: Super Contributor
timestamp: January 16, 2025, 12:14:23 am
content: Hello,no, the Keithley 181 is not suitable. It only has a resolution of 100 nV in the 200 mV range.The Keithley 148 is also not suitable because the least sensitive measuring range is 100 mV.Search for Keithley Low Level Measurements Handbook. You can download it from Tektronix.At 100 nV the thermoelectricity cannot be neglected. There is something in 3.2.2 in the Keithley Low Level Measurements Handbook.The source impedance is also important.Best regardsegonotto

author: Momchilo
author_rank: Regular Contributor
timestamp: January 16, 2025, 12:31:27 am
content: The Keithley 181 has a 10nV resolution in the 200mV range.

author: egonotto
author_rank: Super Contributor
timestamp: January 16, 2025, 12:43:17 am
content: Quote from: Momchilo on January 16, 2025, 12:31:27 amHello,the Keithley 181 has only 6 1/2 digits and a resolution of only 100 nV in the 200 mV range.Best regardsegonotto

author: bdunham7
author_rank: Super Contributor
timestamp: January 16, 2025, 02:31:49 am
content: Quote from: strahd_von_zarovich on January 15, 2025, 09:24:14 pmWhat is the timescale of the measurement? IOW, how quickly (or slowly) do you expect to see this 100nV change? Is it like flipping a switch and then expecting an immediate shift of 100nV or is it adding a drop of electrolyte to a beaker and coming back the next day expecting to see a 100nV change? And how precisely do you need to measure your 100nV change--is 30nV resolution good enough? What would be the thresholds for a postive or negative result?

author: Someone
author_rank: Super Contributor
timestamp: January 16, 2025, 06:46:33 am
content: Quote from: bdunham7 on January 16, 2025, 02:31:49 amOne of the most important questions!As an example below, 34461A 100mV 100NPLC being fed with a modulated 1uV source of 1ΩCould it resolve 100nV ? possibly, but extremely slowly.

author: Momchilo
author_rank: Regular Contributor
timestamp: January 16, 2025, 07:45:21 am
content: Quote from: egonotto on January 16, 2025, 12:43:17 amHi,I disagree. Have a look at the attached data.And here you can see in the video that the 2mV range has a resolution of 10nV and in high res mode, you get a resolution of 1nV.https://www.youtube.com/results?search_query=keithley%20181In my opinion the Keithley 181 is still the best option for OP if money does matter.

author: egonotto
author_rank: Super Contributor
timestamp: January 16, 2025, 05:02:51 pm
content: Hello,What good is the resolution in the 2 mV range if 100 mV to 130 mV are to be measured?Best regardsegonotto

author: Momchilo
author_rank: Regular Contributor
timestamp: January 16, 2025, 05:05:39 pm
content: Not sure what I thoughtOf course you are right, sorry.

author: egonotto
author_rank: Super Contributor
timestamp: January 16, 2025, 06:26:35 pm
content: Hello,when measuring such small voltage differences, it is easy to make mistakes. You should really read the Low Level Measurements Handbook now from Tektronix.Best regardsegonotto

author: strahd_von_zarovich
author_rank: Contributor
timestamp: January 16, 2025, 07:45:32 pm
content: Thank you again for your answers.I collect data using 100 NPLC, and I gather 10 readings to calculate the average. This means each measurement takes about a minute. After that, I change the system parameters, which causes my sensor's output voltage to change by 100 nV.I am now starting to read the Low Level Measurements Handbook from Tektronix.I still haven't figured out why the Keithley 2001 costs twice as much as the DMM7510, even though nearly all the specifications of the 7510 are better. Any ideas?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 08:30:24 pm
content: The K2001 is an old instrument. As a speciallity is offers a high impedance 20 V range, that not many other higher grade DMMs offer.On the downside the K2001 is rather noisy - more on the level of 6 digit meters. The long term reference is also still a LM399, like most 6 digit meters. I would mainly consider the K2001 as a direct replacement in an old system that needs this meter or if one really needs the 20 V range and can not get away with a Sigilent SDM3065 or want to pay for a Fluke 8858.There seem to be still a small market (possibly military ?), that is willing to pay the high price and keep the product alive for those who really need it.Especially the Keithley meters tend to have quite some low frequncy noise. Averaging 10 readings at 100 PLC may not give a much better result than just a single reading. So ideally one would not use so much time in the same setting and better switch between the 2 settings in question and average over multiple cycles. So 10 repeats of 100 PLC with one system setting and 100 PLC of the other system setting, even if this mean loosing some additional time and do less repats (like 8 or 6) in the same overall time. With more averages this may one could also get away with a lower grade meter.

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2025, 12:15:32 am
content: Quote from: strahd_von_zarovich on January 16, 2025, 07:45:32 pmThat sounds like about the best case, your timescale is neither very short nor very long. If is were <1s or >1h you'd have a lot more issues to worry about.The K2001 is more expensive because it often costs more to keep manufacturing a legacy product than it does to make a better one with modern design. I'm not confident you can make this measurement directly with a DMM and you might consider differential methods or some other creative solution. Are you just looking for a go/no-go (yes it changed, no it didn't) or do you need more detail?

author: lowimpedance
author_rank: Super Contributor
timestamp: January 17, 2025, 02:34:17 am
content: Perhaps signal amplification first using one of the off the shelf products from EM electronics.EM Electronics specialises in the design and manufacture of precision, ultra low level DC voltage Amplifiers and ultra low level DC measuring instruments.https://www.emelectronics.co.uk/