11
FN7417.2
January 29, 2008
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 EL8108 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 EL8108 has been designed and specified with
R
F
= 500 for A
V
= +2. This value of feedback resistor yields
extremely flat frequency response with little to no peaking
out to 200MHz. 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
“Typical Performance Curves” beginning on page 4, 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 EL8108 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 EL8108 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
A
V
= +5 is a respectable 200MHz.
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 EL8108.
Driving Cables and Capacitive Loads
The EL8108 was designed with driving multiple coaxial
cables in mind. With 450mA of output drive and low output
impedance, driving six, 75 double terminated coaxial
cables to ±11V with one EL8108 is practical.
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, the back termination series resistor will
decouple the EL8108 from the capacitive cable and allow
extensive capacitive drive.
Other applications may have high capacitive loads without
termination resistors. In these applications, an additional
small value (5 to 50) resistor in series with the output will
eliminate most peaking.
The following schematic show the EL8108 driving 6 double
terminated cables, each an average length of 50 ft.
EL8108
