If you want to use the full 10 bit ADC resolution need to add external amplifiers (on the ATmega 16 side there’s also a Differential Input Channel with optional gain of 10x and 200x, but if 200x gain is used, 7-bit resolution can be expected.)
See some examples for instrumentation amplifiers interface circuits here, on page two:
**broken link removed**
Changing value for R10 you may adjust the gain (in the actual schematics gain is set for a nominal value of 125).