Hi All;
I am sure you thought of this, but perhaps you could isolate the issue by testing your 1K with the 3458A manually in the 10K range? Or 10K in the 100K range.
Cheers;

Tom Knox

SR Test and Measurement Engineer

Phoenix Research

4870 Meredith Way Apt 102

Boulder, Co 80303

Formerly of:
357 Fox Lane
Superior Co 80027

303-554-0307

actast@hotmail.com

"Peace is not the absence of violence, but the presence of Justice" Both MLK and Albert Einstein

From: frank.stellmach@freenet.de frank.stellmach@freenet.de
Sent: Friday, July 22, 2022 11:22 AM
To: volt-nuts@lists.febo.com volt-nuts@lists.febo.com
Subject: [volt-nuts] Re: HP3458 - OHMF and DELAY vs APER and NPLC

Well, meanwhile I think your 3458A might have a bug in its 10k range, if 1k and 100 were measured stable in their corresponding ranges.
Then the HW,  e.g. the current source of the 3458A might be affected.

How does you 3458A measure the 1k in its 10k range, btw.?

I also read through the whole Service Note 12-B, which describes a lot of Firmware bugs between your 4.6 and the latest, 8 or 9.

I found one which is related to OCOMP and Delay > 0.85sec, or so, and another one which deals with slow implicit reading of the buffer, or so, where the 3458A delivers only 1 valid reading. I could not match those directly with your problem, but anyhow there might be another hidden error which might explain.

At RWTH Aachen in 1989, I used one of the first 3458As, also measuring 10k and 100k reference resistors with OCOMP.
I never used the DELAY, as I was not aware of these polarization problems yet, so I did not encounter a behavior as yours… and usually I find every failure on new instruments. Like I did then: That was the calibration constant issue of the PERIOD function, which was resolved very late with Rev. 8.

Frank

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Hello Attila,

A few days ago, I made my regular, monthly comparison of these six 10k and one 1k resistors.
I use Turbo Pascal on an old W98 machine, and here I have one screenshot of the output my program, 1990ties style, I guess.

The resistors all have a known, linear T.C., so I measure the NTC thermometers in parallel, correct the Ohm reading to the nominal 25°C value, and then calculate mean and StD (sigma) statistics using the Welford algorithm. The resistors (VHP202Z, VHP100) and the NTC are coupled tightly inside a small aluminum block.

As you can see, the readings are very stable starting from the very first sample taken, and I achieve a StD / noise value between 0.15 to 0.25ppm each time.
For this measurement I had to sit frozen on my chair and did not breath. You directly see any hand waving in the noise.

Although I’m taking single triggered samples, it is not necessary to skip the first reading, or even more.

Here’s the core of my program, that configuration is everything you need:

.. cmd:= 'OHMF 1000;APER 1;OCOMP ON;TRIG HOLD;DELAY 0.1;';  // for 1kOhm
else cmd:= 'OHMF 10000;APER 1;OCOMP ON;TRIG HOLD;DELAY 1;';  // OCOMP requires settling time, 1s for VHP for 0.1ppm
sendstr(HP3458);

for N_Samp:= 1 to 16 do begin\
	..\
	TRIGHP34401;\
	cmd:= 'TRIG SGL;’;\
	sendstr(HP3458);\
	Temperature:= NTC_2k8(fetch(HP34401))-273.16;\
	..\
	Resistance:= fetch(HP3458); // ASCII read\
	..\
	Resistance_25:= Resistance \* (1 + (25.0-Temperature)\* TC_Factor);\
	Do_Statistics;\
end;\
cmd:= 'TRIG AUTO;';\
sendstr(HP3458);\
Goto_Local(HP34401);\
Goto_Local(HP3458);\
EndCEC;\
..

You are using AZERO ONCE, that is not necessary at all with OCOMP as the DMM in this mode substracts e.m.f. and offsets anyhow.

Then you are using TARM, I think,  that’s also not necessary, maybe even counter productive.

I have found a comment in my DCV acquisition program, that TRIG SGL and TARM creates many ppm of offset, so I skipped that, and let the DMM running free on TRIG AUTO.
For Ohm, TRIG SGL seems to work for my FW 9, maybe that is still a problem on your old FW version.

Background is, that in the DCV specification there is a footnote, that the first sample might have 1ppm deviation, and that seems to apply for halted (armed & triggered) measurements in particular.

So maybe you find some further hints to eliminate your measurement problem.

Frank

As you see, I don’t change anything on AZERO.
If you set OCOMP ON, measurement time will not increase additionally, as the DMM automatically replaces the AZERO by the OCOMP cycle.

Just try that out..

Frank