AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 16

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

Application Note
Wide Bandwidth
Input Signals
Other Error
Sources
16
The reference to input terminal noise must be distinguished between
noise externally superimposed on the input signal lines that is measured
between a system reference and a given input signal (occurs from
capacitive coupling between high-impedance ADC inputs and noisy
signal sources or electro-magnetic interference) and voltage differentials
experienced by different comparator inputs when referenced to each
other. The importance of input terminal noise in this context is the
presence of non-common-mode differential noise between the biasing
networks in the comparator and the input lines. If, under noisy
conditions, the same noise is presented to an input to the comparator
and one of the supply (or other input) terminals, the common-mode
rejection ratio (CMRR) capabilities of the comparator may prevent
performance perturbations; whereas, noise presented to either terminal,
with respect to system ground, may cause havoc (see
Input
be reduced further, is also seen by the other comparator inputs to take
advantage of the CMRR. Of the ADC error sources, this is one of the
most challenging to control in a practical and effective manner.
A certain way to disrupt ADC function is to give the wide bandwidth
comparator something to respond to other than the input signal of
interest. By designing the electronics feeding the ADC inputs to pass
input signals having frequencies that range from dc to purple, ADC
problems are usually guaranteed. Thus, this fourth area is a common
source of ADC malfunction.
Although occurring less frequently and more subtly, other error sources
can also impact ADC performance: rate of conversion requests to a
particular channel and interchannel charge-sharing. These sources and
an estimate of the impact on a given M68HC11 system are presented in
detail in the M68HC11 Reference Manual, Motorola document order
number M68HC11RM/AD.
Freescale Semiconductor, Inc.
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Signals). Efforts should be made to ensure that noise, if it cannot
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Wide Bandwidth
MOTOROLA
AN1058