AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 19

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

Comparator Input
Impedance
AN1740
MOTOROLA
NOTE:
flags, CPF1 and CPF2 in the ASR, may get set during any change in the
INV bit.
The user should use caution also when changing the state of the INV bit
if either flag bit is enabled to trigger a timer input capture or analog
interrupt. Any change to the INV bit should be done with these interrupts
disabled or during a time when an interrupt is not expected before the
flag bits can be cleared.
Changes to the INV, CP1EN, CP2EN, and ICEN bits can cause the
comparator static flags to get set. Always clear the CPF1 and CPF2
static flag bits after changing any configuration of the comparators by
writing a logical 1 to both the CPFR1 and CPFR2 bits in the ASR.
The comparators themselves have a very high input impedance, but
several other circuits connected to their inputs will lower this ideal
impedance. These currents or equivalent impedances are:
The ESD protection (1) and the parasitic capacitances (2) are always
present and will be the limiting factor on high input impedance. The
divider (3) only affects the negative input of comparator 2 and can be
removed from the input by not using the DHOLD bit in the AMUX
register. The last three sources (4, 5, 6) all can be controlled by the
1. Input protection diodes for ESD which may have leakage currents
2. Parasitic capacitances in the input and multiplexer circuitry will
3. The input divider of 2:1 has an impedance of 120 k at DC, but
4. Software-controlled input pulldown devices which can sink up to
5. More than one input multiplexer being on at the same time and
6. Output devices on port B pins being active
Freescale Semiconductor, Inc.
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up to 2 A (equivalent of 2.5 M at 5 volts)
cause the input impedance to decrease with frequency as shown
in
decreases at high frequencies due to the sample capacitor as
shown in
280 A (equivalent of 18 k at 5 volts)
connecting the comparator input to other low impedances
Figure
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Figure
9.
9.
Voltage Comparators
Application Note
19