What are MEGA2560 AINO/AIN1 pins

Hi,
The circuit attached is for ac/dc measurement, hooked the analog comparator inputs 6 & 7 in the picture to MEGA2560 pin 6&7 got the measurement results with a quite large errors.
Hooked 6&7 to MEGA2560's 4&5 which said are the analog comparator pins:
http://man.hubwiz.com/docset/Arduino.docset/Contents/Resources/Documents/www.arduino.cc/en/Hacking/PinMapping2560.html

I got the same measuring results as hooked to 6&7 with error.
why?

Does MEGA2560 has analog comparator function? or are the 4&5 pins the right pin?
Thanks for help.
Adam
The code quite large can upload if need.
circuit:

https://www.google.de/search?as_q=What+are+MEGA2560+AINO%2FAIN1+pins

https://www.google.de/search?q=Does+MEGA2560+has+analog+comparator+function