EL1510
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.
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.
Feedback Resistor Values
The EL1510 has been designed and specified with
R
F
=1.5kΩ for A
V
=+5. This value of feedback resistor yields
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.
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 100Ω line. The average
line voltage is 1.41V
RMS
. The ADSL DMT peak to 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.
+
-
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.
50
1.5k
AFE
300Ω
+
-
TX1
100
1:1
50
1.5k
8
FN7122.2
March 26, 2007
INTERSIL [ INTERSIL CORPORATION ]
