"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Grounding
17-10
CONNECTOR
POWER
SUPPLY
VCC
Figure 17–5. A Careful Board Layout
17.3.4 A Notable Exception
CONNECTOR
ANALOG
DIGITAL
HIGH
LOW
HIGH FREQUENCY
LOW
There is one case where it is necessary to combine analog and digital signals on the analog
ground plane. Analog to digital (A/D) and digital to analog (D/A) converters are packaged
as ICs with analog and digital ground pins coming out of the package. One might
assume, based on the previous discussion, that the digital ground pin should be connected
to digital ground and the analog ground pin to analog ground. That, however, is
not correct.
The pin names analog ground and digital ground refer to internal connections in the IC,
not the plane to which they should be connected. Both should connect to the analog
ground plane. The connection would have been made inside the IC, but it is impossible
to get low enough impedance at the typical geometries inside ICs. The IC designer actually
counts on the end user to supply a low impedance connection outside the IC. Otherwise,
the performance of the converter will be worse than specified.
One might suspect that the digital portions of the converter would make circuit performance
worse by coupling digital switching noise onto the analog ground and power plane.
Converter designers realize this, and design digital portions without a lot of output power
to minimize switching transients. If the converter does not drive large fanouts, this should
not be a problem. Be sure to properly decouple the logic supply for the converter to analog
ground (see the following paragraphs).
FREQUENCY