> how do you pick the appropriate clock rate?

Jason, there are two timng periods. TAQ is the acquisition time, the
time needed to charge the ADC capacitor. TAD is the unit of time
needed for the conversion of the analogue voltage to a digital value

There's a formula to calculate TAQ vs source impedance to ensure
that sufficient time is allowed for charging. For example if you have
a source at the low end of source impedance (say something like a
1k or 500R pot) then you meet the minimum time to charge the input
capacitor. If source impedance is greater then you might have to use
a custom time delay. If the pot was as high as 100k perhaps then
you'd have to calculate how much long for the acquisition. At one time
PICs had only user-defined delays. Now TAQ has settable delay bits
in a register

A TAD is 1.6us, the time for a 1-bit conversion. For a 10-bit ADC the
complete conversion takes 11 TAD. Therefore you use the table or
calculation in the datasheet to determine the ADCS <1:0> setting so
that a TAD > = 1.6us

I believe that TAD < 1.6us results in an incomplete conversion and
that TAD (grossly, for example) > 1.6us is unnecessarily slow

Some background reading. Microchip have plenty of ADC reference
documents too

http://en.wikipedia.org/wiki/Analog-to-digital_converter

http://www.mikroe.com/en/books/picmcubook/ch7/

wbr
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist