[SI-LIST] Re: Question about VGA termination
- From: "Joel Brown" <joel@xxxxxxxxxx>
- To: <olaney@xxxxxxxx>
- Date: Thu, 25 Oct 2007 17:04:23 -0700
Orin,
After reading your explanation its making sense. I am thinking they wanted
to use 50 ohms because it is difficult to fabricate 75 ohms with a typical
motherboard stackup. Also the passive filter at the connector may reduce the
spikes due to the reflections you referred to although I think the main
purpose of the filter is to bandwidth limit the signal for EMI reasons. Also
the filter has some bulk L-R-C values that I am sure mess with the
impedance. Thanks for the help.
Joel
_____
From: olaney@xxxxxxxx [mailto:olaney@xxxxxxxx]
Sent: Thursday, October 25, 2007 4:23 PM
To: joel@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Question about VGA termination
Distributing the near end termination resistance is an attempt to reduce
reflections at the VGA connector. However, it's sloppy. Looking from the
VGA output, the nearby resistor looks like nothing more than a resistor, and
can be ignored. Then you see 50 ohm line, then 150 ohms in parallel with
the 75 ohm coax and far end termination at the monitor. 150 ohms at the
connector in parallel with 75 ohms = 50 ohms. The 50 ohm line is matched
and does not "see" the transition into the 75 ohm line. The nominal video
level for a VGA monitor is .7 VPP, with black at ground. Peak current is
therefore .7/75 = 9.33 mA. The chip is doubly terminated, so must source
twice this, or 18.7 mA per your statement.
All is well if the monitor itself is well terminated. This is not always
the case. Reflections coming back on the 75 ohm line encounter first, 150
ohms paralleled by the 50 ohm line, or 37.5 ohms, yielding a nasty negative
re-reflection. However, at the far end of the 50 ohm line, the signal will
meet the 150 termination at the chip, which makes another nasty but positive
reflection. Keeping the distance between these mismatches short helps to
overlap the reflections to achieve some cancellation. The time skew of
several nonoseconds between them leads to spikes of noncancellation
traveling back to the monitor that are hopefully beyond the monitor
bandwidth, and will be low in amplitude if the pixel rise/fall times are
longer than the spike widths. The 12 inch rule is to maintain adequate
overlap. Beyond that the spikes become wide enough to be increasingly
visible.
In other words, the recommendation is a clever way of being stupid. The
preferred method is 75 ohms at the chip, a 75ohm line to the connector, and
only a few inches at most between the connector and the chip! The fact that
the connector is not itself a good approximation of 75 ohms (usually higher)
might be the driver behind all the rigmarole. It might indicate that the
design was optimized with a network analyzer (not a good idea for video).
Only a TDR knows for sure.
Orin Laney
On Thu, 25 Oct 2007 15:20:56 -0700 "Joel Brown" <joel@xxxxxxxxxx> writes:
> 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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