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This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

DMM7510 7½-Digit Graphical Sampling Multimeter
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Ilya Grudovik
Posts: 25
Joined: October 30th, 2018, 1:54 pm
Country: Belarus

This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Ilya Grudovik » November 15th, 2018, 6:25 am

This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Moreover, for a range of 0.1V, the jump is 6 µV or 60 ppm.

AutoZero can be in any condition.

The input is shorted.
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Andrea C
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Re: This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Andrea C » November 20th, 2018, 2:19 pm

Similar outcome on my unit:
Start with input R set to Auto (10GΩ), change to 10MΩ, back to 10GΩ
img1121_060815.png
Input R Setting effect on reading
img1121_060815.png (50.58 KiB) Viewed 8173 times
A perfect voltmeter would have exactly zero current flowing into or out of it when connected to the DUT.
But actual volt meters have some injection currents.
The higher input R setting can reduce these, gives better isolation from the DUT and lower noise.

The default behavior, to use 10Meg, allows relatively stable readings even with the terminals in open circuit.
See the "DC voltage input impedance" section in the Reference Manual for a little more elaboration.

Ilya Grudovik
Posts: 25
Joined: October 30th, 2018, 1:54 pm
Country: Belarus

Re: This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Ilya Grudovik » November 21st, 2018, 12:54 am

Range 0,1V offset 6 µV = 60ppm. Specifcation only 6ppm. This is 10 times worse than the specification.
Range 1V offset 6 µV = 6ppm. Specifcation 4ppm. This is 1,5 times worse than the specification.

The inputs of the device are shorted.
It seems to me that the bias voltage exceeds that described in the specification.

Andrea C
Keithley Applications
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Re: This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Andrea C » November 26th, 2018, 5:16 am

Check out footnote 4 on the DC Voltage spec.

Ilya Grudovik
Posts: 25
Joined: October 30th, 2018, 1:54 pm
Country: Belarus

Re: This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Ilya Grudovik » November 26th, 2018, 5:50 am

Andrea C wrote:
November 26th, 2018, 5:16 am
Check out footnote 4 on the DC Voltage spec.
Here is this note?: "4. When properly zeroed using the Rel function with external cables."

I considered this note to refer to the cable. That is, I have to take care of the compensation of thermal EMF cable. I was wrong?

But it turns out I should take care of the device and set to zero myself? The device for almost $ 5000 is not able to do it yourself?

Where can I read more about this? What other parameters should I monitor myself?

Ilya Grudovik
Posts: 25
Joined: October 30th, 2018, 1:54 pm
Country: Belarus

Re: This voltage jumps when the input resistance changes on the ranges of 0.1V and 1V at 1NPLC

Post by Ilya Grudovik » December 12th, 2018, 5:40 am

After talking with the engineers, Keithley within email revealed the following:

The device retains its accuracy only if the input impedance AUTO mode is set.
Accuracy in 10 MΩ mode is not verified and is not guaranteed.

One of the engineers suggested using relative measurements to zero the offset manually. But in my opinion there is too little information about the reason for this bias. And as a result, one cannot be sure that it is stable and is compensated for by AZ correctly.

The correct specification as I understand it looks like this:
Снимок3.PNG
Снимок3.PNG (46.81 KiB) Viewed 7582 times

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