AD8123 View Datasheet(PDF) - Analog Devices
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AD8123 Datasheet PDF : 16 Pages
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Data Sheet
APPLICATIONS INFORMATION
BASIC OPERATION
The AD8123 is easy to apply because it contains everything
on-chip needed for cable loss compensation. Figure 20 shows a
basic application circuit (power supplies not shown) with
common-mode sync pulse extraction that is compatible with
the common-mode sync pulse encoding technique used in the
AD8134, AD8147, and AD8148 triple differential drivers. If
sync extraction is not required, the terminations can be single
100 Ω resistors, and the comparator inputs can be left floating.
In Figure 20, the AD8123 is feeding a high impedance input,
such as a delay line or crosspoint switch, and the additional gain
of two that makes up for double termination loss is not required.
COMPARATORS
In addition to general-purpose applications, the two on-chip
comparators can be used to extract video sync pulses from the
received common-mode voltages or to receive differential digital
information. Built-in hysteresis helps to eliminate false triggers
from noise. The Sync Pulse Extraction Using Comparators
section describes the sync extraction details.
AD8123
The comparator outputs have nearly 0 Ω output impedance and
are designed to drive source-terminated transmission lines. The
source termination technique uses a resistor in series with each
comparator output such that the sum of the comparator source
resistance (≈0 Ω) and the series resistor equals the transmission
line characteristic impedance. The load end of the transmission
line is high impedance. When the signal is launched into the source
termination, its initial value is one-half of its source value because
its amplitude is divided by two in the voltage divider formed by
the source termination and the transmission line. At the load,
the signal experiences nearly 100% positive reflection due to the
high impedance load and is restored to nearly its full value. This
technique is commonly used in PCB layouts that involve high
speed digital logic.
Figure 19 shows how to apply the comparators with source
termination when driving a 50 Ω transmission line that is high
impedance at its receive end.
49.9Ω
Z0 = 50Ω
HIGH-Z
Figure 19. Using Comparator with Source Termination
26
ANALOG 27
CONTROL 25
INPUTS 23
POWER DOWN 28
CONTROL
VPEAK
VPOLE
VGAIN
VOFFSET
PD
RECEIVED
RED VIDEO
49.9Ω
31
49.9Ω
32
RECEIVED
GREEN VIDEO
34
49.9Ω
49.9Ω
35
RECEIVED
BLUE VIDEO
49.9Ω
37
49.9Ω
38
RED
GREEN
BLUE
AD8123
18 RED VIDEO OUT
15 GREEN VIDEO OUT
12 BLUE VIDEO OUT
1kΩ
BLUE CMV 2
1kΩ
RED CMV 3
1
8
GREEN
CMV
475Ω
7
2
47pF
47pF
GND REFERENCE
24, 39
4
HSYNC OUT
6
VSYNC OUT
Figure 20. Basic Application Circuit with Common-Mode Sync Extraction
Rev. B | Page 11 of 16
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