>--*--------------- +5V VCC
   |
   |
   |
  ---
  | |
  | |<---------------------[ Va gnd pin of AD converter
  | | 1K                        (the same circuit for Varef)
  ---
   |
   |    Signal >---*-------[ Analog input
   |               |
   |              ---
   |              | |
   |              | | 270 Ohm
   |              | |
   |              ---
   |               |
>--*---------------*--- 0V GND

In some ADs you can select the Vagnd and Varef, you can select where (volts) is the 0 and 255 (if is 8 bist converter)