Detailed Description

The MAX9480/MAX9481/MAX9482 are fully differential

line transceivers for ADSL applications. Each transmit

path has a high-bandwidth, low-distortion, current-feed-

back operational amplifier followed by a fixed-gain

class-G output buffer.

The MAX9480/MAX9481/MAX9482 are class-G devices

and require two dual power supplies, ±5V and ±2.5V.

All preamplifier inputs and outputs are available to

allow external gain configuration. The MAX9480 con-

tains an internal hybrid echo cancellation circuit with

receiver amplifiers set to a fixed gain of 4.6V/V. The

MAX9482 has no internal hybrid, but contains two

uncommitted low-noise op amps for coupling to an

external hybrid network. The MAX9481 has only the

preamp and line-driver circuits.

The two class-G output buffers are internally configured

for an inverting gain of three, and employ an active ter-

mination scheme that presents an 8

Ω load to incoming

signals. The buffers are designed for use with a 1:2.5

line transformer and can deliver 20.4dBm average line

power with a signal crest factor of 5.3 into a standard

100

Ω line. The outputs are designed to be directly DC-

or AC-bridged across the transformer.

The MAX9480/MAX9481/MAX9482 have a low-output

impedance, low-power shutdown mode that is activated

by driving SHDN high. Transmit path amplifiers and

buffers are disabled while the part is in shutdown, and

an 8

Ω resistor is coupled directly between OUT_ and the

output of a three-state buffer referenced to DGND (0V).

The receive amplifiers remain active in shutdown mode.

Applications Information

Theory of Operation

The preamplifiers and receivers are current-feedback

amplifiers; thus, their open-loop transfer function is

expressed as a transimpedance,

The frequency behavior of their open-loop transimped-

ance is similar to the open-loop gain of a voltage-feed-

back amplifier; that is, they have a large DC value that

decreases at approximately 6dB per octave. Analyzing

the follower with gain, as shown in Figure 1, yields the

following transfer function:

≠ 200Ω.

bandwidth is essentially independent of closed-loop

Shutdown

Forcing SHDN high puts the MAX9480/MAX9481/

MAX9482 into low-power shutdown mode. In shutdown

mode, OUT1 and OUT2 are low impedance, and the

power-supply currents drop to less than 10% of their nor-

mal quiescent operating values. When coming out of shut-

down, allow about 1.5µs before commencing operation.

PC Board Layout

Power-Supply Bypassing

The MAX9480/MAX9481/MAX9482 are wide-bandwidth

devices and require careful board layout, including the

possible use of constant-impedance microstrip or

stripline techniques. To realize the full AC performance

of these high-speed amplifiers, pay careful attention to

power-supply bypassing. The PC board should have at

least two layers: a signal and power layer on one side,

and a large, low-impedance ground plane on the other

side. The ground plane should be as free of voids as

possible. With multilayer boards, locate the ground

plane on a layer that incorporates no signal or power

traces. Observe the following guidelines when design-

ing the board. IC sockets increase parasitic capaci-

tance and inductance, and should not be used. Do not

make 90° turns; round all corners. Observe high-fre-

quency bypassing techniques to maintain the amplifier’s

V

V

G

R

R

Equation

OUT

IN

F

G

== +

[]

12

V

V

G

Z

ZG

R

R

Equation

OUT

IN

OL S

OL S

IN

F

=×

+×

+

[]

()

()

()

1

Low-Power, Low-Distortion, Central-Office

ADSL Drivers and Integrated Drivers/Receivers

8

_______________________________________________________________________________________

+1

+1

MAX9480

MAX9481

MAX9482

Figure 1. Current Feedback Amplifier Block Diagram