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AN1775 Datasheet(PDF) 4 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
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AN1775 Datasheet(HTML) 4 Page - Freescale Semiconductor, Inc

 
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Application Note
AN1775 Rev. 1
The
range of an ADC is the difference between its high and low
reference voltages. This means an analog input between VREFH and
VREFL will convert to an 8-bit number, with VREFL converting to $00 and
VREFH converting to $FF.
The
resolution, defined as the range divided by the precision, defines the
analog step that a change in one least significant bit (LSB) represents.
In the case of a 5-volt, 8-bit ADC, the resolution is 5/255 (volts), or
19.6 mV. This means that a change in one LSB in the ADC data register
reflects a change of about 20 mV at the analog input.
Resistor Ladder
Voltage Divider
Consider a resistor ladder connected to an ADC input, as shown in
Figure 3
. Because this arrangement is a voltage divider, each segment
in the ladder can alter the voltage at the input when grounded. If
switches are provided at each segment, one can selectively ground that
segment, altering the composition of the divider, and thus altering the
voltage presented to the ADC pin.
In this way, software can determine which switch in the ladder was
selected by reading the resulting A/D data value. The resistor R0 acts as
a pullup to maintain VDD on the analog input line while no keys are
active. Thus, a conversion value of $FF indicates that no key has been
pressed.
Figure 3. Digital Input Alternative
VDD
VIn
ADC
INPUT
R0
R2
R3
R4
R5
R6
R7
K1
K2
K3
K4
K5
K6
K7
R8
K8
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com


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