Lpc2000

I am currently trying to use the 10 bit a/d converter available on 
the lpc21xx devices.  I have not been able to get a full 10-bit 
resolution out of the device and would appreciate any help.

In the datasheet it specifies a 4.5 MHz or less A/D clock.  I have 
found that contrary to the datasheet I get more resolution if I turn 
up the clock faster the 4.5 MHz.  If I turn it down (slower clock) I 
get less bits.  This is somewhat counter intuitive as usually a 
slower clock gives rise to a better signal? The input to the A/D 
converter is a low impedance source (op amp directly coupled to 
input pin).  


Shane.