Datasheet

AD8611/AD8612
Rev. A | Page 12 of 20
voltage is greater than V
HI
, and does not switch high again until
the input voltage is less than V
LO
, as given in Equation 1:
()
REFREFHI
V
RR
R
VV
+
+
−+=
21
1
5.1V
(1)
21
2
RR
R
VV
REF
LO
+
×=
where V+ is the positive supply voltage.
The capacitor C
F
is optional and can be added to introduce a
pole into the feedback network. This has the effect of increasing
the amount of hysteresis at high frequencies, which is useful
when comparing relatively slow signals in high frequency noise
environments. At frequencies greater than f
P
, the hysteresis
window approaches V
HI
= V+ − 1.5 V and V
LO
= 0 V. For
frequencies less than f
P
, the threshold voltages remain as in
Equation 1.
CLOCK TIMING RECOVERY
Comparators are often used in digital systems to recover clock
timing signals. High speed square waves transmitted over any
distance, even tens of centimeters, can become distorted due to
stray capacitance and inductance. Poor layout or improper
termination can also cause reflections on the transmission line,
further distorting the signal waveform. A high speed
comparator can be used to recover the distorted waveform
while maintaining a minimum of delay.
Figure 25 shows V
OUT
vs. V
IN
as the AD8611 is used to recover a
65 MHz, 100 mV peak-to-peak distorted clock signal into a 4 V
peak-to-peak square wave. The lower trace is the input to the
AD8611, and the upper trace is the QA or QB output from the
comparator. The AD8611 is powered from a 5 V single supply.
TIME (10ns/DIV)
2V/DIV
V
OUT
V
IN
20mV/DIV
06010-022
Figure 25. Using the AD8611 to Recover a Noisy Clock Signal
A 5 V, HIGH SPEED WINDOW COMPARATOR
A window comparator circuit is used to detect when a signal is
between two fixed voltages. The AD8612 can be used to create a
high speed window comparator, as shown in
Figure 26. In this
example, the reference window voltages are set as:
43
4
21
2
RR
R
V
RR
R
V
LO
HI
+
=
+
=
The output of the A1 comparator goes high when the input
signal exceeds V
HI
, and the output of A2 goes high only when
V
IN
drops below V
LO
. When the input voltage is between V
HI
and V
LO
, both comparator outputs are low, turning off both Q1
and Q2, thus driving V
OUT
to a high state. If the input signal
goes outside of the reference voltage window, V
OUT
goes low.
To ensure a minimum of switching delay, the use of high speed
transistors is recommended for Q1 and Q2. Using the AD8612
with 2N3960 transistors provides a total propagation delay from
V
IN
to V
OUT
of less than 10 ns.
Table 8. Window Comparator Output States
V
OUT
Input Voltage
≈ 200 mV V
IN
< V
LO
+5 V V
LO
< V
IN
< V
HI
≈ 200 mV V
IN
> V
HI
V
HI
5V
5V
R1
R2
6
7
3
4
10
1
V
IN
AD8612
1kΩ
V
LO
5V
R3
R4
9
8
11
5
12
14
AD8612
1kΩ
Q1
5
V
V
OUT
1kΩ
500Ω
Q2
500Ω
NOTES
1. Q1, Q2 = 2N3960.
2. PINS 2 AND 13 ARE NO CONNECTS.
A1
A2
0
6010-023
Figure 26. A High Speed Window Comparator