EL2004/EL2004C
350 MHz FET Buffer
Circuit Operation Ð Contd.
General Application Suggestions
Increasing Operating Voltage
and Reducing Thermal Tail
Video DAC Buffer
Many of the available video D to A converters
are unable to directly drive 50X or 75X cables.
The EL2004’s excellent phase linearity at video
frequencies make it an ideal solution. In critical
applications or where line termination is not con-
trolled, a matching pad should be used as shown.
The capacitor should be adjusted for optimum
pulse response. If properly layed out this circuit
will not overshoot.
When driving heavy loads, the changing dissipa-
tion in the output transistors can sometimes
cause temperature gradients in the circuit which
cause a shift in offset voltage and the phenome-
non known as ‘‘thermal tail’’. Bootstrapping the
output as illustrated substantially reduces the
power in the output transistors and mitigates the
effect.
Video DAC Buffer
High Voltage Inputs can be Accommodated
with Bootstrapped Supplies
2004–14
Impedance Matching
The EL2004 provides power gain and isolation
between source and load when used as an active
tap or impedance matching device as illustrated
here. In this example, there is no output match-
ing pad between the 2004 and the 75X line. Such
matching is not needed when the distant end of
the cable is properly terminated as there is no
reflected signal to worry about and the 2004 iso-
lates the source. This technique allows the full
output voltage of the EL2004 to be applied to the
load.
2004–13
Hardware
In order to utilize the full drive capabilities of the
EL2004, it should be mounted with a heatsink,
particularly for extended temperature operation.
Suitable heatsinks include Thermalloy 2240A
(33 C/W), Wakefield 215CB (30 C/W) and
Impedance Converter
§
§
IERC-UP-TO-848CB (15 C/W).
§
The case is isolated from the circuit and may be
connected to system chassis. Sockets are not rec-
ommended as they add substantial inductance
and capacitance which impair the performance of
the device. However, for test purposes they are
unavoidable and precautions such as shielding in-
put from output are suggested.
2004–15
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
