Analog Supply Filtering And Grounding; Gated Mode, Continuous Scan Timing - Motorola ColdFire MCF5281 User Manual

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27.9.4 Analog Supply Filtering and Grounding

Two important factors influencing performance in analog integrated circuits are supply
filtering and grounding. Generally, digital circuits use bypass capacitors on every V
signal pair. This applies to analog 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.
MOTOROLA
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Figure 27-48. Gated Mode, Continuous Scan Timing
Chapter 27. Queued Analog-to-Digital Converter (QADC)
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Signal Connection Considerations
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