13

FN7013.3

March 26, 2007

Applications Information

The EL1507 consists of two high-power line driver amplifiers

that can be connected for full duplex differential line

transmission. The amplifiers are designed to be used with

signals up to 4MHz and produce low distortion levels. A

typical interface circuit is shown in Figure 51 below.

The amplifiers are wired with one in positive gain and the

other in a negative gain configuration to generate a

differential output for a single-ended input. They will exhibit

very similar frequency responses for gains of three or

greater and thus generate very small common-mode outputs

to be adjusted to give similar frequency responses. The

positive-gain driver will generally exhibit more bandwidth and

peaking than the negative-gain driver.

If a differential signal is available to the drive amplifiers, they

may be wired so:

Each amplifier has identical positive gain connections, and

optimum common-mode rejection occurs. Further, DC input

errors are duplicated and create common-mode rather than

differential line errors.

Input Connections

The EL1507 amplifiers are somewhat sensitive to source

impedance. In particular, they do not like being driven by

inductive sources. More than 100nH of source impedance

can cause ringing or even oscillations. This inductance is

equivalent to about 4” of unshielded wiring, or 6” of

unterminated transmission line. Normal high-frequency

construction obviates any such problem.

Power Supplies & Dissipation

Due to the high power drive capability of the EL1507, much

attention needs to be paid to power dissipation. The power

that needs to be dissipated in the EL1507 has two main

contributors. The first is the quiescent current dissipation.

The second is the dissipation of the output stage.

The quiescent power in the EL1507 is not constant with

varying outputs. In reality, 7mA of the 15mA needed to

power the drivers is converted in to output current.

Therefore, in the equation below we should subtract the

Therefore, we can determine a quiescent current with the

equation:

where:

The dissipation in the output stage has two main

contributors. Firstly, we have the average voltage drop

across the output transistor and secondly, the average

output current. For minimal power dissipation, the user

should select the supply voltage and the line transformer

ratio accordingly. The supply voltage should be kept as low

as possible, while the transformer ratio should be selected

so that the peak voltage required from the EL1507 is close to

the maximum available output swing. There is a trade off,

however, with the selection of transformer ratio. As the ratio

is increased, the receive signal available to the receivers is

reduced.

Once the user has selected the transformer ratio, the

dissipation in the output stages can be selected with the

following equation:

where:

FIGURE 51. TYPICAL LINE INTERFACE CONNECTION

-

+

-

+

-

+

-

+

RECEIVE

OUT -

RECEIVE

OUT +

DRIVER

INPUT

R

R

LINE +

LINE -

RECEIVE

AMPLIFIERS

FIGURE 52. DRIVERS WIRED FOR DIFFERENTIAL INPUT

-

+

-

+

P

Dquiescent

V

S

I

S

2I

X

–

()

×

=

P

Dtransistors

2I

O

V

S

2

-------

⎝

⎛

×

×

V

O

–

⎠

⎞

=

EL1507