ADC input resistance - LPC1114

[lpcware]

Content originally posted in LPCWare by sipel_tp on Thu Aug 02 06:26:31 MST 2012
I have to measure a battery voltage using LPC1114 ADC. I would like to use a resistor divider to reduce the imput voltage to the ADC pin. The problem I have is I don't know the imput resistance of the pin so I can't design the resistor divider. Can somebody tell me the imput resistance of the LPC1114 ADC pin? Thank you very much.

[lpcware]

Content originally posted in LPCWare by sipel_tp on Fri Aug 03 04:17:55 MST 2012
Thank you very much

[lpcware]

Content originally posted in LPCWare by jdurand on Thu Aug 02 21:01:02 MST 2012
I don't have a data sheet handy but typically the input to a ADC isn't constant. For measuring things like batteries I usually just put a capacitor to ground on the signal so the ADC pulls current out of the capacitor without changing the voltage by enough to care.

On other processors 1M resistors and a 0.1uF capacitor have worked fine.