TMS320F28377D: isolation between Vss and Vssa

Hi Greg,

We need to get these data from our design team, we will get back to you by next week. While we get these data, may I know what is the concern here?

Best Regards,

Nirav

Hi,

On the board where the TMS is localised, my DGND and AGND (Vss et Vssa) are isolated from each other. So if the TMS dont isolate these ground from each other my isolation is more or less useless.  

Hi Greg,

These grounds are nominally isolated from each other on the die, but current will flow between the two nets if there is a much voltage difference: about 0.3V or more.  You certainly shouldn't expect to be able to do something like connect the cold side of a power system to the digital ground and the hot side to analog ground, but for isolating the digital and analog signals for noise purposes, using separate grounds can make sense. 

The normal design recommendation would be to use separate analog and digital ground planes for best performance, and to join them at a single point directly beneath the device (either with a 0-ohm resistor, or an inductive bead). In this case it is extremely important to route the analog signals completely over the analog ground plane and the digital signals completely over the digital ground plane with no signals crossing the split between planes (otherwise the performance could become much worse than using a single combined ground plane). 

Alternately, using a single ground plane, but taking care to physically separate the analog and digital signals is also acceptable for many applications.