SPMCK16Z4MPV20 datasheet(203/500 Pages) FREESCALE

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M68HC16 Z SERIES

ANALOG-TO-DIGITAL CONVERTER

USER’S MANUAL

8-17

Grounding is the most important factor influencing analog circuit performance in mixed

signal systems (or in stand-alone analog systems). Close attention must be paid to

avoid introducing additional sources of noise into the analog circuitry. Common sourc-

es 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 ap-

proach is to star-point the different grounds at the power supply origin, thus keeping

the ground isolated. Refer to Figure 8-6.

Figure 8-6 Star-Ground at the Point of Power Supply Origin

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.

ADC

VDD

VSS

ANALOG POWER SUPPLY

+5V

AGND

DIGITAL POWER SUPPLY

+5V

PGND

PCB

ADC POWER SCHEM

Freescale Semiconductor, Inc.

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SPMCK16Z4MPV20 Datasheet(PDF) 203 Page - Freescale Semiconductor, Inc