Flebay DMM7510 has Issues - Page 1
Forum section: electronics / testgear

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 02, 2021, 09:14:42 pm
content: I bought a used DMM7510 on the flea-bay and it looks like it has fleas -- in the form of periodic 350uV spikes every 2 seconds on the open-circuit 100mV / 1V / 10V ranges. The spikes go away on the 100V / 1000V ranges or if I attach a low-impedance voltage source, so I suspect the problem is probably charge injection of some kind.I have:* Given it hours to equilibrate temperature* Updated Firmware* Run Autocal* Menu > Info > System Resetwithout effect. Since it was listed as "used" I'm leaning towards sending it back, but I'd like to run it past you fine folk first on the off chance that I've missed a PEBKAC. Thoughts?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 02, 2021, 11:42:50 pm
content: The plot thickens: the periodic spikes go away if I turn on Menu > Settings > Line Sync. Apparently it is off by default after preset. Has anyone else with a DMM7510 seen these spikes before? Is this normal behavior?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 03, 2021, 11:58:51 pm
content: Ew, gross, it's not a faulty unit, it's intentional behavior after a recent updatehttps://forum.tek.com/viewtopic.php?f=363&t=141242&sid=a3128a058304e3c4729d4d396693c0fd

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 12, 2021, 10:02:41 am
content: Quote from: jjoonathan on January 03, 2021, 11:58:51 pmIt was I who opened that topic. I can repeat your settings. Could you please give a complete description of the settings for which this behavior is observed. Those firmware version, mode, all settings ...

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 13, 2021, 03:33:12 pm
content: Sure. The glitches happen on default settings, which is one reason they're so gross. Details (I've set a fixed range for concreteness):Code:Firmware: 1.7.5bMeasure>SettingsRange: 100mVIntegration Unit: NPLCAuto Delay: OnNPLC: 1Display Digits: 7.5 DigitsAuto Zero: OnInput Impedance: 10MCount: 1Unit: VoltLineSync: OffSystem>SettingsAudible Errors: OnKeyClick: OnBacklight Dimmer: 8 HoursBacklight Brightness: 65Time and Date: 08:56Command Set: SCPIPassword: *****Reading Format: PrefixInterface Access: FullLine Frequency: 60HzPhysical Setup:60Hz 120VAC line inNo banana leads attachedAny of the following make the glitches disappear:Disabling Auto ZeroEnabling Line SyncNone of the following make the glitches (400uV spikes every ~2 seconds) disappear:Menu > Info/Manage > System ResetPutting inside a sealed EMC chamberPutting inside a sealed EMC chamber, feeding high purity AC power from IT7321120VAC 60Hz from wall.120VAC 60Hz from IT7321.240VAC 60Hz from IT7321.240VAC 50Hz from IT7321.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 14, 2021, 08:15:24 am
content: I'll check your settings tonight. But in my opinion I saw this and even tried to write in Keithley. It looks like this feature applies to this model of the device and not to your specific sample. He also has a lot of interesting deviations from the ideal noise on other ranges and settings. See the topic about him:https://www.eevblog.com/forum/testgear/all-about-keithley-dmm7510-bugs-and-features-recipes-advice-notes/

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 14, 2021, 07:20:18 pm
content: I can confirm spikes in the following combinations:open input; IMPEDANCE_10M; 0.1Vopen input; IMPEDANCE_10M; 1Vopen input; IMPEDANCE_10M; 10Vopen / closed input; IMPEDANCE_AUTO; 100VHowever, I would not recommend using the 10MOhm input mode in this device in view of the offset that it adds and, as seen above, the spikes.Spikes seem to me to be all about AZ. And most likely the question is at the point in time at which AZ occurs. This is why LineSinc helps.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2021, 08:40:54 pm
content: AFAIk the DMM7511 and some other Keithly meters use a slightly more complicated AZ mode than the HP meters. It is not just signal and zero, but a more mixed sequence like:Signal , Zero , Ref , .... or most of the time Signal , zero, but sometimes a different reading (ref., temperature) instead of the zero.There is some correction for the ADC gain every few seconds - this give the very good warmup drift. However this more complicated mode likely also causes the extra noise in the 1-10 seconds time scale, as the 1/f noise suppression is poor.These extra spikes could be those cases where an extra reading (like the temperature or the reference) is done.Line sync mode would add additional delay before the readings. Chances are the spikes may also be visible on a scope connected to the input, when in then 1 or 10 V range.Many of the other Keithley meters have a buffer at the input, so they would not show such input spikes - the DMM7510 seems to deviate in this aspect and likely has the switching directly at the input, just like most HP meters. It may not have a precharge phase with all the extras like the HP meters to minimize the input spikes.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 14, 2021, 09:16:52 pm
content: Quote from: Kleinstein on January 14, 2021, 08:40:54 pmOh yeah! They are clearly visible in the ranges: 0.1; 1 and 100VAre repetitive. One down then four are the same and the fifth is similar to the first but of a different polarity. In the 0.1 and 1V ranges, they are similar. In the range of 100V, they are slightly different.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 15, 2021, 04:25:34 am
content: Yeah, I see them too. Thanks for putting together that thread with details on all the artifacts!