At 09:31 AM 8/21/02 -0400, you wrote:


>You generally don't need "incredibly complex" calculations.  The true
>thermistor equation derived from the physics is rather complex, but it can
>be modeled to arbitrary accuracy with simpler equations, like polynomials
>for example.  It depends on the temperature range and desired accuracy, but
>a third order polynomial might be good enough.  That's only 5 multiplies and
>3 adds.  Add 2 multiplies and 1 add per polynomial order, so a 5th order
>polynomial is 9 multiplies and 5 adds.  A haven't worked it out, but I
>rather suspect that a 5th order polynomial will be better than other sources
>of error in the system.

Over a halfway wide temperature range, polynomials are quite ill-suited for
this purpose. I've tried it up to about 9th order. It may well be possible
to do a couple of operations first to make the equation more suitable for
polynomial approximation.

Your suggestion of low-order polynomials would probably work if the range
was split up into segments, along the lines of classic spline interpolation
algorithms.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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