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AN1775 Datasheet(PDF) 5 Page - Freescale Semiconductor, Inc

Part No. AN1775
Description  Expanding Digital Input with an A/D Converter
Download  12 Pages
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Maker  FREESCALE [Freescale Semiconductor, Inc]
Homepage  http://www.freescale.com

AN1775 Datasheet(HTML) 5 Page - Freescale Semiconductor, Inc

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Application Note
AN1775 Rev. 1
Using this method, one can theoretically connect 255 input switches to
one ADC pin. However, there are many potential sources of
inaccuracies, which make it impractical to connect so many key inputs.
One should account for some error padding.
Resistor Precision
One inaccuracy is provided by the resistors themselves. Resistors are
categorized according to their variance from a labeled value. The
application should be tolerant to the precision of the resistors being used
(typically, 1 percent or 5 percent). Also, a calculated resistor value might
not be a commonly available value, so the user should plan for a range
of resistor values.
ADC Accuracy
Typically, an 8-bit ADC is accurate within two least significant bits. This
should be accounted for as well. The best way to allow for these
tolerances is to assign to each key switch a range of resulting A/D data
register values.
If an analog input falls within a particular range, one can determine that
the key was pressed. By adjusting the range of ADC results which
represent a given keypress, the user can change the error margin for the
To ensure the best ADC accuracy, the full range of the converter should
be used. In cases where the high reference is not variable, it is typically
fixed at the operating voltage.
A spreadsheet is a good way to determine resistor values and A/D result
ranges. An example of computing values for Figure 3 are shown in
Table 1
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com

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