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AD8145YCPZ-R7 Datasheet(PDF) 20 Page - Analog Devices

Part No. AD8145YCPZ-R7
Description  High Speed, Triple Differential Receiver with Comparators
Download  24 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8145YCPZ-R7 Datasheet(HTML) 20 Page - Analog Devices

 
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AD8145
Rev. 0 | Page 20 of 24
COMPARATORS
In addition to general-purpose applications, the two on-chip
comparators can be used to decode 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 comparator outputs 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 (≈ 20 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, since its amplitude is
divided by two by 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.
An internal linear voltage regulator derives power for the
comparators from the positive supply; therefore, the AD8145
must always have a minimum positive supply voltage of 4.5 V.
SYNC PULSE EXTRACTION USING COMPARATORS
The AD8145 is particularly useful in keyboard, video, mouse
(KVM) applications. KVM networks transmit and receive
computer video signals, which typically comprise red, green,
and blue (RGB) video signals and separate horizontal and
vertical sync signals. Because the sync signals are separate and
not embedded in the color signals, it is advantageous to
transmit them using a simple scheme that encodes them among
the three common-mode voltages of the RGB signals. The
AD8134 triple differential driver is a natural complement to the
AD8145 and performs the sync pulse encoding with the
necessary circuitry on-chip.
The AD8134 encoding equations are given in Equation
4,
Equation
5, and Equation 6.
[
]
H
V
K
V
Red
CM
=
2
(
4)
[
]
V
2
2
= K
V
Green
CM
(
5)
[
]
H
V
K
V
Blue
CM
+
=
2
(
6)
where:
Red VCM, Green VCM, and Blue VCM are the transmitted common-
mode voltages of the respective color signals.
K is an adjustable gain constant that is set by the AD8134.
V and H are the vertical and horizontal sync pulses, defined
with a weight of −1 when the pulses are in their low states, and a
weight of +1 when they are in their high states.
The AD8134 data sheet contains further details regarding the
encoding scheme. Figure 44 illustrates how the AD8145
comparators can be used to extract the horizontal and vertical
sync pulses that are encoded on the RGB common-mode
voltages by the AD8134.
RECEIVED
RED VIDEO
HSYNC
RED CMV
GREEN CMV
BLUE CMV
50Ω
50Ω
1kΩ
1kΩ
VSYNC
RECEIVED
GREEN VIDEO
50Ω
50Ω
RECEIVED
BLUE VIDEO
50Ω
50Ω
RS
RS
475Ω
47pF
47pF
Figure 44. Extracting Sync Signals from Received Common-Mode Signal


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