Datasheet LT1720, LT1721 (Analog Devices) - 28

LT1720/LT1721
TYPICAL APPLICATION Pulse Stretcher
capacitor C now begins charging through R and, at the For detecting short pulses from a single sensor, a pulse end of 100ns, C2 resets low. The output of C1 also goes stretcher is often required. The circuit of Figure 20 acts as low, latching both outputs low. A new pulse at the input a one-shot, stretching the width of an incoming pulse to a of C1 can now restart the process. Timing capacitor C can consistent 100ns. Unlike a logic one-shot, this LT1720-based be increased without limit for longer output pulses. circuit requires only 100pV-s of stimulus to trigger. This circuit has an ultimate sensitivity of better than The circuit works as follows: Comparator C1 functions as 14mV with 5ns to 10ns input pulses. It can even detect a threshold detector, whereas comparator C2 is configured an avalanche generated test pulse of just 1ns duration as a one-shot. The first comparator is prebiased with a with sensitivity better than 100mV.6 It can detect short threshold of 8mV to overcome comparator and system events better than the coincidence detector of Figure 14 offsets and establish a low output in the absence of an because the one-shot is configured to catch just 100mV of input signal. An input pulse sends the output of C1 high, upward movement from C1’s VOL, whereas the coincidence which in turn latches C2’s output high. The output of C2 detector’s 3ns specification is based on a full, legitimate is fed back to the input of the first comparator, causing logic high, without the help of a regenerative one-shot. regeneration and latching both outputs high. Timing 6 See Linear Technology Application Note 47, Appendix B. This circuit can detect the output of the pulse generator described after 40dB attenuation. 5V 15k 0.01μF – OUTPUT PULSE SOURCE C1 50Ω +1/2 LT1720 100ns 51Ω 24Ω R 1k 6.8k C 100pF – 1N5711 C2 1/2 LT1720+ 2k 2k 17201 F20 2k
Figure 20. A 1ns Pulse Stretcher RELATED PARTS 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 17201fc 28 Linear Technology Corporation LT 0908 REV C • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1998