This confusion is why I tried to distinguish digitizer from ADC in my post. I would not consider a single IC with 4 ADCs to be a single ADC.
I looked for detailed information about this particular model and did not find much. Do you have a link?
This discussion thread says it has 4 ADCs in the form of a quad but the photo looks to me like there is a single quad 1 GS/s ADC and the lower sample rate with multiple channels is caused by limited memory bandwidth on the digital side because I see no analog multiplexing which is what I suggested originally:
Ahh... semantics. I assumed most people involved in the discussion were using the term 'ADC' to refer to actual packages - especially since the original poster of the question, poorchava, used the phrase, "...one ADC
unit..." when posing his question.
I also assumed that most of the participants understood that 1 ADC package per 2 channels means a dual channel ADC chip which is internally interleaved for one channel - and 1 ADC package per 4 channels means a quad channel ADC chip which is internally interleaved for one or two channels (as tom66 explained quite clearly). There have been many discussions on this board on this topic - involving the Owons, Rigols, and other brands using similar designs employing TI ADC08D500, ADC08D1000, ADC08D1020, etc, or Chinese-made clones of those chips.