[SI-LIST] Re: Source termination of LVDS drivers
- From: Robert Kezer <Robert.Kezer@xxxxxxxxxxxxx>
- To: "'yost@xxxxxxxxxxxxxxxxxxx'" <yost@xxxxxxxxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Wed, 29 Jan 2003 15:51:59 -0500
I do not believe I said it did not work. I was making the point that for
low voltage operation, if you source terminate LVDS you need to make sure
you have the overhead for proper operation.
-----Original Message-----
From: Boris Yost [mailto:yost@xxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, January 29, 2003 3:08 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Source termination of LVDS drivers
Dear All:
Source termination? I did this, and it works fine. My system may
be a
little different than you 'telecom' types. I have the lower-power
display-oriented LVDS (not Bus-LVDS), and I wanted to use them in a
multidrop chain rather than sender->receiver. So the receivers in the
middle of the cable present a low impedance discontinuity which sends back
nasty reflections. The media is 120-ohm UTP ribbon cable. I have 33 ohms
in series with each source line, 56 ohms in series with each receiver line,
and 2x56 ohms at the end of the cable. The resistors in front of the
receivers keep more of the energy in the bus, not bouncing off the gate
capacitance.
There is one important thing that you didn't ask about, but you
should. If
you 'terminate' the line the way that Brand N or others draw in the data
sheet, the differential impedance is 100 ohms, but the impedance to ground
is 'infinite'. Therefore, if any common mode ends up on your bus, it has
nowhere to go but create EMI like crazy. Of course, there is no common mode
on your signal, because the true and compliment outputs have exactly the
same slew rate (NO!) and they switch at exactly the same time (NO!),
regardless of aging or temperature, or if you use the Precompensation
features (don't go there). So if some common mode ends up on your bus for
some other reason ( :) ), you should have the common mode terminated also.
Boris
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