Keithley 199 Drift - Normal or Fault? - Page 1
Forum section: electronics / testgear

author: WolfK21
author_rank: Contributor
timestamp: December 13, 2021, 06:57:42 pm
content: Hello,as this is my first post I think it is mandatory to introduce myself. My name is Wolfgang and jobwise I'm an automotive electronics engineer (mostly hardware, but also a bit system testing).Like every electronics engineer I own my own lab at home with (as my girlfriend says "far too much") equipment. I have two Tektronix 7k-series Oscilloscopes and roughly 30 plug ins for it, some beefy power supplies and some Tektronik TM500-series mainframes and plug ins. The collection ins completed by some more modern equipment like a Keithley 2000, a Keithley 220, a Keithley 199 and some really modern Rigol test euipment (MSO4014 and DG1062).With all this equipment I developed in the past many different things. But in the past few years time went shorter and shorter, thus I did mostly quick repairs for friends instead of my own projects.But now lets come to the real topic:Like maybe many of you I was forced during the various lockdowns to work from home. Not too bad at all, with the equipment on the desk the home lab was quite easy to do.During one of my testing sessions I discovered that the reading of my Keithley 195 was a bit far off the value it should be. As I didn't have the time to investigate the problem in depth and but it aside and continued working with my Keithley 2000.Now, months later I came back to the problem and discovered in some longer term measurement that the Keithley 199 drifts quite a bit during "warm up".After some tests I found out that all measurement ranges seem to be affected (the only thingh I didn't test is Ohms range).To me this sounds like something with the internal reference is wrong.To illustrate what I mean with "quite a bit of drift", I did some measurement today:I connected the 5V output of my power supply (Tektronix PS503A) to the Keithley 199 (5.1107V) and the 2000 (5.11315V).At this point all instruments were cold!After this I connected the Keithley 2000 to the -10V reference voltage of the Keithley 199 because I thought it is the source of the drift.With the test leads connected to the reference voltage I put the K199 back into its housing (Testleads hanging out of the scanner slot, the rest of the opening was taped to prevent ventilation)Then I started the recording programm and collected roughly one reading every second with the Keithley 199.When I came back from work I stopped the program and took a final measurement with the Keithley 2000 on the power supply's output voltage: Still 5.11317V! (seems to be pretty stable for such a primitive power supply).The Keithley 199 showed 4.8586V at that time!The picture attached shows my recordings:Orange the reading of the Keithley 199 (refers to the left vertical axis)Blue the -10V reference voltage of the K199 measured with the Keithley 2000 (refers to the right vertical axis)By doing the math the drift of the reference voltage is probably not the source of this huge drift. But what else could it be?This leads me to my questions:1. Is this amount of drift normal for the Keitley 199 (I guess not!) or is this really a fault?2. If it is a faut and the reference voltage is probably not the source, what else could it be?3. How much drift can be considered normal?All I all , I think this will be a fun project to repair (finally a repair project for one of my own devices)Best regardsWolfgang.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2021, 07:14:42 pm
content: The drift for the reading looks too high, even though the drift specs are usually only after a warm up for some 1 hour or so.The reference drift may be still ok when the meter gets relatively hot inside and it is just a zener diode. After the initial warm up the drift of the reference is no longer that bad and may be normal.The time dependence looks similar for the reading and reference, but the size does not fit. Changes are it is a temperature effect for both.The 24 hous specs are 40 ppm for the 30 V range and 4 ppm/K. With a 20 K internal temperature change during warm up that would be about a limit of some 80 ppm for the warm up drift.Just to exclude to simple points: is the meter using the auto zero mode ? The non AZ mode may show quite some dift, though still not that much.The 30 V already uses the divider, so the drift without AZ may be pretty high.

author: WolfK21
author_rank: Contributor
timestamp: December 14, 2021, 06:08:04 pm
content: Hello Kleinstein,thank you for your extremely fast reply.As it turns out, you're absolutely right. It's less a fault of the instrument rather than of the user (typical PEBKAC :/ )After I turned on the Muxing the instrument works much better. Here is is plot of the last night.Now the two Keithleys measure the same voltage. The Keithley 2000 was already running the whole day while the K199 was more or less "cold".Thank you very much for your great helpBest regardsWolfgang

author: THDplusN_bad
author_rank: Regular Contributor
timestamp: December 15, 2021, 08:06:26 am
content: Good Morning,may I also add that the Keithley 199 data sheet specifies awarm up time of 2 hours... See the attached excerpt taken from the data sheet.And if I look closely at the time axis of your plot, the large portion of the observed drift happens in the first 2 hours or so.Let's simply assume that Keithley knew exactly what they were doing back then...Grüße,THDplusN_bad