MC68HC16Z1 Datasheet, PDF (203/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series

Freescale Semiconductor, Inc.
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 to
avoid introducing additional sources of noise into the analog circuitry. Common sourc-
es 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 pin. 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 ap-
proach is to star-point the different grounds at the power supply origin, thus keeping
the ground isolated. Refer to Figure 8-6.
ANALOG POWER SUPPLY
DIGITAL POWER SUPPLY
+5V
AGND
+5V
PGND
+5V
VSS
ADC
VDD
PCB
ADC POWER SCHEM
Figure 8-6 Star-Ground at the Point of Power Supply Origin
Another approach is to star-point the different grounds near the analog ground pin 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.
M68HC16 Z SERIES
USER’S MANUAL
ANALOG-TO-DIGITAL CONVERTER
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