8

FN7015.2

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 EL1509 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

1.5k

extremely flat frequency response with little to no peaking

out to 50MHz. 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 amplifier's supply current and consequently

3dB bandwidth drop off at high temperature, the EL1509 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

The EL1509 has been designed to operate with supply

voltages from ±2.5V to ±6V. Optimum bandwidth, slew rate,

and video characteristics are obtained at higher supply

voltages. However, at ±2.5V supplies, the 3dB bandwidth at

Single Supply Operation

If a single supply is desired, values from +5V to +12V 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 EL1509.

ADSL CPE Applications

The EL1509 is designed as a line driver for ADSL CPE

modems. It is capable of outputting 250mA of output current

with a typical supply voltage headroom of 1.3V. It can

achieve -85dBc of distortion at low 7.1mA 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 1:2 is selected. The circuit configuration is as shown

below.

-

+

-

+

TX1

1:2

12.5

12.5

1.5k

1.5k

464

Ω

AFE

100

EL1509