AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 14

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

Application Note
Comparator
Common Mode
Range
14
Case 7 uses comparator 2 as a simple function to detect when the
voltage on PB0 rises above the internal temperature sensing diode. In
this case, port B would be configured with PB0 as an input. Normally, the
user would deactivate the input pulldown on PB0, but there is no internal
reason that it could not remain active if the external voltage source to
PB0 can supply up to 300 A of current to the pulldown.
Case 8 shows how both comparators can be tied to a common signal by
using the internal channel MUX and leaving the HOLD MUX enabled.
Notice that the negative input of comparator 2 can be tied to either the
positive or negative input of comparator 1 by MUX2 or MUX3. If the
positive input of comparator 2 needs to be tied to either input of
comparator 1, then an external connection can be used.
The comparators can determine only which input is higher as long as at
least one input stays within the common mode range from V
V
comparator’s output will be undefined. It is important to constrain at least
one input into the common mode range for reliable operation. Taking the
input above V
divided input to comparator 2.
Care must be taken when using sources ratiometric to V
that the common mode limits are not exceeded as the V
changes. When the supply voltage decreases, the common mode range
decreases at a more rapid rate than the percentage drop in V
example, consider an input source that only varies between 50% and
65% of V
the input will stay between 2.50 to 3.25 volts which are both within the
common mode limits. However, the common mode range is defined by
a fixed voltage drop below V
range of the ratiometric input is now at 2.79 volts, which is right at the
limit of the common mode range. As V
become greater than V
operating limit of 2.7 volts, the input source range is completely above
the common mode range.
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– 1.5 volts. If both inputs should fall outside this range, then the
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– 1.5 volts is only recommended when using the
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– 1.5 volts. As V
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. As V
6, when the V
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falls to 4.29 volts, the upper
falls farther, the input can
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approaches the minimum
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level is at 5.0 volts,
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level
to ensure
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