Freescale Semiconductor, Inc.
Application Note
Implementation
V
DD
R0
REQ
V
In
REQ(N-1)
RN
Figure 4. Voltage Divider
Constants
First, V
DD
(or V
REFH
, the high reference for the converter) and the
number of keys are determined. The converter resolution is found by
dividing V
DD
by the converter precision. The number of voltage steps
needed is V
DD
divided by the number of key switches. An ADC should
always be operated at its full range. The pullup resistor, R0, typically,
should be chosen between 4.7 k (to limit current) to 22 k (to limit time
constant).
Once these constant values are decided, a spreadsheet can be used to
determine the resistor values needed in each segment of the divider
ladder, according to the desired input voltages.
When using an ADC, always use the entire range of the converter
(V
REFH
– V
REFL
). This is the reason for determining the voltage steps
first (V
DD
– n*step value) and then calculating the necessary resistor
values to achieve these voltages through the divider.
For each segment in the ladder, the n
th
segment’s necessary resistor
value needs to be calculated (Rn in the spreadsheet). When a key is
pressed, the equivalent resistance of the included segment resistors
forms a voltage divider with the pullup resistor.
Given the voltage desired at the ADC input (V
In
), the user can determine
the equivalent resistance needed to achieve that voltage by:
Req(n) = (V
In
*R0)/(V
REFH
– V
In
)
The resistor that will form the needed equivalent resistance with the
other resistors in the ladder can be determined, as:
Rn = Req(n) – Req(n – 1)
Freescale Semiconductor, Inc...
Variables
AN1775 Rev. 1
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