12

LT1671

©LINEAR TECHNOLOGY CORPORATION 1998

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

1671fs, sn1671 LT/TP 0499 4K • PRINTED IN USA

PART NUMBER

DESCRIPTION

COMMENTS

LT1016

UltraFast

Industry Standard 10ns Comparator

LT1116

12ns Single Supply Ground-Sensing Comparator

Single Supply Version of LT1016

LT1394

UltraFast Single Supply Comparator

7ns, 6mA Single Supply Comparator

LT1720

UltraFast Dual Single Supply Comparator

Dual 4.5ns, 4mA Single Supply Comparator

UltraFast is a trademark of Linear Technology Corporation.

RELATED PARTS

4MHz Adaptive Trigger Circuit

Line and fiber-optic receivers often require an adaptive

trigger to compensate for variations in signal amplitude

and DC offsets. The circuit in Figure 9 triggers on 2mV to

175mV signal from 100Hz to 4MHz while operating from

a single 5V rail. A1, operating at a gain of 15, provides

wideband AC gain. The output of this stage biases a 2-way

peak detector (Q1 through Q4). The maximum peak is

stored in Q2’s emitter capacitor, while the minimum

excursion is retained in Q4’s emitter capacitor. The DC

value of the midpoint of A1’s output signal appears at the

junction of the 500pF capacitor and the 3M

Ω units. This

point always sits midway between the signal’s excursions,

regardless of absolute amplitude. This signal-adaptive

voltage is buffered by A2 to set the trigger voltage at the

Figure 9. 4MHz Single Supply Adaptive Trigger. Output Comparator’s Threshold Varies Ratiometrically with

Input Amplitude, Maintaining Data Integrity over >85:1 Input Amplitude Range

LT1671’s positive input. The LT1671’s negative input is

biased directly from A1’s output. The LT1671’s output, the

circuit’s output, is unaffected by > 85:1 signal amplitude

variations. Bandwidth limiting in A1 does not affect trig-

gering because the adaptive trigger threshold varies

ratiometrically to maintain circuit output.

Figure 10 shows operating waveforms at 4MHz. Trace A’s

input produces Trace B’s amplified output at A1. The

comparator’s output is Trace C.

5V

–

+

A1

LT1227

A2

LT1006

INPUT

5V

5V

5V

TRIGGER

OUT

1671 F09

500pF

0.1

µF

510

ΩΩ

470

ΩΩ

470

Ω

750

ΩΩ

36

ΩΩ

1

3

2

4

14

13

15

5

6

12

10

11

2k

10

µF

2k

2k

0.1

µF

0.1

µF

0.005

µF

0.005

µF

100

µF

+

Q1, Q2, Q3, Q4 = CA3096 ARRAY: TIE SUBSTRATE (PIN 16) TO GROUND

= 1N4148

LT1671

Q1

Q2

Q3

Q4

3M

3M

Figure 10. Adaptive Trigger

Responding to a 4MHz, 5mV Input.

Input Amplitude Variations from 2mV

to 175mV Are Accommodated

A = 10mV/DIV

B = 50mV/DIV

C = 1mV/DIV

50ns/DIV

1671F10