On Thu, 01 Jan 2004 23:08:09 +0000, LRCR wrote:
You posted your entire question in the subject field. This is very bad
form as the subject field is usually not designed to display so many
characters. For the benefit of others, I have reproduced it below:
Has anyone ever design an auto video termination circuit. When video is
applied a 75ohm is enable to an input video circuit
Also, the question is not entirely clear. Are you trying to design a
circuit where a video receiver detects when a signal is present, then
applies a 75-Ohm termination to the signal? If so, you really just need to
break it into two steps:
1) detect signal
2) enable termination
It would help a lot if you explained why you want to do this. Detecting
the signal should be pretty easy, although you don't specify what kind of
video you want to detect, and I'm not a video expert. Is the video going
to have sync-pulses on it? What is the magnitude of a sync pulse? What
kind of bandwidth do you need?
Anyway, once you have the detection nailed down, enabling the termination
could probably be done with a logic level FET. Although you might run into
problems with the negative part of the signal. So you could use two FET's,
one enabling a 150-Ohm pull-up to the positive rail and one enabling a
150-Ohm pull-down to the negative rail. The driver will never know the
difference.
Good luck.
Mac
