14

LT1394

High Speed Adaptive Trigger Circuit

Line and fibre-optic receivers often require an adaptive

trigger to compensate for variations in signal amplitude

and DC offsets. The circuit in Figure 18 triggers on 2mV to

175mV signals from 100Hz to 45MHz 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 excur-

sion 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,

egardless of absolute amplitude. This signal-adaptive volt-

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

LT1394’s positive input. The LT1394’s negative input is

biased directly from A1’s output. The LT1394’s output, the

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

variations. Bandwidth limiting in A1 does not affect trigger-

ing because the adaptive trigger threshold varies

ratiometrically to maintain circuit output.

Figure 19 shows operating waveforms at 45MHz. Trace A’s

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

comparator’s output is Trace C.

Split supply versions of this circuit can achieve band-

widths to 50MHz with wider input operating range.

A = 0.1V/DIV

B = 0.1V/DIV

C = 5V/DIV

50ns/DIV

AN72 F64

Figure 19. Adaptive Trigger Responding to a 40MHz,

5mV Input. Input Amplitude Variations from 2mV to

175mV Are Accommodated

5V

–

+

A1

LT1227

A2

LT1006

INPUT

5V

5V

5V

TRIGGER

OUT

1394 F18

500pF

0.1

µF

510

Ω

470

Ω

470

Ω

750

Ω

36

Ω

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

LT1394

Q1

Q2

Q3

Q4

3M

3M

Figure 18. 45MHz Single Supply Adaptive Trigger. Output Comparator’s

Threshold Varies Ratiometrically with Input Amplitude, Maintaining Data

Integrity over > 85:1 Input Amplitude Range