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LT1720/LT1721

APPLICATIONS INFORMATION

Figure 19. A 3ns Coincidence Detector

3.9k

0.1

µF

Ω

300

Ω

300

Ω

× 1N5711

MRF501

OUTPUT

GROUND

CASE LEAD

COINCIDENCE COMPARATORS

300ps AND GATE

1720/21 F19

1/2 LT1720

–

1/2 LT1720

Coincidence Detector

High speed comparators are especially suited for interfac-

ing pulse-output transducers, such as particle detectors,

to logic circuitry. The matched delays of a monolithic dual

are well suited for those cases where the coincidence of

two pulses needs to be detected. The circuit of Figure 19

is a coincidence detector that uses an LT1720 and discrete

components as a fast AND gate.

The reference level is set to 1V, an arbitrary threshold. Only

when both input signals exceed this will a coincidence be

detected. The Schottky diodes from the comparator out-

puts to the base of the MRF-501 form the AND gate, while

the other two Schottkys provide for fast turn-off. A logic

AND gate could instead be used, but would add consider-

ably more delay than the 300ps contributed by this dis-

crete stage.

This circuit can detect coincident pulses as narrow as 3ns.

For narrower pulses, the output will degrade gracefully,

responding, but with narrow pulses that don’t rise all the

way to “high” before starting to fall. The decision delay is

4.5ns with input signals 50mV or more above the refer-

ence level. This circuit creates a TTL compatible output but

it can typically drive CMOS as well.

For a more detailed description of the operation of this

circuit, see Application Note 75, pages 10 and 11.

LT1721 Datasheet(PDF) 22 Page - Linear Technology

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