Datasheet

 2011 Microchip Technology Inc. DS39931D-page 387
PIC18F46J50 FAMILY
23.2 Comparator Operation
A single comparator is shown in Figure 23-2, along with
the relationship between the analog input levels and
the digital output. When the analog input at VIN+ is less
than the analog input, V
IN-, the output of the compara-
tor is a digital low level. When the analog input at V
IN+
is greater than the analog input, V
IN-, the output of the
comparator is a digital high level. The shaded areas of
the output of the comparator in Figure 23-2 represent
the uncertainty due to input offsets and response time.
FIGURE 23-2: SINGLE COMPARATOR
23.3 Comparator Response Time
Response time is the minimum time, after selecting a
new reference voltage or input source, before the
comparator output has a valid level. The response time
of the comparator differs from the settling time of the
voltage reference. Therefore, both of these times must
be considered when determining the total response to
a comparator input change. Otherwise, the maximum
delay of the comparators should be used (see
Section 30.0 “Electrical Characteristics”).
23.4 Analog Input Connection
Considerations
Figure 23-3 provides a simplified circuit for an analog
input. Since the analog pins are connected to a digital
output, they have reverse biased diodes to V
DD and VSS.
The analog input, therefore, must be between V
SS and
V
DD. If the input voltage deviates from this range by
more than 0.6V in either direction, one of the diodes is
forward biased and a latch-up condition may occur. A
maximum source impedance of 10 k is recommended
for the analog sources. Any external component con-
nected to an analog input pin, such as a capacitor or a
Zener diode, should have very little leakage current.
FIGURE 23-3: COMPARATOR ANALOG INPUT MODEL
Output
V
IN-
V
IN+
–
+
VIN+
V
IN-
Output
VA
RS < 10k
A
IN
CPIN
5 pF
VDD
VT = 0.6V
VT = 0.6V
RIC
ILEAKAGE
±100 nA
V
SS
Legend: CPIN = Input Capacitance
V
T = Threshold Voltage
I
LEAKAGE = Leakage Current at the pin due to various junctions
R
IC = Interconnect Resistance
R
S = Source Impedance
VA = Analog Voltage
Comparator
Input