MMC2114 MOTOROLA [Motorola, Inc], MMC2114 Datasheet - Page 515

• Manufacturer Part Number: MMC2114
• Description: M CORE Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

19.11.4 Analog Supply Filtering and Grounding
MMC2114 • MMC2113 • MMC2112 — Rev. 1.0
MOTOROLA
Two important factors influencing performance in analog integrated
circuits are supply filtering and grounding. Generally, digital circuits use
bypass capacitors on every V
subsystems and submodules also. Equally important as bypassing is the
distribution of power and ground.
Analog supplies should be isolated from digital supplies as much as
possible. This necessity stems from the higher performance
requirements often associated with analog circuits. Therefore, deriving
an analog supply from a local digital supply is not recommended.
However, if for cost reasons digital and analog power are derived from a
common regulator, filtering of the analog power is recommended in
addition to the bypassing of the supplies already mentioned. For
example, 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 (for example, 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 standalone 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
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.
Freescale Semiconductor, Inc.
For More Information On This Product,
Queued Analog-to-Digital Converter (QADC)
Go to: www.freescale.com
DD
/V
SS
Queued Analog-to-Digital Converter (QADC)
pin pair. This applies to analog
Pin Connection Considerations
Advance Information
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