11

FN7058.3

May 1, 2007

Applications Information

Product Description

The EL2227 is a dual voltage feedback operational amplifier

designed especially for DMT ADSL and other applications

requiring very low voltage and current noise. It also features

low distortion while drawing moderately low supply current

and is built on Elantec's proprietary high-speed

complementary bipolar process. The EL2227 use a classical

voltage-feedback topology which allows them to be used in a

variety of applications where current-feedback amplifiers are

not appropriate because of restrictions placed upon the

feedback element used with the amplifier. The conventional

topology of the EL2227 allows, for example, a capacitor to

be placed in the feedback path, making it an excellent choice

for applications such as active filters, sample-and-holds, or

integrators.

ADSL CPE Applications

The low noise EL2227 amplifier is specifically designed for

the dual differential receiver amplifier function with ADSL

transceiver hybrids as well as other low-noise amplifier

applications. A typical ADSL CPE line interface circuit is

shown in Figure 38. The EL2227 is used in receiving DMT

down stream signal. With careful transceiver hybrid design

and the EL2227 1.9nV/

√Hz voltage noise and 1.2pA/√Hz

current noise performance, -140dBm/Hz system background

noise performance can be easily achieved.

Disable Function

The EL2227 is in the standard dual amplifier package

without the enable/disable function. A simple way to

implement the enable/disable function is depicted below.

When disabled, both the positive and negative supply

voltages are disconnected (see Figure 39)

Power Dissipation

With the wide power supply range and large output drive

capability of the EL2227, it is possible to exceed the +150°C

maximum junction temperatures under certain load and

power-supply conditions. It is therefore important to calculate

applications to determine if power supply voltages, load

conditions, or package type need to be modified for the

EL2227 to remain in the safe operating area. These

parameters are related as follows:

where:

where:

θ

Application

To serve as a guide for the user, we can calculate maximum

allowable supply voltages for the example of the video

shown in Table 1. If we assume (for this example) that we

are driving a back-terminated video cable, then the

FIGURE 38. TYPICAL LINE INTERFACE CONNECTION

-

+

-

+

-

+

-

+

RECEIVE

OUT -

RECEIVE

OUT +

DRIVER

INPUT

R

R

LINE +

LINE -

RECEIVE

AMPLIFIERS

-

+

1µF

4.7µF

1µF

+12V

1k

10k

10k

75k

1k

1k

FIGURE 39.

T

JMAX

T

MAX

θ

JA

PD

MAXTOTAL

×

()

+

=

(EQ. 1)

PD

MAX

2V

S

I

SMAX

V

S

(

V

V

OUTMAX

R

L

----------------------------

×

–

+

×

×

=

(EQ. 2)

EL2227