KEI 2002 high offset at cold start - Page 1
Forum section: electronics / general

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 07, 2017, 05:08:33 pm
content: gents,not sure as to whether this belongs to "metrology" or "repair", so please move this if neccessary...i'm currently in the process of sending my Keithley 2002 to a cal lab, so i want to make sure that it is in healthy condition before shelling out money for nothing. (i gave it the usual treatment of an all-electrolytics exchange and general cleaning)on cold start of the DMM, there is a pretty hefty offset of approx +4 µV (in 200mV range, inputs shorted with short copper wire and plugged with q-tips) that slowly moves to and stabilizes at -1.4µV after about an hour of warm-up.switching through the ranges, that offset is pretty much the same on all ranges. in comparison, my Keithley 2001 is basically "spot-on" right from the start.question to fellow 2002-owners: is that behavior considered normal, or is there the possibility of some hardware fault? any suggestions what i should check? internal supply voltages "look" ok, but the small DC/DC converter that is located at the analog front-end is quite hot to the touch. any thoughts?

author: TiN
author_rank: Super Contributor
timestamp: March 07, 2017, 05:43:00 pm
content: How about 2W short in resistance mode? What is your reading.Could be simply zero offset or bad/dirty F/R switch. My 2002's are busy, can't test now, sorry.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2017, 05:51:45 pm
content: 4 µV and also -1.4 µV after warmup is quite a lot of offset. Checking for dirty switches (front/rear) is one point. Another is checking for input bias current. Excess bias current can also cause an extra offset.

author: Villain
author_rank: Regular Contributor
timestamp: March 07, 2017, 06:52:23 pm
content: they will do short adjustment anyway is my guess so it doesnt matter right?

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 07, 2017, 07:06:54 pm
content: Quote from: TiN on March 07, 2017, 05:43:00 pm2WOhms Rear: approx. -6 mOhms, Front: approx. -2mOhms, which is in line with my KEI2001. however switch is a bit touchy, so definitely a another candidate for replacement.200 mVDC range offsets at front and rear are about 500nV apart, so that's probably because i'm using different shorts at front and rear, and the front panel is warmer than the rear.i am not worried about the constant offset "per se" after warm-up, because the instrument is off-cal anyway. it's a Rev. A04, cal due 1995. however, the drift from cold to equilibrium seems to be rather high.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 07, 2017, 07:08:59 pm
content: Quote from: Kleinstein on March 07, 2017, 05:51:45 pmyep, will look into bias current. as for the -1.4µV after warm-up: i'm not too worried about that atm. that offset should be gone after cal.edit: with 132MOhms on inputs, range 200mVDC reads -4.4 mV. so input bias current is -33pA. factory spec claims < 100 pA (absolute value, i suppose), hence should be good.as a side note: built-in automatic selftest directly after cold start doesn't throw an error.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 09, 2017, 06:18:43 am
content: I just fired mine up, but it really needs more time to stabilize the last three digits. I think the book says 4 hours...I did recently measure the standard deviation with a cal short on the 20V range, it came out to about 600nV - which is decent. Seems in line with other reports (about double some of the other 8.5 digit machines, but it has double the range).I do worry about that switch. I just flushed the switch on a K2000 with switch cleaner, followed by a little deoxit, followed by another flush and then a final drop of deoxit. I got quite a lot of junk, mostly - i think - from the paint on the spring. That cures all the stickiness, and the front and back readings are consistent. If the unit is calibrated with a dirty switch, that's buying trouble for the future.I also used the self test 200.1, 200.2 and 200.3 to extract the last significant digit of the raw A/D data. Over 10 samples (it is a pain to get them) i was seeing an SD again of about 5 counts, or 500nV on the 10V range. A bit less than the reading done through the front end, but still OK.My Advantest units give me about 300nV on a short test, which also includes front end noise and banana plugs. That's similar to TiN's results for every HP except Dr. Frank's.

author: TiN
author_rank: Super Contributor
timestamp: March 09, 2017, 01:30:21 pm
content: I'd say little offsets doesn't matter, as they are easy to REL out, as soon as they are stableYou got lucky, as I had no time to setup another batch of resistors for TCR test (which takes 45 hours a run), so here's today 13 hour span of both 2002's with copper+Qtip short.There is a typo on graph, it's NPLC20, not 50. My previous tests show no difference from 50. Snake setup config:Code:# Setup SCPI DMMself.inst.clear()self.inst.write("*RST")self.inst.write("*CLR")self.inst.write(":SYST:AZER:TYPE SYNC")self.inst.write(":SYST:LSYN:STAT ON")self.inst.write(":SENS:FUNC 'VOLT:DC'")self.inst.write(":SENS:VOLT:DC:DIG 9;NPLC 20;AVER:COUN 10;TCON MOV")self.inst.write(":SENS:VOLT:DC:RANG 0.2")self.inst.write(":FORM:ELEM READ")

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 09, 2017, 02:00:40 pm
content: wow, awesome. thx 1e6, Tin! will use that as reference and try to do the same.btw, does that diagram include the cold-start of the KEI2002s, or is that showing data after warm-up?after warm-up, my meter basically behaves the same as the ones you have in the diagram.

author: TiN
author_rank: Super Contributor
timestamp: March 09, 2017, 02:03:37 pm
content: My meters are never off, have mercy, we are in metrology section here

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 09, 2017, 02:09:54 pm
content: no prob, i hear you!