8

FN7122.2

March 26, 2007

Capacitance at the Inverting Input

Due to the topology of the current feedback amplifier, stray

capacitance at the inverting input will affect the AC and

transient performance of the EL1510 when operating in the

non-inverting configuration.

In the inverting gain mode, added capacitance at the

inverting input has little effect since this point is at a virtual

ground and stray capacitance is therefore not “seen” by the

amplifier.

Feedback Resistor Values

The EL1510 has been designed and specified with

extremely flat frequency response with no peaking out to

40MHz. As is the case with all current feedback amplifiers,

wider bandwidth, at the expense of slight peaking, can be

obtained by reducing the value of the feedback resistor.

Inversely, larger values of feedback resistor will cause rolloff

to occur at a lower frequency. See the curves in the Typical

Performance Curves section which show 3dB bandwidth and

peaking vs frequency for various feedback resistors and

various supply voltages.

Bandwidth vs Temperature

Whereas many amplifiers’ supply current and consequently

3dB bandwidth drop-off at high temperature, the EL1510

was designed to have little supply current variations with

temperature. An immediate benefit from this is that the 3dB

bandwidth does not drop off drastically with temperature.

Supply Voltage Range and Operation

The EL1510 has been designed to operate with supply

voltages from ±2.5V to ±12V. If a single supply is desired,

values from +5V to +24V can be used as long as the input

common mode range is not exceeded. When using a single

supply, be sure to either 1) DC bias the inputs at an

appropriate common mode voltage and AC couple the

signal, or 2) ensure the driving signal is within the common

mode range of the EL1510.

ADSL CPE Applications

The EL1510 is designed as a line driver for ADSL CPE

modems. It is capable of outputting 400mA of output current

with a typical supply voltage headroom of 1.8V. It can

achieve -85dBc of distortion at low 7.5mA of supply current

per amplifier.

The average line power requirement for the ADSL CPE

application is 13dBm (20mW) into a 100

Ω line. The average

ratio (crest factor) of 5.3 implies peak voltage of 7.5V into the

line. Using a differential drive configuration and transformer

coupling with standard back termination, a transformer ratio

of 1:1 is selected. The circuit configuration is as shown

below.

-

+

-

+

100

TX1

1:1

50

50

1.5k

1.5k

300

Ω

AFE

EL1510