Am I right in thinking that inputting 1 to DMODE (Bit0) in the configuration register will connect the 1024th cell to the 1st cell?
Yes, as desceribed in the manual .
Also, let's assume that after sampling 1024 cells on channel 0, 200 cells are sampled in the second week.
Does the readout of 1024 cells start from cell 0?
Or does it start from the 200th cell?
I don't understand what you mean by "second week".
Normally, you run the chip continously (DMODE=1) until you receive a trigger. Then, you typically read out the chip from the stop position by using a RSRLOAD pulse (as described unter "REGION-OF-INTEREST READOUT MODE".