Finally 7.5 Digit - My "new" Keithley DMM7510

[tonyalbus]

You can usually download the cal certificates directly: https://www.tek.com/en/calibration-services/certificate-download

ETA: I dunno if that will work for the meter. I thought there was somewhere to get it tho. 🤔

Thanks,
Josh

pitty : A Calibration certificate was not found

EDIT: Strange also not for my DMM6500 that was bought from Welectron last year

[tszaboo]

8.5 digits is really quite a lot.
Above all, I imagine that calibrating such a meter would be difficult.
A 7.5 digit meter can probably still be calibrated with an 8.5 digit meter, but an 8.5 digit meter itself...
Checking whether a used, older 8.5 meter is still within its tolerances is also rather difficult with the usual “private” references.
That's one of the reasons why I'm considering sending my 7510 in for calibration, so that I can be sure that everything is still OK, because I can't find out with the means I have here, quite the contrary.
With the DMM7510, I can calibrate/adjust my references, but not vice versa.

You can calibrate a 3458A with just a 10K resistor (like WEkom) and a Fluke 732A.
But then, you turned one problem into two.
The 3458A is really nice, I had one near my workspace for a few years. You know - you can measure the voltage drop of an LED being turned on, on a solid ground plane - kind of nice.
I wouldn't mind a follow-up for that, but by the looks of it there isn't a willingness in any companies to make a modern 8.5 digit meter.

[Martin72]

It describes how to measure DC as accurately as possible. I must have forgotten to set two parameters earlier, so I'll do that tomorrow.

Not much has changed since yesterday, which is a relief because, to put it bluntly, with these settings, you can go and have a coffee before anything happens.
(Filter: 100, PLC: 10, Range: Manual)

[tszaboo]

It describes how to measure DC as accurately as possible. I must have forgotten to set two parameters earlier, so I'll do that tomorrow.

Not much has changed since yesterday, which is a relief because, to put it bluntly, with these settings, you can go and have a coffee before anything happens.
(Filter: 100, PLC: 10, Range: Manual)

Try 1000 NPLC with auto zero. 40 seconds before anything on the VFD.

[Martin72]

i Asked Welectron also, then not Marco but someone else answered the email telling in different words: they only calibrate items they sell , so it was a NO
silly thing is, the calibration, with those two options you mention, are actually items on their website..

In addition, I know that Welectron does not calibrate the devices itself, but sends them to a specialist laboratory.
However, I will only pursue this further if Tek refuses to calibrate the device or offers to do so at an excessive cost.

[KungFuJosh]

However, I will only pursue this further if Tek refuses to calibrate the device or offers to do so at an excessive cost.

You'll be fine. Have you registered your DMM in an account on their site yet?

[Martin72]

Does that still make sense for a used device?

[KungFuJosh]

Does that still make sense for a used device?

100%. I did, and the service I got was great. And I had to do it all through the website, with the registered device. Then I was also able to track the progress of the repair service.

But I sent you some referral info, you might be able to get a 20% discount that way. So try that first.

[temperance]

Try 1000 NPLC with auto zero. 40 seconds before anything on the VFD.

I'm a speed reader. That's why I solely use 2 digit meters with flash converters.

[2N3055]

 

Try 1000 NPLC with auto zero. 40 seconds before anything on the VFD.

I'm a speed reader. That's why I solely use 2 digit meters with flash converters.

[temperance]

If I'm not mistaken, I see a switching power supply in there. That part on its own is interesting because that can't be a standard off the shelf power supply. Does anyone know what is in there?

Or maybe someone can try to measure the common mode noise between earth and the input terminals?

[Kleinstein]

The switched mode power supply is somewhat special as it uses coaxial cables to get near perfect shielding at the transformer. There are some threads about the DCDC converter already. https://www.eevblog.com/forum/metrology/dmm7510-coax-transformer/msg4840400/#msg4840400
Because the coax cable takes up quite some volume the SMPS part needs a relatively large core and is expected to get a not so good efficiency.

The DMM7510 like many of the Keithley meters have some issue with low frequency noise. So it is good for short integration (like 1 or 10 PLC), but averaging over longer time or using the digital fitler is not improving things as much (e.g. way less that the 1/sqrt(N) for independent readings).
https://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/msg1366061/#msg1366061
So doing the 100 to 1000 PLC conversions (40 seconds) is a range where the meter is not especially good.

[Martin72]

So it is good for short integration (like 1 or 10 PLC)

This is likely why the manual recommends 10 PLC.
However, the corresponding setting can only be set to 12 anyway.

[Kleinstein]

It is common for many DMMs with multi-slope ADC that the integration time in one converion is limited. Insteady of > 10 PLC conversion often averaging over multiple AZ cycles are used. This is to reduce the 1/f noise contribution as the speed of the AZ loop sets the lower frequency limit. There may be a balance between noise that gets better with averaging fast conversions and INL and switching artifacts for the input that gets better with a bit longer integration at a time.

The extra LF noise in the DMM7510 is however also presenst for the average over multiply 5 or 10 PLC converions, like with the digitial filter. It is still not 100% clear what causes this noise, but it looks like some not ideal software is involved as extra trigged readings can improve on it a little.

[temperance]

Interesting transformer. Thanks Kleinstein for the link.

https://youtu.be/uvgJ2zAxgAY?t=2840

51min22 you can see the transformer.

The logic board is powered by a regular SMPS and the ADC board is powered from the regular SMPS trough this transformer with coaxial cable windings. The communication between the logic board and the ADC board seems to have been done with ISO7242 digital isolators with only 2 pF barrier capacitance, or in total 6 pF with three of those. (located on the "back plane")

Can someone measure for me the capacitance between the neg. input terminal and the chassis? I wonder what the total capacitance looks like.

The transformer core looks like a smaller core mounted into a larger core or is this "smaller core' a plastic spacer?

Well, I'm now tempted to buy one just to investigate how this thing works...not because I need those digits. Thank you lads.

[Hydron]

Well, I'm now tempted to buy one just to investigate how this thing works...not because I need those digits. Thank you lads.

Not gonna lie, I have the same temptation...

[KungFuJosh]

Goooood, let the TEAS consume you. 

[tonyalbus]

Playing with a ADR1001 ref.  man i love this new(old) DMM7510

[Martin72]

However, I will only pursue this further if Tek refuses to calibrate the device or offers to do so at an excessive cost.

I got a reply from Tek...
ISO calibration including return shipping: €670, with measurement report: €800.
I need to think about it some more...

[tonyalbus]

Whoops... i thought about it already very quick ... its a no for me.
but without ISO ?

[Martin72]

There is nothing under ISO, that is basically the standard. Above that, there is DakkS, which Tek also offered me for almost 1000....

Where did you get the evaluation board? Don't say Mouser or Digikey...

[tonyalbus]

Where did you get the evaluation board? Don't say Mouser or Digikey...

Send by a friedly viewer.

[Martin72]

4W measurement on an Isabellenhütte shunt, 50mOhm.
For a quick and dirty job, the result is good, even if it is outside the tolerance range (0.5%), but then again, the shunt is not cooled.

[Sorama]

Where did you get the evaluation board? Don't say Mouser or Digikey...

Send by a friedly viewer.

You fried a viewer?
You should make a review of that 

[bson]

As an ignorant I'm asking what is the use case of a 7 1/2 digit DMM? What are you usually measuring that needs that level of accuracy/resolution?

One nice use case is to characterize the linearity of a 20-bit DAC or ADC to within a few percent without the fuss of range switching.