With a 1:2.5 transformer, the two 8

Ω impedances real-

ized by the active-feedback network form a perfect line

termination. The MAX9480 family’s active termination

design makes it possible to use a ±5V power supply

instead of ±12V or ±15V, significantly increasing driver

efficiency.

Hybrid Considerations

The MAX9480 includes an internal hybrid coupling cir-

cuit to realize the receive function with no external com-

ponents. The hybrid couples the transmitted signal

from the line-driver port (Tx port) to the line port and

cancels the echo in the receiving port (Rx port). The

hybrid circuit is detailed in Figure 6. If using the

MAX9481 or the MAX9482, external circuitry must be

added to realize a receive function. The MAX9482

includes two uncommitted op amps for this purpose.

A traditional hybrid network with a 2:1 resistor ratio

must be replaced with a 1.2:1 ratio to achieve nominal

echo cancellation. Additionally, the use of a synthe-

sized output impedance has the side effect of prevent-

ing a “virtual-ground” condition at the driver output

(BOUT), as seen by the receive signal. Hence, the

resulting hybrid circuit exhibits an increased attenua-

tion of the receive signal with respect to a traditional

case with no synthesis. For central-office ADSL appli-

cations, the noise specifications allow this trade-off to

be made.

Pseudo-Class-G Amplifier

The driver consists of two stages: the current-feedback

preamplifier and the voltage-feedback buffer. To save

power, the preamplifier uses a power supply of only

±2.5V. The output swing of the preamplifier is about

amplifier with a fixed gain of -3V/V. This buffer stage

employs two power supplies: a lower voltage supply,

±2.5V, and a higher voltage supply, ±5V. In the differ-

ential driver, two parallel amplifiers provide the output

current to the load, as shown in Figure 7.

In this pseudo-class-G amplifier, there is no abrupt

supply switchover between the higher voltage and the

lower voltage amplifiers, as in a traditional class G. It is

a seamless transition that depends only on the ampli-

tude of the input signal and the gain. The lower voltage

With a low input voltage, the lower voltage amplifier pro-

vides most of the output current to the load. As the volt-

age of the input signal increases, the lower voltage

amplifier starts to saturate and the higher voltage ampli-

fier begins to drive the output.

Low-Power, Low-Distortion, Central-Office

ADSL Drivers and Integrated Drivers/Receivers

______________________________________________________________________________________

11

Ω

LINE -

LINE +

1:2.5

Ω

LINE PORT

Tx PORT

Ω

Ω

Figure 5. Output Equivalent Circuit

OUT2

OUT1

RXP

RXM

R2

1.6

Ω

Rx

PORT

R3

1k

Ω

R5

1k

Ω

R4

1.2k

Ω

R6

1.2k

Ω

LINE

PORT

Tx

0.8Tx

Rx

0.8Rx

-Tx

-0.8Rx

-0.8Tx

-Rx

0Tx + 0.18Rx

0Tx - 0.18Rx

(a) HYBRID CIRCUIT IN MAX9480

(b) TRADITIONAL HYBRID CIRCUIT

R1

1.6

Ω

Tx

PORT

MAX9480

OUT2

OUT1

RXP

RXM

8

Ω

Rx

PORT

1k

Ω

1k

Ω

2k

Ω

2k

Ω

LINE

PORT

Tx

0.5Tx

Rx

0Rx

-Tx

-0.0Rx

-0.5Tx

-Rx

0Tx + 0.67Rx

0Tx - 0.67Rx

8

Ω

Tx

PORT

Figure 6. Hybrid Circuit in the MAX9480