On Tue, 26 Apr 2005, Paul Hutchinson wrote:

> I've seen some thermistors with very poor performance when used for
> thermometers. The reason was that the manufacturer had designed them for
> uses other than temperature measurement. I seem to recall there used to be
> some thermistors that intentionally had hysteresis to make them more useful
> as temperature error compensators in certain circuits that have hysteresis
> in the temperature error.
>
> Check the manufacturers data sheet for the specs on hysteresis, the spec may
> be labeled repeatability. If there is no spec for hysteresis then that
> should be included in the overall accuracy specification.
>
> Also repeat your tests with an ohm meter instead of your circuit to
> eliminate any possible error in your circuit.

I did that. The other suspect part is a 0.1uF decoupling cap that is not 
of high quality. It intervenes in the timing chain (RC constant). It 
could be my reason. I will put in a higher quality cap later and see.

thanks,
Peter
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