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Will this 2V signal damage my bench multimeter over time?
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Amaruk:

--- Quote from: eloso on December 31, 2021, 05:15:00 pm ---Here's a challenge  - does any other Bench Meter on the planet do this when set to a manual range (excepting when a range is exceeded of course, which isn't the case here) ?
Eloso

--- End quote ---

Eloso, I did a measurement along the lines of your experiment above. In my case I used my Keithley 236 SMU as a voltage source as it can measure very low currents while putting out a voltage. I also set it to 4-wire sense and joined force and sense leads at the Siglent SDM3045X terminals (see image below). This is what I found out:

For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.

Below is an image of my Siglent SDM3045X in action when I presented it with a 2.3V from the Keithley power supply and sensing at the input terminals. Impressive!

And to answer your question: My Keysight 34461A DMM does not have this type of relay in the manual voltage range.

Finally, I should just also add that the cabling from my Keithley include some in-expensive coax cables from the power supply to a breakout box with 4mm banana connectors so the isolation is not as good as it could be if I had used proper triax cables.
eloso:

--- Quote ---Eloso, I did a measurement along the lines of your experiment above. In my case I used my Keithley 236 SMU as a voltage source as it can measure very low currents while putting out a voltage. I also set it to 4-wire sense and joined force and sense leads at the Siglent SDM3045X terminals (see image below). This is what I found out:

For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.

--- End quote ---


Well that should be a lot more accurate than anything I could do - thanks.   Doing the sums seems to indicate that at less than 2.2V the DC input resistance of the Siglent is 9.5Mohm and above 2.2V it is 22Mohm.

Surely, surely thats a bug ?   I know we all have to be cogniscent at all times of the impact of our instruments on the DUT but it is particlarly unhelpful for this impact to change across a definite threshold.  What if you were to ever measure voltage across a very high impedance sensor, tracking it against time ? Oh, that's a nice jump in my figures, just where it crosses 2.2V  ::)

Regards


Eloso
Kleinstein:
This really looks like something is totally messed up. A bit like getting wrong firmware (e.g. from the 3055) so that at 2 V another 10 M resistors / divider is added in parallel to 10 M already there below 2 V.

Is the test with the 600 mV range impedance set to 10 M or high Z ?  There is a chance this may make a difference.

Given the relatively small price difference between the SDM3045 and SDM3055, the 3045 does not look that atractive anyway.
bdunham7:

--- Quote from: Amaruk on December 31, 2021, 05:55:19 pm ---For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.

--- End quote ---


That's not kosher.  It also appears not to meet the published specification for DC input impedance which is stated to be 10M +/- 2%.  And is a non-trivial problem--I can think of multiple situations where this would make the meter completely unusable for certain tasks.
Amaruk:

--- Quote from: bdunham7 on December 31, 2021, 07:14:25 pm ---That's not kosher.

--- End quote ---
And you are absolutely right. My data was actually wrong. This time I hooked up my 34461A in series as a current meter and used my second Keithley 236 power supply as a voltage supply using its 2-wire sensing mode. And now it all checks out just fine-both before the relay activates (1.9V) and after (2.3V). As you can see in the statistics in the images below, the input impedance is steady at around 10Mohm both at 1.9V and at 2.3V. Thus, the relay does not seem to affect input impedance. That is good news!! Sorry for the confusion.

I should also add that my Keithley's do not have valid calibrations but both my Siglent DMM and my Keysight DMM do. That is another reason this second set of data is better - the readings come from calibrated instruments. But I am still not sure what happened in my first set of measurements. A good example of why it is good to re-check measurements that do not make sense.
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