Signal Connection Considerations

TRIG1 (GATE)

EOC

QS

CWP

CSPQ1

Q1 RES

CF1

TOR1

0

LAST

LAST

XX

CCW0

8

CCW1

 

CCW2

 

CCW3

 

CCW0

CCW0

CCW1

CCW2

CCW3

R0

R1

R2

R3

Queue Restart

Queue Restart

CCW3

CCW2

R2

CCW0

CCW3

R3

Figure 27-48. Gated Mode, Continuous Scan Timing

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 VDD/VSS 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

Chapter 27. Queued Analog-to-Digital Converter (QADC)

27-67

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Motorola MCF5281, MCF5282 user manual Analog Supply Filtering and Grounding, Gated Mode, Continuous Scan Timing