12
LT1671
LINEAR TECHNOLOGY CORPORATION 1998
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
1671fs, sn1671 LT/TP 0499 4K • PRINTED IN USA
TYPICAL APPLICATION
U
PART NUMBER DESCRIPTION COMMENTS
LT1016 UltraFast
TM
Precision Comparator 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 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
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
0.1µF
510ΩΩ
470ΩΩ
470Ω
750ΩΩ
36ΩΩ
1
3
2
4
14
13
15
5
6
12
10
11
2k
10µF
2k
2k
+
0.1µF
0.1µF
0.005µF
0.005µF
100µF
+
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
