Datasheet

AD8564
Rev. B | Page 10 of 12
USING HYSTERESIS
Hysteresis can easily be added to a comparator through the
addition of positive feedback. Adding hysteresis to a comparator
offers an advantage in noisy environments where it is not desirable
for the output to toggle between states when the input signal is
near the switching threshold.
Figure 17 shows a method for
configuring the AD8564 with hysteresis.
COMPARATOR
SIGNAL
R1
R2
V
REF
C
F
01103-017
Figure 17. Configuring the AD8564 with Hysteresis
The input signal is connected directly to the inverting input of
the comparator. The output is fed back to the noninverting
input through R2 and R1. The ratio of R1 to R1 + R2 and the
output swing establishes the width of the hysteresis window,
with V
REF
setting the center of the window or the average
switching voltage. The output switches high when the input
voltage is greater than V
HI
and does not switch low again until
the input voltage is less than V
LO
, as given in Equation 2.
(
)
REFREFHI
V
R2R1
R1
VVV
+
−−=
+
1
(1)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−=
R2R1
R1
VV
REF
LO
1
(2)
where
V
+
is the positive supply voltage.
The C
F
capacitor may also be added to introduce a pole into
the feedback network. This has the effect of increasing the
amount of hysteresis at high frequencies. This can be useful
when comparing a relatively slow signal in a high frequency
noise environment.
At frequencies greater than
R2C
f
F
P
π
=
2
1
, the hysteresis
window approaches
V
HI
= V
+
– 1 V and V
LO
= 0 V.
At frequencies less than
f
P
, the threshold voltages remain as it is
in Equation 1.