Keithley 2000 Reading Greater then Zero When Input Shorted. - Page 1
Forum section: electronics / repair

author: lugaw
author_rank: Regular Contributor
timestamp: February 27, 2023, 09:38:45 am
content: I recently got this Keithley 2000 and it had a few errors but after recapping and re-flowing the input resistors it seems to be ok but unlike my 34401A it never get to zero when shorting the inputs.The image shown is having its input shorted for more than 8hours. It has no errors when doing the full built in test.Is it normal or is there something wrong with the it? I tested all the reading and its reading properly on all settings except for that tiny DC and AC values when shorted.

author: TizianoHV
author_rank: Regular Contributor
timestamp: February 27, 2023, 10:03:02 am
content: DC offset is fine, it changes with ambient temperature.AC offset is a bit high but still fine (my K2790 has around 90uV).Remember that AC offset is due to RMS converter noise and it shouldn't be nulled with rel function since it has an impact only for low values.

author: lugaw
author_rank: Regular Contributor
timestamp: February 27, 2023, 10:09:58 am
content: So the Keithley doesn't have an auto zero function like the 34401A?DC and AC on my 34401A is both zero when shorted.Ambient temperature here right now is cold, 16 to 18C.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2023, 10:25:09 am
content: The Keithley meter has an AZ buffer at the input and the auto zero function only works after that buffer. There should be a procedure (part of calibration ?) to also subtract the small offset one gets with the external short. Normally the offset should be quite stable, but over the years of use there may be a little, possibly depending on the supply voltage / mains frequency.It is also possible that the last adjustment was just not good (e.g. unusual thermal environment).AFAIK even the HP meters that have the AZ function essentially all the way to the input (after the protection) still have an offset that is subtracted by the software to allow for offsets from thermal EMF, e.g. in the protection, terminals and switches.The residual reading the in the AC range looks a bit high for my feeling. It may depend on the range and environment.

author: Gyro
author_rank: Super Contributor
timestamp: February 27, 2023, 10:25:49 am
content: The Keithley is the more correct in this case. As TizianoHV points out, it is displaying it's own internal noise whereas the HP is blanking it. This means that it will display a zero reading up to a certain low level AC voltage (hidden in the %Range + %Reading accuracy spec), whereas the Keithley allows evaluation of input voltages near its noise figure.For DC, autozero can't distinguish between thermocouple effects at the front-end and very low level input voltages. Again the Keithley is more correct in displaying them. In the DC case, they can be nulled using the Rel Key of course.A zero reading isn't necessarily an accurate or good thing.

author: TizianoHV
author_rank: Regular Contributor
timestamp: February 27, 2023, 10:26:32 am
content: DCV: 34401A has better zero stability and tempco, i saw that in my 34970A too(similar to 34401A).ACV: 34401A null (hide) AC zero offset with some math, but in reality the offset is still there. Probably they did that to avoid confusion.Ciaoo

author: lugaw
author_rank: Regular Contributor
timestamp: February 27, 2023, 10:37:43 am
content: Ok thanks for the feedback!I am new to this meter and don't know how it behaves. I couldn't find the auto zero feature mentioned in the manual.The DC offset is very consistent as you all mentioned probably because it was in storage for a long time and the last calibration was 1995!The AC offset is also very consistent and doesn't change when shorted even after long hours.I will probably just leave it on for a while and observe it some more and clean the INPUT selector switch if the offset remains after a week.Is there a way to turn off the display?

author: exe
author_rank: Unknown
timestamp: February 27, 2023, 10:51:01 am
content: Quote from: lugaw on February 27, 2023, 10:37:43 amthere is an SCPI command for this (requires rs232 connection). Though it won't completely turn it off (afaik filament will be powered), but it won't display digits.

author: TizianoHV
author_rank: Regular Contributor
timestamp: February 27, 2023, 11:07:47 am
content: Unluckily SYST:AZ: 0/1 and DISP:ENAB 0/1 are available only from remote interface.Still, from local, you can use the shift+FILTER(TEST), DISPLAY, ENTER, ENTER... function were it will turn on and off all segments, in theory reducing wear.A trick to "disable" AZERO from local (you need to send a remote command once):K2000 permit to save and recall setups, that means that you can disable AZ by remote (with a common RS32 dongle) and save that setup in memory (shift+OPEN(SAVE), SAVE0, ENTER). Now every time you need AZ disabled you simply recall the desired setup were you saved some of the inaccessible settings (shift+CLOSE(SETUP), RECALL:SAVE0, enter).I wouldn't expect much change from DC offset, after all these years looks goodIf you want you can do such simple test ->https://www.eevblog.com/forum/metrology/tempco-and-calibration-of-a-dmm-simple-data-post-processing/msg4644232/#msg4644232to see how stable it is. In my case I saw that zero offset changes quite a bit with temperature. That means that doing a complete zero calibration wouldn't be that useful since it would drift again with temperature (in my lab winter-summer temperature goes from 16 to 30°C)

author: lugaw
author_rank: Regular Contributor
timestamp: February 27, 2023, 11:33:05 am
content: Quote from: TizianoHV on February 27, 2023, 11:07:47 amThanks for the recall setting tips that's a good one.I also have a wide temperature range of 12 to 32C

author: gamalot
author_rank: Super Contributor
timestamp: February 27, 2023, 12:06:35 pm
content: Comparing to my K2000 and K2015, your K2000 seems to have no problem with the DC offset, but the AC offset seems to be a little bit high.