[sdiy] resistor temperature coefficient measurement

[ASSI]

On Friday 14 February 2003 13:42, Czech Martin wrote:
> Perhaps the +-50ppm spec is the maximum deviation
> that some production lots may have on a bad day
> and the average is much better than this?

The definition of TKR may seem somewhat un-intuitive: the most common 
definition of +-50ppm means that the actual TKR may lie in a region 
bounded by the two lines +50ppm and -50ppm, going through zero at 20°C. 
This definition allows for different materials to be used in any actual 
incarnation of that resistor. This means that for the same spec and 
value you may get quite different behaviour for resistors made by 
different manufacturers or even from the same manufacturer over time. 
Most certainly some resistance values will exhibit different behaviour 
than other resistance values unless they're made with exactly the same 
materials and process. The resistors from the same lot will of course 
be pretty consistent.

Pure metal films will exhibit a consistently positive TKR which may be 
somewhat curved (higher TKR at higher temperature). Alloys and strained 
films (based on different thermal expansion of the substrate and 
resistive film) can be made to change sign of TKR, which generally 
gives less overall change in resistivity and an almost zero TKR in a 
band about the extremal point (which may or may not lie at room 
temperature), but the change of sign may not be welcome in all 
applications. And yet other materials will give you negative TKR.

As always, if you ask more of the device than is normally speced and 
shown in the datasheets, you either need to extend the specification or 
experiment to get at whatever data you are interested in. If you know 
the exact curve, you can for instance combine resistors with positive 
and negative TKR to come up with TKR zero via compensation. This trick 
is especially neat for capacitors, because single caps with a TKC of 
about 100...200ppm are easily obtained in either polarity.


Achim.
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