ADS7230 / Thermal pad, AGND, BGND grounding

Hi Ryuji,

Per the datasheet, if a split Digital and Analog GND planes are used,  the preferred connection is to tie the pad to the quiet analog GND.  However, this is also dependent on how the GND scheme is implemented in the board layout.  The user still needs to connect the DGND and AGND in some place of the board since they must be at the same potential.  A single solid GND plane board layout can also work properly, and in the case of a single GND plane, connecting the pad to the GND plane should work fine.  However, the user must be careful when routing the signals. An optimal board layout keeps the digital and noisy signals away from the sensitive analog signals; and the digital signals should not cross paths with the sensitive analog traces.   The bypass supply capacitors should be in close proximity to the supply pins.  The reference bypass capacitor should be in close proximity to the reference pins and the REF- should be connected to its own via to GND with the shortest, low inductance connection. There are different ways to accomplish a good grounding scheme in the board layout.  Please find below a link with a detailed discussion covering board layout recommendations and ground considerations.   If you wish to post the board layout we could review.

Best Regards,

Luis 

http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/grounding-techniques.aspx