What is wrong with arduino due inputs?

Question

Arduino

There is an arduino due, it has a large number of digital inputs / outputs.
Something strange is happening, the outputs (digital) work fine, but the inputs constantly show a high signal, if, for example, the contact is grounded with a finger, it will be zero, what happens? I tried to turn on the built-in pull-up resistor, the story is similar ...

Answer 1

ADC inputs without a signal source connected will show zero only in theory. in practice, this cannot be obtained and there will always be pickups, most often simple 50 Hz from a 220 network.

Answer 2

Most likely electromagnetic interference. For example, some powerful transformer is located nearby.