On Wed, 25 Jan 2006 15:21:25 +0100, you wrote:

>Hi!
>
>I am looking at controlling the frequency of a wienbridge oscillator using a
>digital potmeter. My problem being that digital logarithmic potmeters don't
>come in anything > 256 steps. Because of that I have decided that I want to
>add a "pad-resistor" that is connected to one end of the potentiometer and
>the wiper, to *simulate*  a log taper. [ See
>http://www.edn.com/article/CA46509.html ]. All I need to do is implement a
>correction-table to *correct* any possible deviations from a logarithmic
>scale, or that's my plan anyway!
>
>Are there any better ways to make a correctional table other than using a
>lookup table that *jumps*? I would like to be able to calculate the
>correction on the fly and return the corrected value. Is that a good idea?
>Any tips?

You have to balance the space/time of the code used to calculate the answer against the space needed
for a lookup table. 
For some functions, table size can be hugely reduced by simple linear interpolation between entries
- this works very well for things like PT100 and thermocouple linearisation which only require small
corrections, and the code is simple if you used fixed-point maths with the point at a multiple of 8
bits.
Excel is a useful tool to evaluate the errors from various table sizes, to determine the optimum
step size for the accuracy required. 






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