Hi Attila,
I don't know if you saw the last couple messages I sent a while back, or
if they didn't show up on list. Here's one again, just in case. The
other will follow if I can find it - it especially addresses some of the
resistor issues.
Yes, those are very nice cables - I looked them up. But, they are still
banana plugs, relying on spring contact. From the description, I'd guess
there are two contacts in series - one from the plug core to the banana
or basket spring, and the other from there to the inner surface of the
jack. Another type is machined directly from springy material, with
cross cuts, and profiled to compress on insertion - these have only one
contact involved, so can be the best of the banana plug class. The fancy
materials and plating help greatly with contact issues, but may still
not be enough. I didn't see any specs on the datasheet other than "low
EMF," so I wonder if maybe they offer related low EMF jack/binding posts
too. Maybe the combination is specified with numbers. Also, what does HP
say in the 3458A manual about interconnect and fixturing?
Regarding dielectric absorption, what is the reference node? If you
connect the wire pairs at the resistor, there is very low resistance and
voltage drop between them at the "open" (3458A) end, so I would think
the insulation between them shouldn't matter too much. The other part of
dielectric absorption would be from each wire pair to the other, and to
everything else beyond. This should be dominated mostly by very small
(due to distance of separation) air capacitance, in series with the the
C from the conductors to the insulation/air boundary. I suppose that you
may be at the resolution level where the effects are noticeable, but my
gut feel is that the wire insulation characteristics are still minute in
scale. The main thing is to maximize the distance between the wire
pairs, and to everything else.
It appears the measuring process includes AC, with the various auto-cals
and zeroing going on, so the resistor and wiring would be jumping to
different voltages in the process. Is it possible to just set up for
pure DC, with constant current in the resistor? This may help to
separate the capacitive from DC effects. Speaking of timing, as you
noted, the few seconds of effects are about right for dielectric
absorption, but also a good profile for thermal or contact effects too.
Especially if the test current to the resistor is being turned on and
off during the process, everything may respond over some time span in
seconds.
One last thing occurred to me. DMMs typically have a switch or post to
connect the low input (guard) to earth ground. Have you tried different
settings for this? I would think grounded should be best, but maybe not.
Maybe you have some subtle ground loops in the system, that could add DC
offsets or AC from the activity. If the grounding choice makes a
difference, check the CMRR specs. That invokes one more last, last thing
- the DUT itself. I don't know what it looks like, but I'd guess it's in
a metal can. If so, there's of course parasitics to it from the
resistor. Is the can grounded, guarded, or isolated? Could there be
significant leakage current into the can? Could the can be filled with
tar or other potting material? this could give you yet another
dielectric absorption issue to worry about.
Ed
On 1/30/2022 4:59 AM, Attila Kinali wrote:
Hoi Ed,
On Fri, 28 Jan 2022 15:02:46 -0800
ed breya <[email protected]> wrote:
I'm not familiar with the 3458A, so don't know how the connections are
made up front. Presuming it's working right, I would suspect the
interconnect setup. The thermal voltages and contact noise may be the
main problem, especially using any sort of banana plugs.
These are not "any sort" of banana plugs. These are low-EMF from Pomona.
I doubt that wires would perform significantly better. But it's worth
a shot.
Put the
source current and high sense on one piece, and the low source and sense
on the other. In operation, the Kelvin connections tie each twin lead
pair together at the resistor, so even crappy insulation won't matter.
This I doubt. The effect I am seeing looks awfully like what I would
expect from a settling effect that takes a few seconds. And dielectric
absorption is one of those that are exactly in that time-range.
Before changing anything around, a little experiment may shed some
light. Instead of doing resistance measurements, set it to measure the
voltage on the resistor, leaving the whole setup as it was. In the most
sensitive ranges, with all those digits, I'd think you'll see the
effects of the input bias current, the thermal voltages, and some
resistor noise. The thermals should be easy to spot - just puff some air
at the connections, or wave your hand around.
That's a good idea. I'll set it up next.
Attila Kinali
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