16

LT1719

1719f LT/TP 0300 4K • PRINTED IN USA

©LINEAR TECHNOLOGY CORPORATION 2000

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

High Performance Sine Wave

to Square Wave Converter

Propagation delay of comparators is typically specified for

a 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 8a is a simple LT1719-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 cross

talk. Figure 8b shows the time delay vs input amplitude

with a 10MHz sine wave. The LT1719 delay changes just

0.65ns over the 26dB amplitude range; 2.33

° at 10MHz.

The delay is particularly flat yielding excellent AM rejection

Figure 8a. LT1719-Based Sine Wave to Square Wave Converter

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

450

µA Single Supply Comparator

LT1720/LT1721

Dual/Quad 4.5ns, Single Supply 3V/5V Comparator

Dual/Quad Comparator Similar to the LT1719

RELATED PARTS

from 0dBm to 15dBm. If a 2:1 transformer is used to drive

the input differentially, this exceptionally flat zone spans

– 5dBm to 10dBm, a common range for RF signal levels.

Similar delay performance is achieved with input frequen-

cies as high as 50MHz. There is, however, some additional

encroachment into the central flat zone by both the small

amplitude and large amplitude variations.

With small input signals, the hysteresis and dispersion

make the LT1719 act like a comparator with a 12mV

10MHz will barely toggle the LT1719, with 90

°ofphaselag

or 25ns additional delay.

crease due to internal capacitive feed-forward in the input

stage. Unlike some comparators, the LT1719 will not

falsely anticipate a change in input polarity, but the feed-

forward is enough to make a transition propagate through

the LT1719 faster once the input polarity does change.

LT1719

50

Ω

1719 F08a

SINE WAVE

INPUT

SQUARE WAVE

OUTPUT

5V

–5V

3V

INPUT AMPLITUDE (dBm)

–5

0

1

2

3

4

5

0

510

15

1719 F08b

20

25

25

°C

10MHz

Figure 8b. Time Delay vs Sine Wave Input Amplitude