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test_equipment:precision_z-foil_vishay_resistors [2013/01/08 19:00] |
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| + | ===== Precision Z-Foil Vishay Resistors ===== | ||
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| + | //The following was captured from the [volts]nut] mailing list// | ||
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| + | ---- | ||
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| + | The experiments and the following discussion with Vishay took nearly one year, until now I am able to present a final result. | ||
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| + | Further details about the measurements and the setup will be given later. | ||
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| + | I now have a personal, thorough understanding of Vishays Z-foil technology and its capabilities. | ||
| + | This can also be found between the following lines. | ||
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| + | '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 | ||
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| + | I ordered 5 EA 9k9998 +/- 10ppm, to be trimmed to nominal 10000.00 Ohm +/1 ppm by low ohmic thin film resistors. | ||
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| + | Vishay always performs measurements on VHP types, i.e. absolute resistance and TCR. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | The different sources of instablities, especially thermally induced ones, could then be separated, quantified and reduced. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | I complained at Vishay PG about the unexpected behaviour. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | Conclusion: | ||
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| + | The ensemble of 5 precision resistors represents a median standard resistance of 10k +/- 1ppm, if thermally stabilized to 25.00°C precisely. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | 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.' | ||
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| + | Best regards - Frank | ||
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