Hoi Illya,
On Fri, 28 Jan 2022 20:22:56 -0500
Illya Tsemenko <[email protected]> wrote:
First off, are you confident that your 3458A is properly working and has no
stability
issues due to A3 U180 drift?
Yes. My meter has not been in use for quite long, so first thing I did was
monitor
what CAL 72 did. As you can see from the first plot, it took about half a year
of
continuous running until it settled. Since new year, CAL72 has moved up by a
ppm.
So I guess that the ADC board is ok.
As for your APER vs NPLC timings and noise. Keep in mind that when you select
NPLC larger
than 10 meter internally configure ADC to perform not single but multiple
conversions
with max NPLC 10, and then average out readings before giving to user. So
running NPLC
100 (typical sweetspot for 3458s) meaning that meter makes 10 internal readings
and give
you average of those as result. OCOMP and Delay is also applied after -each-
reading, so
you get longer than expected acquisition times.
Yes. I know. That's what explains the sqrt(5) difference in noise between APER
1 and
NPLC 50. That also explains why NPLC is more succeptible to the settling I see
than
APER is, because NPLC switches every 200ms. I.e. if the settling isn't over
after DELAY,
NPLC will see the residue, while APER will give it an other 800ms to settle and
give
the average over that period.
I join the suggestion to ditch banana connectors altogether and use direct spade
to post > wiring.
I am not convinced that it's EMF. At least not caused by the cables.
I measured an 100Ω resistor earlier and it did not show anything like this
(measured with NPLC 100, OCOMP OFF, AZERO On and whatever the default DELAY is).
If anything, if EMF was a problem it should be 10 times worse with 100Ω as
the voltage applied to the resistor is 1/10th than with 10kΩ. But as you can
see from the first plot, the 100Ω shows only the ~6.7ppm/°C tempco I would
expect it to.
And once this tempco is removed, the combined temperature correction and
measurement
uncertainty is about 0.5ppm.
Meanwhile, the 10kΩ (measured with APER 1, DELAY 2, OCOMP On AZERO Once) shows
jumps
of >1ppm between measurements. So much that, with the little data I have, I
wouldn't
even attempt to calculate a temperature coefficient. The cables are the same, so
the EMF should have the same effect (with the excpetion of the resistor side,
that
is now of course different).
My guess is, that most of this uncertainty is from the settling I am seeing,
which
I can't really figure out. I think it is dielectric absorption, but I am not
sure.
Unfortunately, I don't have PTFE cables at hand.
Do you have any timescale data plots or CSV files with your setup? You should
see flat
nice and steady data with specified meters noise, without random jumps or
excessive
noise. If there is large slope over time, then its good idea to check
temperature
stability. If your resistor has 10ppm/K TCR then to get 1ppm stability you must
maintain
environment within ±0.05C. That is not trivial without airbath chamber on any
decently
long timescale (hours).
Yes. I looked at the 200 measurements of each run and they are, up to noise,
flat.
The noise is also pretty well Gauss distributed, at least for the 100Ω case. For
the 10kΩ case I see a slight e^-t slope at the beginning, but only if I
correlate
multiple measurements together, Otherwise it's below the noise. (With APER 1
DELAY 2)
I have found some old test with 3458A with 10kOhm measurement, while ambient
temperature allowed to go crazy:
https://xdevs.com/sl935_10k_sep/
One can see that loosing 10ppm due to temperature excursions is not hard at all,
even with <0.1ppm/K resistance standard (Fluke SL935 has internal oven for
resistor elements).
Ah.. Ok. So you have some effects. But at least it seems to be mostly
temperature related.
Your noise is about a factor 10 below what I see and is where I would expect it
to be.
If you have another more stable standard, that would be good alternate check.
Sometimes
these decades long standards go bad too (from user abuse or mishandling) and
become
unstable no matter what.
Unfortunately not a 10kΩ standard only a 100Ω and a 1kΩ. I will see what these
show
with the new settings I have.
Attila Kinali
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