AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 18

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
18
NOTE:
Therefore, the comparator will now change states if the voltage on V
near 15 mV.
In this discussion, the offset voltage has been described with respect to
the negative input of the comparator, but in practice this offset may be
with respect to either input of the comparator. Therefore, the resulting
output states for each state of the INV bit gives the user an indication of
the relative voltages on V
The two conditions where the comparator outputs change states based
on the state of the INV bit can be further analyzed to show that each of
these conditions is unique to a given polarity of the input offset voltage.
The user, therefore, can determine the polarity of the input offset voltage
for any particular comparator by simply connecting both of its inputs to
the same voltage and then checking the comparator output state for
each state of the INV bit versus
V
connected to a source impedance greater than 2 k . In this case, the
user should first clear the associated port B data bit and then set the
associated data direction bit. Another approach to tie together the inputs
of comparator 1 is to set the MUX2 and MUX3 bits and clear the HOLD,
DHOLD, MUX1, MUX4, and VREF bits in the AMUX register. This will
connect the PB2 and PB3 pins by connecting them both to the channel
multiplexer, which also has been isolated from other sources.
When operating with one comparator input near V
that the input always be tested with the INV bit both set and clear. If the
results are different, then the inputs are both within V
It is impossible to control the outputs of the comparators when changing
the state of the INV bit since it interchanges the inputs and inverts the
output of both comparators at the same time. Therefore, the comparator
SS
Freescale Semiconductor, Inc.
by using the associated port B pins as outputs, if the inputs are
For More Information On This Product,
Output state of comparators
INV = 0
0
1
Table 2. Results Using INV Bit on Comparators
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INV = 1
P
and V
1
0
Table
N
as given in
2. The inputs can both be tied to
Case B, negative, V
Case A, positive, V
Polarity of input offset
Table
(see
SS
, it is recommended
Figure
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2.
of V
P
P
= V
7)
= V
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MOTOROLA
N
.
N
+ V
– V
AN1740
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IO
P
is