ADC error

[elmo2000]

if the input of a adc channel is connected to AGND analog ground the samples should contain zero but it varies from 0 to 8
causing an +/- 4 bit jitter

[Ttelmah]

Some error will be the result of the quantisation error in the sampling process, and noise inside the chip (this is why for the lowest noise sampling, you want to stop other devices inside the processor). However the amount you are getting is well beyond this (I'd normally expect to see perhaps one or two counts 'worst case', if the chip is working while sampling). I'd be looking at noise on the connection, on the Vref source, and that Vref is at a legitimate voltage (you get large readings, if Vref falls below about 2v, and the minimum for the full converter resolution, is typically 3v - check the data sheet for your chip).

Best Wishes