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Signal Connection Considerations
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 associated current can return to ground through the analog ground.
It is this 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 pins. The end result is that the ground observed by the analog
circuit is no longer true ground and thus skews converter performance.
Two similar approaches to improving or eliminating 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 27-49.
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.
Analog Power Supply
+5 V
PCB
Figure 27-49. Star-Ground at the Point of Power Supply Origin
Other suggestions for PCB layout in which the QADC is employed include:
27-68
NOTE
AGND
+5 V
V
SS
QADC
V
DD
MCF5282 User's Manual
Digital Power Supply
PGND
+5 V
MOTOROLA
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