LT1715
20
1715fa
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
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www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
LT 1008 REV A • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
LT1016 UltraFast Precision Comparator Industry Standard 10ns Comparator
LT1116 12ns Single Supply Ground-Sensing Comparator Single Supply Version of LT1016
LT1394 7ns, UltraFast, Single Supply Comparator 6mA Single Supply Comparator
LT1711/LT1712 4.5ns, 3V/5V/±5V Single/Dual Rail-to-Rail Comparators UltraFast Rail-to-Rail Input and Output Comparator
LT1713/LT1714 7ns, Low Power, 3V/5V/±5V Single/Dual Rail-to-Rail Comparators Rail-to-Rail Input and Output Comparator
LT1719 4.5ns Single Supply 3V/5V Comparator Single Comparator Similar to the LT1715
LT1720/LT1721 Dual/Quad 4.5ns, Single Supply 3V/5V Comparator Dual/Quad Comparator Similar to the LT1715
High Performance Sine Wave
to Square Wave Converter
Propagation delay of comparators is typically specifi ed fora
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 12a is a simple LT1715-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 crosstalk.
Figure 12b shows the time delay vs input amplitude with a
10MHz sine wave. The LT1715 delay changes just 0.65ns
over the 26dB amplitude range; 2.33° at 10MHz. The delay
is particularly fl at yielding excellent AM rejection 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 fre-
quencies 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 LT1715 act like a comparator with a 12mV hysteresis
span. In other words, a 12mV
P-P
sine wave at 10MHz will
barely toggle the LT1715, with 90° of phase lagor 25ns
additional delay.
Above 5V
P-P
at 10MHz, the LT1715 delay starts to decrease
due to internal capacitive feed-forward in the input stage.
Unlike some comparators, the LT1715 will not falsely an-
ticipate a change in input polarity, but the feed-forward is
enough to make a transition propagate through the LT1715
faster once the input polarity does change.
Figure 12a. LT1715-Based Sine Wave to Square Wave Converter Figure 12b. Time Delay vs Sine Wave Input Amplitude
–
+
1/2 LT1715
50Ω
1715 F12a
SINE WAVE
INPUT
SQUARE WAVE
OUTPUT
5V
–5V
3V
INPUT AMPLITUDE (dBm)
–5
0
TIME DELAY (ns)
1
2
3
4
5
0
51015
1715 F12b
20 25
632mV
P-P
2V
P-P
6.32V
P-P
25°C
V
CC
= 5V
V
EE
= –5V
+V
S
= 3V
10MHz
