Thanks all for your explanations, but I still don't quite get it completely.
I'm using the I/O channels as analog inputs which gives me back a value varying from 0 to 3.3V. The unit that i'm using (an x-IMU) also has an 3.3 V power output, 100 mA. Conneting this to one of the analog input pins directly gives a signal of 3.3V. Though, in my application I'm using a part of the hand, a couple of fingers, to conduct the power output to the analog input. The skins resistance obviously causes the voltage measured is less than 3.3V, let's say half or 1/3 of it, but still... a clear signal (which is what I want). But the analog input only reads about 0,05V, more or less (depending on how tough the grip is).
Do I understand it correctly if I say that: the voltage measured is so low because it can only sink upto 4mA? If so, is there any way that I can somehow increase this signal?
Thanks again in advance.