Dont know if this is of interest to others, but it answers a question from a previous thread on interfacing to the Panda II ADC inputs, and I could not find similar data in searching the forum.
Several forum threads and the LPC 2387 datasheet point out that an over-driven ADC input, i.e. greater than 3.3 volts, can affect the conversion accuracy of adjacent channels, but the effect was not quantified. This left some uncertainty about just how big a problem this is if you have a signal that is only slightly above the max of 3.3 volts.
Since my application needed to quantify this affect I took some measurements that give some idea of what happens to adjacent channels when one is over driven. The data was taken using two batteries and voltage dividers to provide a 2.4 to 3.8 volt level on channel AD0.0 and a constant 1.6 volt level on channel AD0.1.
As the AD0.0 voltage was increased up to the 3.3 volt max there was very little error introduced on the AD0.1 channel – a slight rise, but less than +4%. However, when the voltage was increased to 3.8 volts the error in the adjacent channel AD0.1 increased to +25%. A plot is attached showing the effect.
I guess the take away is that if you keep below the 3.3 volt max – no worries if up to 3 % error on adjacent channel is OK. Going much above, like > 3.8 volts is problematic.
A caveat is that this is DC data and may be different from what would be measured with an AC signal.
