AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 15

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

Comparator Input
Offset Voltage
AN1740
MOTOROLA
NOTE:
Be sure to calculate the common mode range over the range of voltage
levels expected for V
comparators within the common mode range.
In practice, the common mode range can extend below V
internal input protection diode starts to conduct at about –0.3 volts (at
125 C). If the input is taken further negative, the diode conduction can
become excessive which can seriously affect analog functions. In a like
manner, the inputs can be taken up to 0.3 volts above V
upper input protection diode starts to conduct.
The voltage comparator design used in the MC68HC05JJ/JP series of
MCUs was selected for its simplicity and small silicon area on the device.
Basic comparators like this can have input voltage offsets of up to 15
millivolts. This amount of offset can be significant, especially when
monitoring signals near V
below V
change state and the comparator will look “stuck” even though both
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Figure 6. Ratiometric Sources vs. Common Mode Range
SS
to overcome the V
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6.0
5.0
4.0
3.0
2.0
1.0
DD
and always keep one input to the voltage
1.0
65% V
SS
. If one of the comparator inputs must go
DD
IO
2.0
, then the comparator’s output will not
65% V
EXCEEDS VCMR
DD
V
3.0
DD
(VOLTS)
4.0
50% V
EXCEEDS VCMR
DD
5.0
Voltage Comparators
MAX VCMR
(V
50% V
DD
DD
–1.5)
SS
Application Note
6.0
DD
before the
until the
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