Vishay Z202 Foil Resistors for LTZ1000 Reference Board - Where to get?

[Andreas]

Is it thermal lag between resistor and sensor or actual a shift in resistance value as the result of thermal cycling?
It was speculated it had something to do with mechanical tension between the epoxy package and the substrate of the resistor?

Hello,

in my opinion it is mainly mechanical lag (creeping) between the substrate the cement and the metal foil bonded to the substrate
+ influence from the epoxy package and absorbed humidity.

What I typically see with plastic molded resistors with low T.C.:
The resistance somewhere starts "in the middle".
With temperature excursion there is some hysteresis.
After half a day at initial temperature the resistance returns to the middle.
Pure thermal lagging would have much smaller time constants (in the minutes range).

Sometimes you can also see that on subsequent temperature cycles the hysteresis gets smaller.
Perhaps with some drift.
(the absorbed humidity gets out of the epoxy package).

with best regards

Andreas

[Martin72]

If you rely on parts lists...
C1, C2, C3 and C9 are Kemet 100nF/63V film capacitors and “huge”, but the footprints on the board are only for 1206, I could still fit one capacitor halfway, the other positions are simply impossible in terms of space.
There seems to be a discrepancy between the parts list, the circuit diagram and the board.
C9 is specified as X7R type in the circuit diagram and not as a film capacitor.
C1 to C3 are connected in parallel to the large tantalum capacitors.
The question is, do they have to be film capacitors at all, or would 100nF X7R in a 1206 package size suffice for all positions?
Personally, I see no electrical reason for this, but then I'm no metrologist.

[Alex Nikitin]

I would avoid using X7R dielectric as much as possible, it is leaky, unstable and microphonic. NP0 (C0G) capacitors are much better for this kind of application, preferably from TDK.

Cheers

Alex

[Martin72]

The resistors from Digikey arrived today.
It looks unspectacular for the money...
The LTZ1000ACH also arrived today, so if I can find another 100n NP0, I can finish the assembly.
I had also brought an insulating washer from work that fits the LTZ; it holds the component at a slight distance from the circuit board – there's no reason not to use that, is there?

[Andreas]

Hello,

my LTZ1000A samples had different T.C. depending on lead length. (see also LTZ1000 thread)
So I could trim the T.C. partially by shortening the legs.
(with the non-A version I was not so successful).

I personally would keep the leads (KOVAR) as long as possible to keep the soldering joints more equal in temperature.
Further you need to thermally isolate the leads (on both sides of the PCB) from air drafts to reduce noise.

with best regards

Andreas

[Korchahin]

....
Absolute error over 30 deg C is comparable to the Z201 measurements.
...

On the Mouser.de website I noticed the Z201T and VART series resistors.
Z201T
https://www.mouser.de/datasheet/2/428/VIPGS01681_1-2572870.pdf
VART
https://www.mouser.de/datasheet/2/428/zseries-3247554.pdf

Are they suitable for assembling LTZ1000 references?

[Andreas]

Hello,

The Z-series is suitable.

The VAR series is more a "gag" for the audiophiles.
They are very brittle and have less humidity protection than the z-series.

I think I measured 2 samples in the Resistor thread:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462296/#msg462296

with best regards

Andreas

edit:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467

[Rax]

Martin72 - I recommend reading this thread: https://www.eevblog.com/forum/metrology/agilent-keysight-03458-66509-dcv-reference-pca-for-3458a/msg5195181/#msg5195181..

Different design than what you're using, but you may still find it useful, I wager. The idea was to organize more systematically information around building an LTZ1000 kit, including all calculations, parts, etc. The core of it is Franks' board and overall design.

Information on the resistors to order from TCC is also in that thread. Not sure how efficient or expensive it is to use them for you, but the capacity to order, say, two resistors to your exact specification (including value to XY.ZT kohm, resulting from your own calculations per your exact LTZ part) at that level or performance and for $20-30 is amazing.

Hopefully this helps a bit.