Video-Out Coupling


If you want to connect a video signal to
several destinations, you need a distribution amplifier to match the
75-ohm video cable. A distribution amplifier terminates the incoming
cable in 75 ohms and provides several outputs, each with 75-ohm output
impedance. Since this is usually achieved by putting a 75-ohm series
resistor in the output lead of each video opamp (current-feedback
amplifier), the opamps must be set up for a gain of 2 in order to
achieve an insertion gain of 1 (0 dB). The disadvantage of this
arrangement is that if the amplifier or its power supply fails, no
signal is available at any of the outputs. This can be remedied by using
a high input impedance amplifier, which can be tapped into a video line
without having to have its own 75-ohm termination resistor.

In order to eliminate hum interference and voltage differences
between the cable screen and the circuit earth, the circuit exploits the
common-mode rejection of the opamp. This can be optimized with resistor
RG1. With the indicated LT1396 video opamp, more than 40 dB of
common-mode rejection can be achieved. The signal bandwidth of the
circuit can be optimized using the trimpots. It reaches to more than 10
MHz, which is quite acceptable for video signals. Thanks to the
high-impedance connection to the video line, the video signal is not
affected when the power for the coupled amplifier is switched off. You
can learn more about the LT1396 from its data sheet at http://www.linear-tech.com.


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