AD8145 datasheet(20/24 Pages) AD

Part #	AD8145

Description	High Speed, Triple Differential Receiver with Comparators

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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

−

= 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

1kΩ

VSYNC

RECEIVED

GREEN VIDEO

50Ω

RECEIVED

BLUE VIDEO

50Ω

RS

475Ω

47pF

Figure 44. Extracting Sync Signals from Received Common-Mode Signal

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