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Autocalibration changes input impedance setting but doesn't always show it properly

DMM7510 7½-Digit Graphical Sampling Multimeter
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Joined: May 16th, 2020, 3:12 am
Country: United Kingdom

Autocalibration changes input impedance setting but doesn't always show it properly

Post by essele » May 22nd, 2020, 1:52 am

First things first ... the manual ... can I leave the leads connected or not??

On page 3-44 of the manual:

You can run auto calibration with input cables connected. At the start of the auto calibration process, the front terminals are monitored. If more than 30 V DC or 1 V AC is detected on the front-panel inputs, auto calibration is not run and an event message is displayed.

Then in a box three paragraphs later:

To prevent instrument damage, verify that no test voltages are connected to the input terminals when performing auto calibration.

But that's not the bug ...

If you are using high impedance voltage measurements (Input Z = AUTO) then the input impedance is >10G on the 10V range. If you then run an auto calibration the input impedance immediately switches to 10M for quite a few seconds before you hear the relay click and it gets disconnected. (I'm watching this happen on a K2000 connected to the 7510.)

That bit I can half understand ... given the warnings about input voltages I suspect it switches to a higher voltage range (hence the 10M impedance) to check the voltage at the input. Although quite why you wouldn't just disconnect the inputs I don't know.

Once the auto calibration is complete, and you go back "Home" I've seen a two different situations:

1. It drops you back to the range you were on (10V in my case) but leaves you at 10M impedance (i.e. it's changed that setting) ... really not great, but at least the screen is correctly displaying what has happened.

2. It drops you back at 1000V range, still in AUTO mode (which is 10M in this range), but when you switch to the 10V range it still says "AUTO" but actually stays at 10M ... therefore displaying one thing and doing another (again!)

The second one has happened to me at least twice, but I cannot reproduce it now ... I'm starting to get very frustrated about the strange and inconsistent behaviour of this meter. It really feels like the firmware does not properly handle "state" very well and settings persist when they shouldn't and interfere with other measurements or activities.

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