Flebay DMM7510 has Issues

[jjoonathan]

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 Reset

without 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?

[jjoonathan]

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?

[jjoonathan]

Ew, gross, it's not a faulty unit, it's intentional behavior after a recent update 

https://forum.tek.com/viewtopic.php?f=363&t=141242&sid=a3128a058304e3c4729d4d396693c0fd

[MegaVolt]

Ew, gross, it's not a faulty unit, it's intentional behavior after a recent update 

https://forum.tek.com/viewtopic.php?f=363&t=141242&sid=a3128a058304e3c4729d4d396693c0fd

It 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 ...

[jjoonathan]

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: [Select]

Firmware: 1.7.5b

Measure>Settings
    Range: 100mV
    Integration Unit: NPLC
    Auto Delay: On
    NPLC: 1
    Display Digits: 7.5 Digits
    Auto Zero: On
    Input Impedance: 10M
    Count: 1
    Unit: Volt
    LineSync: Off

System>Settings
    Audible Errors: On
    KeyClick: On
    Backlight Dimmer: 8 Hours
    Backlight Brightness: 65
    Time and Date: 08:56
    Command Set: SCPI
    Password: *****
    Reading Format: Prefix
    Interface Access: Full
    Line Frequency: 60Hz

Physical Setup:
    60Hz 120VAC line in
    No banana leads attached

Any of the following make the glitches disappear:

• Disabling Auto Zero
• Enabling Line Sync

None of the following make the glitches (400uV spikes every ~2 seconds) disappear:

• Menu > Info/Manage > System Reset
• Putting inside a sealed EMC chamber
• Putting inside a sealed EMC chamber, feeding high purity AC power from IT7321
• 120VAC 60Hz from wall.
• 120VAC 60Hz from IT7321.
• 240VAC 60Hz from IT7321.
• 240VAC 50Hz from IT7321.

[MegaVolt]

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/

[MegaVolt]

I can confirm spikes in the following combinations:
open input; IMPEDANCE_10M; 0.1V
open input; IMPEDANCE_10M; 1V
open input; IMPEDANCE_10M; 10V

open / closed input; IMPEDANCE_AUTO; 100V

However, 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.

[Kleinstein]

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.

[MegaVolt]

Chances are the spikes may also be visible on a scope connected to the input, when in then 1 or 10 V range.

Oh yeah! They are clearly visible in the ranges: 0.1; 1 and 100V
Are 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.

[jjoonathan]

Yeah, I see them too. Thanks for putting together that thread with details on all the artifacts!