[SI-LIST] Question about VGA termination
- From: "Joel Brown" <joel@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 25 Oct 2007 15:20:56 -0700
I am working on a design with VGA analog video interface. The mfg of the
graphics chip recommends 150 ohm termination at the graphics chip and
another 150 ohm termination at the VGA connector.
They also recommend 50 ohm trace impedance between the two 150 ohm
resistors, the resistors may be up to 12 inches apart. There is a passive
filter between the second 150 ohm resistor and VGA connector. A monitor
which has 75 termination resistors plugs into the VGA connector via a 75 ohm
video cable. So this brings up some questions:
Why not use a single 75 ohm termination at the graphics chip?
What is the purpose of having a 150 ohm termination resistor close to the
VGA connector and / or filter?
Why 50 ohm trace impedance (and not 75 ohm) between the 150 ohm resistors?
Why is there a limitation of 12 inches between the 150 ohm termination
resistors?
I have been running some spice simulations on a filter circuit to be used on
this interface, however there is no spice model for the analog video outputs
of the graphics chip.
So I am simply using a current source with 0.0187 Amp full scale, is this a
reasonable approach?
Thanks - Joel
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