AD8122(RevPrC) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD8122
(Rev.:RevPrC)

Triple Differential Receiver with 300m Adjustable Line Equalization

Analog Devices 

AD8122

APPLICATIONS INFORMATION

BASIC OPERATION

The AD8122 is easy to apply because it contains everything

on-chip needed for cable loss compensation. Figure 8 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, AD8142, 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.

INPUT OVERDRIVE RECOVERY AND PROTECTION

Occasional large differential transients can occur on the cable

due to a number of causes, such as ESD and switching. When

operating the AD8122 at G = 1, a differential input that exceeds

+3.4V or -3.4V will cause the output to “stick” at the associated

power supply rail (positive rail for positive overdrive, negative

rail for negative overdrive). The AD8122 recovers from this

condition when the magnitude of the differential input falls

below 200 mV. Most video signals return to nominally zero

volts during the blanking intervals, therefore recovery from the

overdriven condition in systems that use these signals would

occur during the first blanking interval that occurs after the

overdrive event has passed. In systems with G = 1 that employ

video signals that do not return to zero, such as those that

include DC offsets, it is necessary to prevent the overdrive

condition from occurring. In these cases the protection circuit

illustrated in Figure 6, which limits the differential input voltage

to a little over ±2V, should be placed between the termination

resistors and each AD8122 differential input. The overdrive

condition does not occur in applications with G = 2.

49.9

123

HN2D02FUTW1T1

TERMINATION

RESISTORS

654

49.9

456

AD8122

INPUT

HN2D02FUTW1T1

321

Figure 6. Required Input Protection For Applications With G = 1

COMPARATORS

While the two on-chip comparators are most often used to

extract video sync pulses from the received common-mode

voltages as shown in Figure 8, they may also be used to recover

sync pulses in sync-on-color applications, to receive differential

digital information received on other channels such as the fourth

UTP pair , or as general purpose comparators. Built-in hysteresis

helps to eliminate false triggers from noise. The Sync Pulse

Extraction Using Comparators section describes the sync

extraction details.

Preliminary Technical Data

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 7 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 7. Using Comparator with Source Termination

Rev. PrC | Page 10 of 13

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