LT1720 Linear Technology, LT1720 Datasheet - Page 14

• Manufacturer Part Number: LT1720
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
LT1720/LT1721
fast enough that the absolute dispersion of 2.5ns
(= 7 – 4.5) is often small enough to ignore.
The gain and hysteresis stage of the LT1720/LT1721 is
simple, short and high speed to help prevent parasitic
oscillations while adding minimum dispersion. This in-
ternal “self-latch” can be usefully exploited in many
applications because it occurs early in the signal chain, in
a low power, fully differential stage. It is therefore highly
immune to disturbances from other parts of the circuit,
either in the same comparator, on the supply lines, or
from the other comparator(s) in the same package. Once
a high speed signal trips the hysteresis, the output will
respond, after a fixed propagation delay, without regard
to these external influences that can cause trouble in
nonhysteretic comparators.
The input trip points are tested using the circuit shown in
the Test Circuits section that precedes this Applications
Information section. The test circuit uses a 1kHz triangle
wave to repeatedly trip the comparator being tested. The
LT1720/LT1721 output is used to trigger switched capaci-
tor sampling of the triangle wave, with a sampler for each
direction. Because the triangle wave is attenuated 1000:1
and fed to the LT1720/LT1721’s differential input, the
sampled voltages are therefore 1000 times the input trip
voltages. The hysteresis and offset are computed from the
trip points as shown.
Crystal Oscillators
A simple crystal oscillator using one comparator of an
LT1720/LT1721 is shown on the first page of this data
sheet. The 2k-620 resistor pair set a bias point at the
comparator’s noninverting input. The 2k-1.8k-0.1 F path
sets the inverting input node at an appropriate DC average
level based on the output. The crystal’s path provides
resonant positive feedback and stable oscillation occurs.
Although the LT1720/LT1721 will give the correct logic
output when one input is outside the common mode
range, additional delays may occur when it is so operated,
opening the possibility of spurious operating modes.
Therefore, the DC bias voltages at the inputs are set near
the center of the LT1720/LT1721’s common mode range
and the 220
14
V
TRIP
Test Circuit
resistor attenuates the feedback to the
U
INFORMATION
U
W
U
noninverting input. The circuit will operate with any AT-cut
crystal from 1MHz to 10MHz over a 2.7V to 6V supply
range. As the power is applied, the circuit remains off until
the LT1720/LT1721 bias circuits activate, at a typical V
of 2V to 2.2V (25 C), at which point the desired frequency
output is generated.
The output duty cycle for this circuit is roughly 50%, but
it is affected by resistor tolerances and, to a lesser extent,
by comparator offsets and timings. If a 50% duty cycle is
required, the circuit of Figure 7 creates a pair of comple-
mentary outputs with a forced 50% duty cycle. Crystals
are narrow-band elements, so the feedback to the nonin-
verting input is a filtered analog version of the square wave
output. Changing the noninverting reference level can there-
fore vary the duty cycle. C1 operates as in the previous
example, whereas C2 creates a complementary output by
comparing the same two nodes with the opposite input
polarity. A1 compares band-limited versions of the out-
puts and biases C1’s negative input. C1’s only degree of
freedom to respond is variation of pulse width; hence the
outputs are forced to 50% duty cycle. Again, the circuit
operates from 2.7V to 6V, and the skew between the edges
of the two outputs are shown in Figure 8. There is a slight
duty cycle dependence on comparator loading, so equal
capacitive and resistive loading should be used in critical
2.7V TO 6V
V
CC
620
Figure 7. Crystal Oscillator with Complementary
Outputs and 50% Duty Cycle
2k
220
1/2 LT1720
1/2 LT1720
+
–
+
–
CRYSTAL (AT-CUT)
1MHz TO 10MHz
C2
C1
0.1 F
GROUND
CASE
1.8k
2k
1k
0.1 F
LT1636
A1
+
–
100k
100k
0.1 F
OUTPUT
OUTPUT
1720 F07
CC