LT1720/LT1721
28
17201fc
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 1998
LT 0908 REV C • 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
LT1671 60ns, Low Power, Single Supply Comparator 450μA Single Supply Comparator
LT1715 4ns, 150MHz Dual Comparator Similar to the LT1720 with Independent Input/Output Supplies
LT1719 4.5ns Single Supply 3V/5V Comparator Single Comparator Similar to the LT1720/LT1721
Pulse Stretcher
For detecting short pulses from a single sensor, a pulse
stretcher is often required. The circuit of Figure 20 acts as
a one-shot, stretching the width of an incoming pulse to a
consistent 100ns. Unlike a logic one-shot, this LT1720-based
circuit requires only 100pV-s of stimulus to trigger.
The circuit works as follows: Comparator C1 functions as
a threshold detector, whereas comparator C2 is configured
as a one-shot. The first comparator is prebiased with a
threshold of 8mV to overcome comparator and system
offsets and establish a low output in the absence of an
input signal. An input pulse sends the output of C1 high,
which in turn latches C2’s output high. The output of C2
is fed back to the input of the first comparator, causing
regeneration and latching both outputs high. Timing
capacitor C now begins charging through R and, at the
end of 100ns, C2 resets low. The output of C1 also goes
low, latching both outputs low. A new pulse at the input
of C1 can now restart the process. Timing capacitor C can
be increased without limit for longer output pulses.
This circuit has an ultimate sensitivity of better than
14mV with 5ns to 10ns input pulses. It can even detect
an avalanche generated test pulse of just 1ns duration
with sensitivity better than 100mV.
6
It can detect short
events better than the coincidence detector of Figure 14
because the one-shot is configured to catch just 100mV of
upward movement from C1’s V
OL
, whereas the coincidence
detector’s 3ns specification is based on a full, legitimate
logic high, without the help of a regenerative one-shot.
6
See Linear Technology Application Note 47, Appendix B. This circuit can detect the output of the
pulse generator described after 40dB attenuation.
–
+
–
+
PULSE SOURCE
C1
1/2 LT1720
C2
1/2 LT1720
50Ω
51Ω
6.8k
1N5711
24Ω
15k
R
1k
2k
2k
17201 F20
2k
C
100pF
0.01μF
OUTPUT
100ns
5V
Figure 20. A 1ns Pulse Stretcher
