Please go through this AN it may be useful:
There is Forum thread which can be useful:
Hello Yasuki san,
Please check the first AppNote Ankita has attached. In section 6 Modifying TD dynamically there are two methods. Using Indexed/Nested DMA, you can update the transfer count parameter to achieve dynamic update of TD.
Could you elaborate more about your concern over DMA and ADC timing ?
Thank you for the information. I will study.