11

FN7058.4

September 14 ,2010

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 in Figure 39. 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 the

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 in Equation 1:

where:

Equation 2:

where:

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 maximum

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. IMPLEMENTATION OF ENABLE/DISABLE

FUNCTION

×

()

+

=

(EQ. 1)

(

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

×

–

+

×

×

=

(EQ. 2)

EL2227