[volt-nuts] Review: Vishay Z foil resistors as Secondary Ohm Standards

[Rob Klein]

Frank,

Unfortunately, your results closely match my findings on a batch of four 
VHP202ZT's that I bought almost two years ago with
the same purpose in mind.

While the initial accuracy and the stability with respect to thermal 
hysteresis are both excellent, I too found TC's that are nowhere near
the 'typical' value specified by Vishay. Neither could I find any trace 
of the parabola curve.

Very similar results were found for a group of 10 Z201's (the 
non-hermetic version of the VHP202).

Having since acquired an SR104, the actual 'need to build' has gone, but 
I must say I was (and still am) severely disappointed
with these findings. Although, as an experienced electronics engineer, I 
should know better than to follow 'typical' values on
a datasheet, I feel there is a certain element of false advertising 
involved here. I find it very hard to swallow that, out of 14 units
tested, not a single one comes even close to the performance we're led 
to believe they are capable of.
One wonders what Vishay's definition of "typical" is.


- Rob.

Op 19-11-2011 16:57, Frank Stellmach schreef:
> Hello Volt-Nuts,
>
> The experiments and the following discussion with Vishay took nearly 
> one year, until now I am able to present a final result.
> Further details about the measurements and the setup will be given later.
> I now have a personal, thorough understanding of Vishays Z-foil 
> technology and its capabilities.
> This can also be found between the following lines.
>
>
>
> 'The specification for the hermetically sealed, oil filled metal foil 
> Z resistors (VHP202ZT) imply a usage as a secondary Ohm standard:
> Longterm stability of 2ppm/6 years and a low TCR of 0.05ppm/K between 
> 0..60°C typically,
> vanishing @ 25°C, given by a parabola form of resistance over 
> temperature: http://www.vishaypg.com/docs/63120/hzseries.pdf
>
> I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed to nominal 10000.00 Ohm 
> +/1 ppm by low ohmic thin film resistors.
>
> Vishay always performs measurements on VHP types, i.e. absolute 
> resistance and TCR.
> So I also ordered the measurement protocol for a small additional charge.
> The complete batch of resistors were measured by Vishay at 25°C to a 
> precision of about +/- 1ppm, each one 3 times.
>
> After delivery, I compared each resistor of the ensemble against each 
> other by a HP3458A.
> The initial setup was sufficiently stable (order of 1 ppm) to 
> determine the relative resistance deviation, and to fine tune the 
> trimming resistors to the desired precision level.
>
> It became apparent during these first measurements, that the setup, 
> and the resistors were not as stable over temperature, as needed for 
> sub ppm level accuracy, and as expected.
>
> First, the measurement setup (i.e. environmental conditions, HP3458A 
> and resistor assembly) was greatly improved to get a verifiable 
> transfer accuracy of < 0.2ppm over one hour, although the HP3458A is 
> not specified in this regard.
>
> Each resistor was assembled into an aluminium bar, together with a 
> precision NTC.
> The bar then is mounted into a small aluminium box, which carries Au 
> plated CuTe plugs to provide 4 pole measurement for the resistor, and 
> 2 plugs for the NTC and case grounding.
>
> The different sources of instablities, especially thermally induced 
> ones, could then be separated, quantified and reduced.
>
>
> One resistor out of the ensemble was thermally cycled from 25°C to 
> about 0°C, back to 25°C, and then to 60°C and back.
> As the resistance value at 25°C was exactly reproduced every time to 
> within <0.2ppm, obviously no thermal hysteresis occurred.
>
> During the measurement, the HP3458A was frequently checked by another 
> VHP202 being at constant temperature.
> This demonstrated that the instrument stayed stable to within 0.2ppm 
> transfer accuracy.
>
> The differential TCR (dR/dT) of the DUT over this temperature range 
> was calculated to be between -0.5 .. -2.6 ppm/K.
> That is an order of magnitude above the expected, "typical" TCR in the 
> Vishay specification.
> Also, the upwardly directed parabolum of R vs. T could not be 
> detected, which should be typical for the Z foil technology,
> according to: "Zero TCR Foil Resistor Ten Fold Improvement in 
> Temperature Coefficient", R. Goldstein, J. Szwarc @ Visahy.
>
> Coming from the same batch, all resistors have the same T.C.; this was 
> checked briefly for another DUT.
> This excludes the possibility to improve TCR by a serial/parallel 
> assembly of 4 resistors.
>
>
> I complained at Vishay PG about the unexpected behaviour.
>
> Vishay had made TCR measurements at -55 and +125 before delivery and 
> repeated this on one resistor I had resent them. They found a TCR of 
> -0.4..-0.6ppm/K.
> Although they found a similar result, my complaint was completely 
> rejected.
> They claimed, that this TCR is well within specification.
>
> Although most of the datasheets about Z-foil resistors and several 
> other publications about the Z-foil technology imply a more optimistic 
> picture, only in Table 1 of the specification, a max. upper limit 
> ('spread') of +/- 2.2ppm/K TCR over the complete temperature range is 
> specified. This is a barn door, 10 times bigger than the typical 
> values over the complete temperature range.
>
> Additionally, Vishay defines the TCR as a "Chord Slope", i.e. as 
> (Rx-R25)/(Tx-T25), not as a differential one.
> Recalculation of my results then gave a TCR of -0.5 .. -1ppm/K, still 
> very high.
>
> Also, the parabolum with vanishing TCR at 25°C, (or at any other 
> center temperature) is "typical" only, and does not have to occur on 
> real components.
>
>
> Conclusion:
>
> The ensemble of 5 precision resistors represents a median standard 
> resistance of 10k +/- 1ppm, if thermally stabilized to 25.00°C precisely.
>
> The 3458A in combination with this stable resistor ensemble achieves a 
> repeatability / transfer stability of <0.2ppm over 1 hour and an 
> absolute stability of < 1ppm over days for the HP3458A alone.
>
> The longterm stability of the ensemble should be less than 2ppm/6 
> years, which greatly improves the mediocre longterm stability of the 
> internal Ohm reference of the HP3458A.
> This is to be checked during the coming years.
>
> A goal which could not be achieved, is the usage of the VHP202Z 
> resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K 
> is unexpectedly too high.'
>
> Best regards - Frank
>
>
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