[SI-LIST] : LVDS questions

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From: Tom Zimmerman (zimmerman@fnal.gov)
Date: Tue Feb 01 2000 - 12:13:01 PST


Hello SI experts,

I have a few questions about LVDS and I am hoping a few of you would be kind
enough to help me out. I'm not very familiar with off-chip driver
techniques -- I'm mostly involved in building custom analog IC's for high
energy physics applications.

1. My basic understanding of LVDS (which may be incorrect) is that a
constant current of several mA is supplied to one of the output pads (pads
are terminated in approx. 100 ohms) in order to supply a voltage
differential across the load of several hundred mv. In order to establish a
common mode voltage (approx. 1.2 V), the other pad must effectively be
connected to a voltage source. Is this really the way LVDS is done? This
configuration seems inherently unbalanced, since one output pad is low
impedance, the other high impedance. It seems that this would reduce common
mode noise rejection. Wouldn't a balanced configuration be more desirable??

2. How does LVDS compare to other schemes such as PECL or CML (is CML just
essentially 2 open collectors which switch a current to one output pad or
the other??). Is there some consensus as to a superior scheme, or does each
scheme have its own advantages and disadvantages depending on the
application?

3. Some time ago I was asked to provide low-level differential LVDS-like
digital outputs on a chip I designed. I ended up
simply switching the output pads between 2 equal valued on-chip resistors
(hundreds of ohms), one connected to ground, the other to a positive low
impedance voltage source. In other words, this scheme is like LVDS except
it uses 2 equal valued current sources of rather low output impedance (the
resistors). The common mode output voltage then lies at half the positive
voltage source value. I was attracted to this scheme (perhaps naively)
because of its inherent simplicity and its balanced nature. The users of
this chip report that it works just fine and that they are pleased with the
performance. I would like to know what the disadvantages of this approach
might be (since I am considering using it again). Is this a silly thing to
do, and if so, why? Isn't a balanced output preferable to unbalanced
(if LVDS is indeed unbalanced)?

Thanks in advance for any and all responses. Greatly appreciated,

Tom Zimmerman
Fermilab
zimmerman@fnal.gov

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