LT1719
22
1719fa
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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FAX: (408) 434-0507
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www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2000
LT 0809 REV A • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
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LT1671 60ns, Low Power, Single Supply Comparator 450μA Single Supply Comparator
LT1713/LT1714 Single/Dual 7ns, Rail-to-Rail Comparator Rail-to-Rail Inputs and Outputs
LT1720/LT1721 Dual/Quad 4.5ns, Single Supply 3V/5V Comparator Dual/Quad Comparator Similar to the LT1719
High Performance Sine Wave
to Square Wave Converter
Propagation delay of comparators is typically specifi ed for
a 100mV step with some fraction of that for overdrive. But
in many signal processing applications, such as in com-
munications, the goal is to convert a sine wave, such as
a carrier, to a square wave for use as a timing clock. The
desired behavior is for the output timing to be dependent
on the input timing only. No phase shift should occur as
a function of the input amplitude, which would result in
AM to FM conversion.
The circuit of Figure 9a is a simple LT1719S8-based sine
wave to square wave converter. The ± 5V supplies on the
input allow very large swing inputs, while the 3V logic
supply keeps the output swing small to minimize cross
talk. Figure 9b shows the time delay vs input amplitude
with a 10MHz sine wave. The LT1719 delay changes just
0.65ns over the 26dB amplitude range; 2.33° at 10MHz.
The delay is particularly fl at yielding excellent AM rejection
Figure 9a. LT1719-Based Sine Wave to Square Wave Converter
from 0dBm to 15dBm. If a 2:1 transformer is used to drive
the input differentially, this exceptionally fl at zone spans
–5dBm to 10dBm, a common range for RF signal levels.
Similar delay performance is achieved with input frequen-
cies as high as 50MHz. There is, however, some additional
encroachment into the central fl at zone by both the small
amplitude and large amplitude variations.
With small input signals, the hysteresis and dispersion
make the LT1719 act like a comparator with a 12mV
hysteresis span. In other words, a 12mV
P-P
sine wave at
10MHz will barely toggle the LT1719, with 90° of phase
lag or 25ns additional delay.
Above 5V
P-P
at 10MHz, the LT1719 delay starts to decrease
due to internal capacitive feed-forward in the input stage.
Unlike some comparators, the LT1719 will not falsely an-
ticipate a change in input polarity, but the feed-forward is
enough to make a transition propagate through the LT1719
faster once the input polarity does change.
–
+
LT1719S8
50Ω
1719 F09a
SINE WAVE
INPUT
SQUARE WAVE
OUTPUT
5V
–5V
3V
INPUT AMPLITUDE (dBm)
–5
0
TIME DELAY (ns)
1
2
3
4
5
0
51015
1719 F09b
20 25
632mV
P-P
2V
P-P
6.32V
P-P
25°C
V
CC
= 5V
V
EE
= –5V
+V
S
= 3V
10MHz
Figure 9b. Time Delay vs Sine Wave Input Amplitude
