[volt-nuts] Precision resistors

[Tony Holt]

On 06/08/2015 16:25, David C. Partridge wrote:
 > Looking for a set of precision resistors for calibration purposes.
 >
 > The crucial factor isn't that they be *exactly* the values above, so 
I don't necessarily need 0.001% parts.  Low TCR is important, and I will 
need to know that actual values to 10ppm or better.
 >

Presumably long term stability is more important than TCR - not many 
resistors are guaranteed to drift less than 25ppm/year, even unpowered. 
E.g. Welwyn 4800 metal foil 25ppm/year, Caddock USF ultra stable 200 & 
300 series 20ppm/year dropping to 10ppm/year after 10 years. Short of an 
SR104 or the like I doubt you can do better than the 2ppm 6 year life 
stability of the hermetically sealed Vishay metal foil resistors 
including the HZ series you mentioned or the VHP101, VHP203 etc.

Frankly I find it hard to differentiate between the Vishay sealed foil 
resistors - they seem to have almost identical specs. For example is the 
VHP203's .05ppm typical TCR (0 to 60C) better than the 10ppm max for the 
VHP101 (15 to 45C)? I've no idea but given that the maximum TCRs are 
usually at least 10 times worse than the typical figures, maybe not.

Is the 'VHP101 < 10 ppm (+ 15 °C to + 45 °C)' even a maximum given that 
Figure 4 says typical TCR is .3ppm and has a 'typical' curve showing 
approx 8.5ppm max change from 15 to 45C (and has a TCR of .55ppm from 15 
to 20C)? It looks to me that the 10ppm figure isn't specifying a maximum 
TCR at all which means you could get almost anything! Interestingly a 
note in the HZ series datasheet says 'For maximum TCR < 1 ppm/°C, see 
VHP100 and contact application engineering'

The VH102Z seems to have the best guaranteed TCR of .6ppm maximum (100 
to 100k ohms) compared to 2ppm for the VHP203 and the HZ series.

Edwin Pettis states his resistors drift is typically better than 2ppm in 
the first year, so pretty good but you'd still need to have them 
measured every few years. If you have to get them professionally 
calibrated it may be cheaper to buy the Vishay parts. Edwin could 
provide the measured values as could Vishay if you bought directly from 
them.

The spec for the Fluke 5450A's 10k is 6.5ppm max drift/year, but an old 
one may be a good solution as hopefully the resistors will be very 
stable by now. The only way you could be sure though is to have it 
calibrated at least twice to see how much it actually drifts which 
wouldn't be cheap.

An alternative is to buy one or more 10k VH102Z as a standard and buy or 
build a 10:1 and perhaps a 100:1 Hamon divider and use the 10k reference 
to calibrate 1M, 100k, 1k and 100R resistors in a bridge arrangement 
using a null meter or suitable DVM. Adding a 2:1 would address the 20K, 
200K etc. requirement. This approach might not be suitable for very high 
or low resistances, but would allow you to have confidence in many of 
your resistors relative to your 10k standard. The divider could also be 
useful for voltage calibrations or checking your other calibrators.

Dr Frank describes the one he built, achieving uncertainties of 0.2 / 
0.5 ppm for 10:1 / 100:1 releative to output, in reply #8 and #10 here:

[url]http://www.eevblog.com/forum/testgear/hp34401-measurement-of-linearity/?nowap[/url]

I thought he had a more detailed description somewhere but I can't find 
it just now. You could probably get away with using cheaper resistors at 
the cost of slightly increased uncertainty.