Motorola MPC533 Reference Manual page 515

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 13-49.
Another approach is to star-point the different grounds near the analog ground signal on the
microcontroller by using small traces for connecting the non-analog grounds to the analog
ground. The small traces are meant only to accommodate DC differences, not AC
transients.
This star-point scheme still requires adequate grounding for
digital and analog subsystems in addition to the star-point
ground.
Other suggestions for PCB layout in which the QADC64E is employed include:
• Analog ground must be low impedance to all analog ground points in the circuit.
• Bypass capacitors should be as close to the power signals as possible.
The analog ground should be isolated from the digital ground. This can be done by cutting
a separate ground plane for the analog ground
• Non-minimum traces should be utilized for connecting bypass capacitors and filters
to their corresponding ground/power points.
• Distance for trace runs should be minimized where possible
MOTOROLA Chapter 13. Queued Analog-to-Digital Converter Legacy Mode Operation
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
QADC64E Integration Requirements
NOTE
13-71