Hiya,
I've been having conversations with colleagues about the best (lowest noise?) ways to connect AGND to DGND on primarily "analog" devices such as the ADS127L01.
The datasheet says this:
"The ground plane can be split into an analog plane (AGND) and digital plane (DGND), but this split is not necessary. Place analog signals over the analog plane and digital signals over the digital plane. As a final step in the layout, completely remove the split between the analog and digital grounds. If ground plane separation is necessary, make the connection between AGND and DGND as close to the ADC as possible."
Fair enough, I've read this other places as well, that on an "analog" chip with digital I/O, connect AGND and DGND together as close as possible to the chip on the "analog GND" plane.
BUT... The ADS127L01EVM doesn't appear take this advice. See Figure 19 of: www.ti.com/.../sbau261b.pdf, there are two planes, one connected to DGND, and a separate one for AGND under the ADS127L01. They appear to be shorted together through the board using the plane on the bottom and the GND testpoints which are not near the ADSL127L01. So, why did eval board layout not the datasheet advice? It is not a single physical plane and it is not connected close to the ADC (or am I reading the eval board incorrectly?).
Thanks!