MMC2107 Datasheet, PDF (492/618 Pages) –

Freescale Semiconductor, Inc.
Queued Analog-to-Digital Converter (QADC)
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 economic 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 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 approach is to star-point the different grounds at
the power supply origin, thus keeping the ground isolated. Refer to
Figure 18-50.
Another approach is to star-point the different grounds near the analog
ground pin 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.
Technical Data
492
Queued Analog-to-Digital Converter (QADC)
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MMC2107 – Rev. 2.0
MOTOROLA