I'm not affected by caffeine either and I have a diagnosis of ADHD. I've had some nurses in the psychiatric unit say they weren't surprised because they've met many ADHD people who aren't affected by caffeine.
When I used to drink soda/pop, I would drink an entire 2L bottle before I fell asleep every night. In fact, it sometime had the opposite affect than caffeine should on most people. I would find it easier to fall asleep.