[volt-nuts] Review: Vishay Z foil resistors as Secondary Ohm Standards
Marvin E. Gozum
marvin.gozum at jefferson.edu
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?
At 10:57 AM 11/19/2011, Frank Stellmach wrote:
>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.
>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
>volt-nuts mailing list -- volt-nuts at febo.com
>To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
>and follow the instructions there.
More information about the volt-nuts