Again:

Please pick TWO SEPARATE and arbitrary points on your digital ground plane. TWO SEPARATE points, please. Not ONE point as in your schematic. Maybe one point is on the left side of digital ground plane and the other is on the right side of digital ground plane.

Now assume, that a digital ground return current flows between THESE TWO points, as result of 'communication' between two separate digital chips.

And now think about, WHERELSE this current can flow between THESE TWO SEPARATE points located on digital ground plane. Of course, there's this one straight path right on the digital ground plane, where this current can flow and where this current is intended to flow. But you will find, that there's another path where this current, or at least a certain portion of this current can flow, namely from the left point (see above!) on DIGITAL ground plane to 0V terminal of +5V connector (the 'power connector' sitting on DIGITAL plane), through the interwinding capacitance to 0V terminal of +5V connector (the 'power connector' sitting on ANALOG plane), and via the connection path under your ADC back to the right point (see above!) on DIGITAL ground plane!

You will see, that the wrong placing of supply connectors on analog plane and digital plane, namely far away from the ONE AND ONLY connection between analog and digital ground is the cause of digital ground return currents flowing on analog plane. This must be avoided, by just connecting all your many and individual grounds to the 'one and only' ground point, which should be located there, where analog ground pin and digital ground pin of your ADC are connected to each other.

By the technique of this star point grounding you can force all digital ground return currents to EXCLUSIVELY flow across digital ground plane. No digital ground return current (noisy, spiky, dirty!) can then flow across analog ground plane anymore and eroding of your analog ground by these noisy, spiky and dirty digital ground return currents is prevented! That's the whole mystery of star point grounding...

Kai