Minimising errors in my setup  - Page 1
Forum section: electronics / general

author: AndyTT
author_rank: Newbie
timestamp: April 18, 2020, 10:56:41 am
content: Hi All,I have recently joined the forum in hope of learning how to improve the accuracy of my measurements by means of minimising sources of error in my setup. In my job I use instruments daily, however until recently have never needed to measure uV to any kind of set accuracy. My aim is to minimise the sources of error as far as practically possible with some help from the experts in this forum. I have another thread running on the meter I have chosen (3458A) to measure uV in my application.I have read lots of posts so far but they have made me to ask more questions. If anyone can help answer them it would be of great help.I am trying to characterise a precision zener diode (6V). I need to measure this diode down to a resolution of 1uV.E.g 6.000001V. The meter (3458A) assuming its calibrated and working to spec should be able to do this no problem.The questions I have:1) At 6V is this still classed as a low voltage measurement where I would see benefit from using low thermal emf cables?2) I have read through the forum and found lots of posts on low emf cables and connectors. These cables seem to be made from Teflon and shielded. However I read a white paper from fluke saying cables show better immunity to noise and generating emf by twisting the cable. If you have to choose, twisting cable is better than shielding cable. I have yet to find any low emf cable that comes as twisted pair. Am I missing something here?3) my test setup consists of a rack of instruments where the rear terminals are used to connect to the device under test. The cables run from the rear of the meter down the side of the cabinet and into a panel mounted banna jack. From the front I connect from banana to dut. This keeps things tidy from cables but I am now concerned this could be bad for generating errors. I have a rack mount fan inside the cabinet to keep the temperatures regulated, the air flow blows from bottom to top to push hot air out the top and suck in the cold air from bottom. My cables and rear terminals are in the path of the air flow. I am not running a calibration lab here, it is a working environment. What are your thoughts on this setup? I will take pictures of the setup as soon as I am able.Thank youAndy

author: ramon
author_rank: Regular Contributor
timestamp: April 18, 2020, 03:49:31 pm
content: Is that possible? I mean 1uV resolution is possible, but according to specs I am not sure that you can do 1uVaccurateand repeatable measurement. At least, that is what I can read from 3458a accuracy specs.Wonder if anyone here can provide some example, like a raw GPIB output (and, by raw I meanraw) from a 3458a that shows a fixed 1uv output from some voltage reference for around just 10 days?

author: TiN
author_rank: Super Contributor
timestamp: April 18, 2020, 04:59:04 pm
content: First I'd suggest keeping all in one thread, so people get familiar with your setup/lab experiments, instead of guessing which thread is relevant.As for questions.1. Thermal EMF is just one of the factors. So your question in nothing to do with amplitude of signal. You may have 1V signal and not care about EMF at all (for example if it's delta-mode measurement setup where your signal alternates in polarity and removing EMF error). Or you can have 10V signal that you trying to transfer between two low noise references where thermal EMF somewhat important to take care of. Or you measure high-current shunt signal, where thermal EMF is worst error contributor due to large thermal gradient over terminals.2. Teflon is needed for resistance measurements, starting from about 10kOhm and higher due to insulation leakage concern. It's not needed and actually can be worse for voltage measurements (e.g. high voltage). And yep, you missing very own Fluke's cables used for typical calibration workload:5440A-7003- these are twisted low-thermal (not Teflon though).3. Usually I see most precision setups avoiding rear connections, as that's where you have more airflow from instrument fans/etc, but it's setup specific thing. Airflow (no matter front or rear connections) is generally bad for sensitive measurements.There is no magic cable/setup that works best for every possible case. Even with 3458A you may want to use different type cables/connections or sometimes NO cable at all if you pushing the measurement limits. I use PTFE cables for resistance measurements, above mentioned Belden/Fluke type cable for general work, guarded triaxial for low current measurements, and double shield coaxial cables for AC measurements. For sensitive AC measurements use very short (<10cm) twisted cable or coax cables. In extreme cases use no cable and direct coax adapters/interconnect between two instruments, oriented face to face (connector to connector).Before you worry about all these rabbit holes, outline your requirements and how much effort you want to go. If you just want to test 6V zener diode, most likely your error from ambient/diode temperature change will be significantly bigger than any error from the cable or 3458A itself. That would make spending efforts into minimizing 30 uV of error from thermal EMF a waste of time, if your DUT zener changed 2000 uV due to 1C temperature change from airflow/or airconditioner kick in

