EL2003C, EL2033C
100MHz Video Line Driver
Driving Cables
There are at least three ways to use the EL2003C and
EL2033C to drive cables, as shown in the adjacent fig-
ure. The most obvious is to directly connect the cable to
the output of the buffer. This results in a gain determined
by the output resistance of the EL2003C or EL2033C
and the characteristic impedance of the cable, assuming
it is properly terminated. For RG-58 into 50W the gain is
about -1dB, exclusive of cable losses. For optimum
response and minimum reflections it is important for the
cable to be properly terminated.
Direct Drive
Double Matched
Back Matched
Double termination of a cable is the cleanest way to
drive it since reflections are absorbed on both ends of the
cable. The cable source resistor is equal to the character-
istic impedance of the cable less the output resistance of
the EL2003C and EL2033C. The gain is -6dB exclusive
of the cable attenuation.
Back matching is the last and most interesting way to
drive a cable. The cable source resistor is again the char-
acteristic impedance less the output resistance of the
EL2003C and EL2033C; the termination resistance is
now much greater than the cable impedance. The gain is
0dB and DC levels waste no power.
Op Amp Booster
The EL2003C or EL2033C can boost the output drive of
almost any monolithic op amp. Because the phase shift
in the EL2003C and EL2033C is low at the op amp's
unity gain frequency, no additional compensation is
required. By following an op amp with an EL2003C or
EL2033C, the buffered op amp can drive cables and
other low impedance loads directly. Even decompen-
sated high speed op amps can take advantage of the
EL2003C’s or EL2033C’s 100mA drive.
An additional EL2003C or EL2033C make a good
receiver at the terminating end. Because an unterminated
cable looks like a resonant circuit, the receiving
EL2003C or EL2033C should have an isolating resistor
in series with its input to prevent oscillations when the
cable is not connected to the driver. Of course if the
cable is always connected to the back match, no resistor
is necessary.
WARNING: ONE END OF A CABLE MUST BE
PROPERLY TERMINATED. If neither end is termi-
nated in the cable characteristic impedance, the cable
will have standing waves that appear as resonances in
the frequency response. The resonant frequencies are a
function of the cable length and even relatively short
cables can cause problems at frequencies as low as
1MHz. Longer cables should be terminated on both
ends.
Op Amp Booster
Driving capacitive loads with any closed loop amplifier
creates special problems. The open loop output imped-
ance works into the load capacitance to generate phase
lag which can make the loop unstable. The output
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