Motorola ColdFire MCF5281 - Manual (Page 599)

Queued Analog-to-Digital Converter (QADC)
Freescale Semiconductor 28-61
28.9.4 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
DD
/V
SS
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.
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.
Two similar approaches to improving or eliminating the problems associated with grounding excess
transient currents involve star-point ground systems. One approach is to star-point the different grounds at
the power supply origin, thus keeping the ground isolated. Refer to
Figure 28-49 .
Another approach is to star-point the different grounds near the analog ground signal 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.
NOTE
This star-point scheme still requires adequate grounding for digital and
analog subsystems in addition to the star-point ground.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3