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2020, 05:00:25 pm
content: I suspect that noise will override the voltage. The diode breakdown isn't the same over the area of the die, for one thing, so the current needs to be regulated very closely and of course have very little noise. Add the temperature coefficient of the breakdown voltage and this becomes very uncertain.Thermal emfs can be estimated, especially if you know the composition of the electrodes and wiring.To get to the best resolution of the voltmeter, buck out a fixed value with a good power supply to measure the difference voltage on the most sensitive range.

author: AndyTT
author_rank: Newbie
timestamp: April 18, 2020, 08:41:07 pm
content: Thank you all for your detailed responses. I will try and explain a little better what I am trying to achieve.I am trying to characterise the long term drift of a zener voltage. E.g take measurement, put in oven, wait some time, take measurement, calculate drift in ppm, repeat for X amount of cycles. I expect to see over time the delta to converge towards zero ( in theory) but in reality if I can get below 5ppm repeatability I would be happy. The more cycles through the oven the more stable the zener becomes.At these accuracies I would expect error from temperature and cable setup to seriously screw with my experiment.The 3458A is an 8.5 digit meter so 1uV is very achievable. The meter itself can drift with temperature amongst other issues which my other post is regarding. Apologies I did not know if keeping it in one thread was better than creating another post. If a mod wants to merge the two posts please feel free.I might be getting too carried away here with what I think is realistically achievable in terms of accuracy and repeatability. I feel like I am starting to chase ppm. If anything, it’s providing me with knowledge I previously didn’t have on this subject.Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2020, 09:00:15 pm
content: For a more normal zener diode there will be quite some TC and also noise. So it would also need a good temperature reading (at least repeatable). Because of the noise it may take more than just 1 reading, may be more like a data set over 30 minutes, in case there are random jumps.The thermal EMF gives an additive error. So it is a questing of the required absolute uncertainty, not so much about the voltage value wether one should take care.

author: TiN
author_rank: Super Contributor
timestamp: April 18, 2020, 09:49:15 pm
content: 3458A (and any other 8.5 digit meter) are not able to provide 1 uV accurate readings by itself, period. Own measurement noise of 3458A on 12VDC range is about same or more as that 1uV. Sounds like you are confusing accuracy and resolution, which are very different things.If your 3458A is good and stable, it's capable of testing almost any usual zener output with minimal errors already, if you use commonly used precision measurements technique, such as using short good shielded cables, repeatable and stable thermals (both DMM and DUT), good connections (no banana plug or crocodile crap!) and proper programming of operation settings.Suggest you to actually setup your experiment with some datalogging, and just try it, instead of guessing. Precision measurements is the field that benefits from experimentation like nothing else. There is no such thing as bad results, only bad interpretation.QuoteIt does not quite work this way. If heat treatment could make zeners more and more stable, Fluke and co would not be spending so much effort into predicting and stability studies of zener-based references, and overpaying for all that battery-powered express shipping to try preserve that drift rate stability.

author: splin
author_rank: Frequent Contributor
timestamp: April 19, 2020, 12:14:13 am
content: Quote from: AndyTT on April 18, 2020, 08:41:07 pmHmm, which theory was that? Presumably you mean a hypothesis - but as Tin says, it's not so simple - unfortunately.You might find the links in this post interesting:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg614899/#msg614899

author: AndyTT
author_rank: Newbie
timestamp: April 21, 2020, 11:04:35 am
content: Quote from: TiN on April 18, 2020, 09:49:15 pmHi Tin,You are absolutely correct, Reading back over my post, I have somehow confused resolution and accuracy.Things I am looking at now:1) Check Meter (Stability) SN18A check first - not looking good so far :-(2) Ensure proper cabling technique3) Operation SettingsCould you elaborate on operation settings a little. What settings would you typically use?I am using 10 PLC, Auto Zero enabled. Are there any other settings you would suggest?also, with regard to cabling....It seems almost impossible to source any Belden 8719 cable here in the UK without spending £600+. Is the cable really critical or is it the terminations that see the most benefit. For my particular use, DCV, twisted pair and shielded with good low emf terminations would enough?Here are a couple pictures of the test setup at the moment. Cabling not ideal situated in air flow of fan and much longer than probably needed. going to look into moving connections to front of meter.ThanksAndy

author: dl1640
author_rank: Regular Contributor
timestamp: April 21, 2020, 01:07:50 pm
content: Simply use a LAN cable and strip both ends to fresh bare copper , then hook it up onto the binding posts , use sponge to cover the binding posts to prevent drafts .