DS90CR216MTDX Datasheet, PDF (14/23 Pages) Texas Instruments – DS90CR215/DS90CR216 +3.3V Rising Edge Data Strobe LVDS 21-Bit Channel Link - 66 MHz

DS90CR215, DS90CR216
SNLS129D – MARCH 1999 – REVISED APRIL 2013
AN = ####
AN-1041 (SNLA218)
AN-1035 (SNOA355)
AN-806 (SNLA026)
AN-905 (SNLA035)
AN-916 (SNLA219)
Topic
Introduction to Channel Link
PCB Design Guidelines for LVDS and Link
Devices
Transmission Line Theory
Transmission Line Calculations and Differential
Impedance
Cable Information
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CABLES
A cable interface between the transmitter and receiver needs to support the differential LVDS pairs. The 21-bit
CHANNEL LINK chipset (DS90CR215/216) requires four pairs of signal wires and the 28-bit CHANNEL LINK
chipset (DS90CR285/286) requires five pairs of signal wires. The ideal cable/connector interface would have a
constant 100Ω differential impedance throughout the path. It is also recommended that cable skew remain below
150 ps (@ 66 MHz clock rate) to maintain a sufficient data sampling window at the receiver.
In addition to the four or five cable pairs that carry data and clock, it is recommended to provide at least one
additional conductor (or pair) which connects ground between the transmitter and receiver. This low impedance
ground provides a common mode return path for the two devices. Some of the more commonly used cable types
for point-to-point applications include flat ribbon, flex, twisted pair and Twin-Coax. All are available in a variety of
configurations and options. Flat ribbon cable, flex and twisted pair generally perform well in short point-to-point
applications while Twin-Coax is good for short and long applications. When using ribbon cable, it is
recommended to place a ground line between each differential pair to act as a barrier to noise coupling between
adjacent pairs. For Twin-Coax cable applications, it is recommended to utilize a shield on each cable pair. All
extended point-to-point applications should also employ an overall shield surrounding all cable pairs regardless
of the cable type. This overall shield results in improved transmission parameters such as faster attainable
speeds, longer distances between transmitter and receiver and reduced problems associated with EMS or EMI.
The high-speed transport of LVDS signals has been demonstrated on several types of cables with excellent
results. However, the best overall performance has been seen when using Twin-Coax cable. Twin-Coax has very
low cable skew and EMI due to its construction and double shielding. All of the design considerations discussed
here and listed in the supplemental application notes provide the subsystem communications designer with many
useful guidelines. It is recommended that the designer assess the tradeoffs of each application thoroughly to
arrive at a reliable and economical cable solution.
BOARD LAYOUT
To obtain the maximum benefit from the noise and EMI reductions of LVDS, attention should be paid to the
layout of differential lines. Lines of a differential pair should always be adjacent to eliminate noise interference
from other signals and take full advantage of the noise canceling of the differential signals. The board designer
should also try to maintain equal length on signal traces for a given differential pair. As with any high speed
design, the impedance discontinuities should be limited (reduce the numbers of vias and no 90 degree angles on
traces). Any discontinuities which do occur on one signal line should be mirrored in the other line of the
differential pair. Care should be taken to ensure that the differential trace impedance match the differential
impedance of the selected physical media (this impedance should also match the value of the termination
resistor that is connected across the differential pair at the receiver's input). Finally, the location of the CHANNEL
LINK TxOUT/RxIN pins should be as close as possible to the board edge so as to eliminate excessive pcb runs.
All of these considerations will limit reflections and crosstalk which adversely effect high frequency performance
and EMI.
UNUSED INPUTS
All unused inputs at the TxIN inputs of the transmitter must be tied to ground. All unused outputs at the RxOUT
outputs of the receiver must then be left floating.
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