Motorola MPC533 Reference Manual page 596

QADC64E Integration Requirements
An RC low pass filter could be used to isolate the digital and
analog supplies when generated by a common regulator. If
multiple high precision analog circuits are locally employed
(i.e., two A/D converters), the analog supplies should be
isolated from each other as sharing supplies introduces the
potential for interference between analog circuits.
Grounding is the most important factor influencing analog circuit performance in mixed
signal systems (or in stand-alone analog systems). Close attention must be paid not to
introduce additional sources of noise into the analog circuitry. Common sources of noise
include ground loops, inductive coupling, and combining digital and analog grounds
together inappropriately.
The problem of how and when to combine digital and analog grounds arises from the large
transients which the digital ground must handle. If the digital ground is not able to handle
the large transients, the current from the large transients can return to ground through the
analog ground. It is the excess current overflowing into the analog ground which causes
performance degradation by developing a differential voltage between the true analog
ground and the microcontroller's ground signal. The end result is that the ground observed
by the analog circuit is no longer true ground and often ends in skewed results.
Two similar approaches designed to improve or eliminate the problems associated with
grounding excess transient currents involve star-point ground systems. One approach is to
star-point the different grounds at the power supply origin, thus keeping the ground
isolated. Refer to Figure 14-48.
14-72
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
NOTE
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