AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 7

• Manufacturer Part Number: AN1058
• Description: Reducing A/D Errors in Microcontroller Applications
• Manufacturer: Freescale Semiconductor / Motorola

Appropriate Design Techniques
AN1058
MOTOROLA
perfectly fast comparator, implying that the response time (characterized
by bandwidth) must be reasonably fast even in the slowest ADC types.
Consequently, a wide bandwidth comparator will appropriately respond
not only to input/reference signals but also to any other signal present at
the comparator input terminals (including noise components
superimposed upon the signals of interest).
The typical comparator uses some form of differential front end. The
operation of the differential front end is dependent upon biasing
networks that are ultimately connected to the supply terminals of the
comparator. Therefore, the comparator should be considered as a
5-terminal device — two differential inputs, one output, and two inputs to
the biasing networks — for the purposes of designing with the ADC. The
implication is that signals present at the supply terminals of an ADC,
particularly the high-frequency signals typically superimposed on the
power supply in digital systems, can affect comparator and ADC
operation.
Due to the high bandwidth of the comparator found in ADCs, the
designer of a given system should be extremely careful about the type
and amount of signals allowed to reach the comparator stage of the
ADC, particularly the power supply terminals. For this reason, some of
the more mundane and overlooked aspects of electrical product design,
such as printed circuit design and circuit interconnection, become
increasingly critical to the success of the MCU/ADC system.
Most of the MCU is digital. As seen in
MC68HC11E9, a representative MCU, is digital circuitry. Thus, it is
reasonable to assume that digital design practices will generally be
employed when designing with the MCU.
With an analog-based subsystem, such as the M68HC11 ADC, normally
accepted digital design practices may not be sufficient to ensure
satisfactory performance of the converter.
Freescale Semiconductor, Inc.
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Figure
Appropriate Design Techniques
2, a major portion of the
Application Note
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