DS92001_06 Datasheet, PDF (10/12 Pages) National Semiconductor (TI) – 3.3V B/LVDS-BLVDS Buffer

Application Information (Continued)
Avoid 90˚ turns (these cause impedance discontinuities).
Use arcs or 45˚ bevels.
Within a pair of traces, the distance between the two traces
should be minimized to maintain common-mode rejection of
the receivers. On the printed circuit board, this distance
should remain constant to avoid discontinuities in differential
impedance. Minor violations at connection points are allow-
able.
TERMINATION
Use a termination resistor which best matches the differen-
tial impedance or your transmission line. The resistor should
be between 90Ω and 130Ω for point-to-point links. Multidrop
(driver in the middle) or multipoint configurations are typically
terminated at both ends. The termination value may be lower
than 100Ω due to loading effects and in the 50Ω to 100Ω
range. Remember that the current mode outputs need the
termination resistor to generate the differential voltage.
Surface mount 1% - 2% resistors are the best. PCB stubs,
component lead, and the distance from the termination to the
receiver inputs should be minimized. The distance between
the termination resistor and the receiver should be < 10mm
(12mm MAX).
PROBING LVDS TRANSMISSION LINES
Always use high impedance (> 100kΩ), low capacitance
(< 2 pF) scope probes with a wide bandwidth (1 GHz)
scope. Improper probing will give deceiving results.
FAILSAFE FEATURE
The BLVDS receiver is a high gain, high speed device that
amplifies a small differential signal (30mV) to BLVDS ouput
drive levels. Due to the high gain and tight threshold of the
receiver, care should be taken to prevent noise from appear-
ing as a valid signal.
The receiver’s internal fail-safe circuitry is designed to
source/sink a small amount of current, providing fail-safe
protection (a high level output voltage ) for floating, termi-
nated or shorted receiver inputs.
1. Terminated Input. If the driver is disconnected (cable
unplugged), or if the driver is in a power-off condition,
the BLVDS outputs will again be in a HIGH state, even
with the end of cable 100Ω termination resistor across
the input pins. The unplugged cable can become a
floating antenna which can pick up noise. If the cable
picks up more than 10mV of differential noise, the re-
ceiver may see the noise as a valid signal and switch. To
insure that any noise is seen as common-mode and not
differential, a balanced interconnect should be used.
Twisted pair cable will offer better balance than flat
ribbon cable.
2. Shorted Inputs. If a fault condition occurs that shorts
the receiver inputs together, thus resulting in a 0V differ-
ential input voltage, the BLVDS outputs will remain in a
HIGH state. Shorted input fail-safe voltage range is 0V
to 2.4V.
3. External Biasing. External lower value pull up and pull
down resistors (for a stronger bias) may be used to
boost fail-safe in the presence of higher noise levels.
The pull up and pull down resistors should be in the 5kΩ
to 15kΩ range to minimize loading and waveform distor-
tion to the driver. The common-mode bias point should
be set to approximately 1.2V (less than 1.75V) to be
compatible with the internal circuitry. Please refer to
application note AN-1194 “Failsafe Biasing of LVDS In-
terfaces” for more information.
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