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LT1721IS

Dual/Quad, 4.5ns, Single Supply 3V/5V Comparators with Rail-to-Rail Outputs

Linear Technology 

LT1720/LT1721

APPLICATIONS INFORMATION

and 100µV/°C baseline uncertainty. Switching delay is

less than 15ns and the minimum sampling window width

for full power response is 30ns.

The input waveform is presented to the diode bridge

switch, the output of which feeds the LT1227 wideband

amplifier. The LT1720 comparators, triggered by the sample

command, generate phase-opposed outputs. These sig-

nals are level shifted by the transistors, providing comple-

mentary bipolar drive to switch the bridge. A skew

compensation trim ensures bridge-drive signal simulta-

neity within 1ns. The AC balance corrects for parasitic

capacitive bridge imbalances. A DC balance adjustment

trims bridge offset.

The trim sequence involves grounding the input via 50Ω

and applying a 100kHz sample command. The DC balance

is adjusted for minimal bridge ON vs OFF variation at the

output. The skew compensation and AC balance adjust-

ments are then optimized for minimum AC disturbance in

the output. Finally, unground the input and the circuit is

ready for use.

Voltage-Controlled Clock Skew Generator

It is sometimes necessary to generate pairs of identical

clock signals that are phase skewed in time. Further, it is

desirable to be able to set the amount of time skew via a

tuning voltage. Figure 18’s circuit does this by utilizing

the LT1720 to digitize phase information from a varactor-

tuned time domain bridge. A 0V to 2V control signal

provides ≈ ±10ns of output skew. This circuit operates

from a 2.7V to 6V supply.

CLOCK

INPUT

VCC

2.7V TO 6V

10ns

TRIM

“FIXED”

2k

2.5k*

2k*

14k

“SKEWED”

12pF†

MV-209

VARACTOR

DIODE

36pF†

0.005µF

= 1N4148

= 74HC04

* 1% FILM RESISTOR

** SUMIDA CD43-100

† POLYSTYRENE, 5%

INPUT

0V TO 2V ≈

±10ns

SKEW

+

A1

LT1077

–

+

C1

1/2 LT1720

–

VCC

2.5k

Q

FIXED

OUTPUT

2.5k

+

C2

1/2 LT1720

–

1M

0.1µF

Q′

SKEWED

OUTPUT

1M

47µF

VCC

+

2.2µF

6.2M*

1.82M*

L1**

VIN

SW

LT1317 FB

VC

GND

200pF

1.1M

100k

1720/21 F18

Figure 18. Voltage-Controlled Clock Skew

21

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