ADC voltage divider, seeking help for interface and input resistance

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by Harrie on Fri Jul 24 03:52:59 MST 2015
For a project I would like to use the ADC from a LPC11C24FBD48 to measure the voltage of 12V and 24V battieries.

However, I'm struggeling with finding the right resistor values.
So far I have come to the following result:

To get an output voltage of 3.3 V with and input of 30 volt, using:
R1 = 71.2 kOhm
R2 = 8.8 kOhm
C   = 10 nF

From the datasheet I found that the following:
Rvsi      voltage source interface resistance = 40 kOhm
Ri          input resistance  = 2.5 MOhm

Can someone please advice me how to take the interface and input resistance into account?

For a schematic overview of the divider please following the link below.
Circuit drawing