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LT1671CMS8 Datasheet PDF : 12 Pages
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LT1671
TYPICAL APPLICATION
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 3MΩ 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
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.
A = 10mV/DIV
B = 50mV/DIV
C = 1mV/DIV
5V
2k
2k 750ΩΩ
5V
10µF +
0.1µF
5V
+
A1
LT1227
–
510ΩΩ
36ΩΩ
+
100µF
0.1µF
3
1
Q1
2
6
5
Q2
4
3M
0.005µF 500pF
13
Q3 14
15
0.005µF
10
3M
12
Q4
11
2k
INPUT
Q1, Q2, Q3, Q4 = CA3096 ARRAY: TIE SUBSTRATE (PIN 16) TO GROUND
= 1N4148
5V
+
A2
–LT1006
50ns/DIV
1671F10
Figure 10. Adaptive Trigger
Responding to a 4MHz, 5mV Input.
Input Amplitude Variations from 2mV
to 175mV Are Accommodated
470ΩΩ
0.1µF
470Ω
+
– LT1671
TRIGGER
OUT
1671 F09
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
RELATED PARTS
PART NUMBER
DESCRIPTION
LT1016
UltraFastTM Precision Comparator
LT1116
12ns Single Supply Ground-Sensing Comparator
LT1394
UltraFast Single Supply Comparator
LT1720
UltraFast Dual Single Supply Comparator
UltraFast is a trademark of Linear Technology Corporation.
12 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX:(408)434-0507 q www.linear-tech.com
COMMENTS
Industry Standard 10ns Comparator
Single Supply Version of LT1016
7ns, 6mA Single Supply Comparator
Dual 4.5ns, 4mA Single Supply Comparator
1671fs, sn1671 LT/TP 0499 4K • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 1998
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