ADA4432-1WBCPZ-R7 datasheet(17/28 Pages) AD

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Data Sheet

ADA4432-1/ADA4433-1

Rev. A | Page 17 of 28

APPLICATIONS INFORMATION

METHODS OF TRANSMISSION

Pseudo Differential Mode (Unbalanced Source

Termination)

The ADA4432-1 can be used as a pseudo differential driver

with an unbalanced transmission line. Pseudo differential mode

uses a single conductor to carry an unbalanced data signal from

the driver to the receiver, while a second conductor is used as a

ground reference signal.

The positive conductor connects the ADA4432-1 output to the

positive input of a differential receiver, such as ADA4830-1. The

negative wire or ground conductor from the source circuitry

connects to the negative input of the receiver. Match the impedance

of the input termination at the receiver to the output termination

of the ADA4432-1 (see Figure 43).

Figure 43. Pseudo Differential Mode

Pseudo Differential Mode (Balanced Source Impedance)

Pseudo differential signaling is typically implemented using

unbalanced source termination, as shown in Figure 43. With this

arrangement, however, common-mode signals on the positive

and negative inputs receive different attenuation due to unbalanced

termination at the source. This effectively converts some of the

common-mode signal into a differential mode signal, degrading

the overall common-mode rejection of the system. System

common-mode rejection can be improved by balancing the output

impedance of the driver, as shown in Figure 44. Splitting the source

termination resistance evenly between the hot and cold conductors

results in matched attenuation of the common-mode signals,

ensuring maximum rejection.

Figure 44. Pseudo Differential Mode with Balanced Source Impedance

Fully Differential Mode

The ADA4433-1 is designed to be used as a fully differential driver.

The differential outputs of the ADA4433-1 allow fully balanced

transmission using twisted or untwisted pair cable. In this

configuration, the differential output termination consists of two

source resistors, one on each output, and each equal to half the

receiver input termination. For example, in a 75 Ω system, each

output of the ADA4433-1 is back terminated with 37.5 Ω resistors

that are connected to a differential resistance of 75 Ω at the receiver.

An illustration of this arrangement is shown in Figure 45.

Figure 45. Fully Differential Mode

PRINTED CIRCUIT BOARD (PCB) LAYOUT

As with all high speed applications, attention to PCB layout is of

paramount importance. Adhere to standard high speed layout

practices when designing with the ADA4432-1 and ADA4433-1.

A solid ground plane is recommended. Place a 0.1 µF surface-

mount, ceramic power supply decoupling capacitor as close as

possible to the supply pin.

Connect the GND pin(s) to the ground plane with a trace that is

as short as possible. Use controlled impedance traces of the shortest

length possible to connect to the signal I/O pins and do not run the

traces over any voids in the ground plane. A 75 Ω impedance level

is typically used in video applications. All signal outputs of the

ADA4432-1 and ADA4433-1 should include series termination

resistors when driving transmission lines.

When the ADA4432-1 or the ADA4433-1 receives its inputs from a

device with current outputs, the required load resistor value for

the output current is most often different from the characteristic

impedance of the signal traces. In this case, if the interconnections

are sufficiently short (less than 2 inches), the trace does not

need to be terminated in its characteristic impedance.

INN

INP

ADA4830-1

Ω

−

Ω

POSITIVE WIRE

NEGATIVE WIRE

DRIVER PCB

ADA4432-1

INN

INP

ADA4830-1

Ω

−

37.5

Ω

37.5

Ω

POSITIVE WIRE

NEGATIVE WIRE

DRIVER PCB

ADA4432-1

INN

INP

ADA4830-1

Ω

−

37.5

Ω

37.5

Ω

POSITIVE WIRE

NEGATIVE WIRE

DRIVER PCB

ADA4433-1

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