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

Mon Nov 21 12:06:31 UTC 2011

Thanks Frank for a most wonderful report.
Would you be able to share what the cost of each VHP202ZT you ordered?

best wishes,

-marv

At 10:57 AM 11/19/2011, Frank Stellmach wrote:
>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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