mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 225

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

MC68HC16Y3/916Y3
USER’S MANUAL
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 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 10-6.
PCB
Figure 10-6 Star-Ground at the Point of Power Supply Origin
+5V
ANALOG POWER SUPPLY
Freescale Semiconductor, Inc.
AGND
ADC
For More Information On This Product,
ANALOG-TO-DIGITAL CONVERTER
Go to: www.freescale.com
+5V
VDD
VSS
DIGITAL POWER SUPPLY
PGND
+5V
MOTOROLA
ADC POWER SCHEM
10-17