Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia
Programmable video termination
Posted: Tue Jun 22, 2004 8:25 pm
In a project I am connecting to a baseband composite video signal (usually the Video-Out port of the TV). The video decoder chip used here is Phlipps SA7115. I am having problems with termination...depending on the set I may/may not need to terminate the video soure with a 75ohms resistor (for example if the device is conencted to VCR rather than a TV). I want to avoid putting a physical switch on the device for this purpose, so I'm looking at a programmable termination circuit.
I currently have a 2N7002 in the design to provide this switching. I'm concerned about the added capacitance and its effects on the video. It's typically speced at 28pF for VGS=0(off) and VDS=10-25V. The VDS spec is way too high for our application (~1Vpp) and the curves show that the pF rise sharply for lower voltages. And, the ON pF are not speced -- I'm hoping this means that it's so low it's not mentioned.
An alternative is to use a small relay as the terminating switch. Does anyone have any other ideas? Has anyone else ever built a rogrammable video terminator?
Thanks.
dyeatman
Joined: 06 Sep 2003 Posts: 1934 Location: Norman, OK
Solid state termination switch
Posted: Tue Jun 22, 2004 9:47 pm
Maxim has a number of video/RF switches you might take a look at.
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