Hi
I've got this circuit
If EN is low ( < 0.5V ) will the FET be non-conducting? And if EN is high ( > 4V ), does the FET just act like an analog switch?
Sorry if this is such a basic question. No, it's not homework.
Ta.
On Sep 2, 1:16=A0am, "Grumps" wrote: > Hi >
Not a good way to switch video. You have a built in 6dB signal loss if you attempt to drive another cable. The off isolation is likely to be poor. There are much better ways to do this using chips from Maxim, Intersil, Analog Devices, TI and others.
G=B2
Thanks. Why is there an inherent 6dB loss?
The original circuit was intended to provide some form of video driver protection if the output was connected to the wrong point. Some of our installers have been known to connect (maybe accidentally) a 24V power line to the video outputs. The output then dies. We can obviously detect an abnormal voltage present and turn off the FET.
I am waiting for replies from the manufacturers you quoted, but do you know offhand if any of them have a video buffer that has some sort of protection like this?
Transmission lines need to be terminated in their characteristic impedance. This is typically done by inserting a resistor (usually 75 ohms minus the actual impedance of the amplifier) in series with the cable and another shunting the output to ground - as you showed in your circuit. To get 1 volt, you need to start with 2 at the amplifier output so there's your 6dB loss.
My guess is that by the time your circuit detects a fault, it's already too late. I take it it isn't feasible to use a different type of connector on the power pack so that accidents can't happen. You could possibly put some low voltage zener diode crowbars on the video output but you might blow the zener(s) or the output terminator resistor when incorrectly connected. Remember that no what you design, nothing is idiot proof.
You expect them to reply? Why don't you just go to their websites and check out the chips? Keep in mind they do not build complete video switching systems. They make ICs that switch video but need to be integrated into a finished product. These chips have maximum ratings and I don't think you'll find video amplifiers/switches that can take
24 volts where it doesn't belong. You'll still need to add some sort of protection but unique connectors have always been a good route to avoiding errors.G=B2
Actually they all did. Took 'em long enough though!
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