[SI-LIST] Re: LVDS cable
- From: "Hofmann, Mark" <Mark.Hofmann@xxxxxxxxxxxxxxx>
- To: "'vinayg@xxxxxxxxxxxxxxx'" <vinayg@xxxxxxxxxxxxxxx>,"'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 19 Sep 2003 15:41:57 -0400
Vinay,
You might want to consider using twisted, shielded pairs to ensure the
successful transmission of your LVDS signals. In one application, we =
were
able to successfully carry LVDS signals 1m (3ft.) The setup that was =
used
included controlled impedance PC boards, 3M MDR style connectors and =
3644B
cable from 3M. This should get you the performance you need. (I'm not
saying that 3M is the best solution, it was just what was available and =
it
worked)
There are people on this list that are far more qualified than I to =
discuss
the reasons why normal ribbon cable is a bad idea. Suffice to say that =
you
should consider a twisted pair solution that provides impedance control =
and
the bandwidth necessary to carry LVDS signals. As the data sheet says:
"The intended application of this device and signaling technique is for
point-to-point baseband data transmission over controlled impedance =
media
of approximately 100 =A7=D9. The transmission media may be =
printed-circuit board
traces, backplanes, or cables."
Another key distinction is the term "high speed". While your data bus
signals are only 20MHz, the LVDS drivers are running much faster. Rise
time is what'll cause you to have problems and the LVDS drivers are =
running
1.2ns maximum, typical numbers are closer to 0.5ns! That's "high =
speed" in
my book.
Mark Hofmann
PerkinElmer Life and Analytical Sciences
mark.hofmann@xxxxxxxxxxxxxxx
-----Original Message-----
From: Vinay G [mailto:vinayg@xxxxxxxxxxxxxxx]=20
Sent: Friday, September 19, 2003 1:59 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] LVDS cable
Hi Gurus,
I have an application where in the databus running at around 20MHz =
needs =3D
to be taken out of the board to a display sitting 2 ft(<1m) away. I am =
=3D
planning to make it differential & send it to the other board (which =
will =3D
help me to move the distance more than 2 feet).=3D20
I am Looking at HIGH-SPEED DIFFERENTIAL LINE TRANSCEIVER from TI =3D
-SN65LVDM1677. =3D20
What type of connector & cables should I use for this type of =
application?=3D
As it is not very high speed & it is short distance (<1m) can I use =
=3D
Ribbon Cable for my application?? Has any body tried such a cable for =
=3D
short distance differential application=3D20
=3D20
Suggestions required,
Thanks in advance
Vinay G
>>> "Boris Yost" <yost@xxxxxxxxxxxxxxxxxxx> 01/30/03 01:37AM >>>
Dear All:
Source termination? I did this, and it works fine. My system may =3D
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 =
=3D
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 =3D
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 =
=3D
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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list=3D20
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at: =3D20
//www.freelists.org/archives/si-list=3D20
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages=3D20
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu=3D20
=3D20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at: =20
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages=20
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
=20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
