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AD8611/AD8612
Rev. A | Page 12 of 20
voltage is greater than VHI, and does not switch high again until
the input voltage is less than VLO, as given in Equation 1:
()
REF
HI
V
R
V
+
+
=
2
1
5
.
1
V
(1)
2
1
2
R
V
REF
LO
+
×
=
where V+ is the positive supply voltage.
The capacitor CF 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 fP, the hysteresis
window approaches VHI = V+ 1.5 V and VLO = 0 V. For
frequencies less than fP, 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 VOUT vs. VIN 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
VOUT
VIN
20mV/DIV
06
01
0-
02
2
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:
4
3
4
2
1
2
R
V
R
V
LO
HI
+
=
+
=
The output of the A1 comparator goes high when the input
signal exceeds VHI, and the output of A2 goes high only when
VIN drops below VLO. When the input voltage is between VHI
and VLO, both comparator outputs are low, turning off both Q1
and Q2, thus driving VOUT to a high state. If the input signal
goes outside of the reference voltage window, VOUT 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
VIN to VOUT of less than 10 ns.
Table 8. Window Comparator Output States
VOUT
Input Voltage
≈ 200 mV
VIN < VLO
+5 V
VLO < VIN < VHI
≈ 200 mV
VIN > VHI
VHI
5V
R1
R2
6
7
3
4
10
1
VIN
AD8612
1k
VLO
5V
R3
R4
9
8
11
5
12
14
AD8612
1k
Q1
5V
VOUT
1k
500
Q2
500
NOTES
1. Q1, Q2 = 2N3960.
2. PINS 2 AND 13 ARE NO CONNECTS.
A1
A2
0
60
10
-02
3
Figure 26. A High Speed Window Comparator