DS90LV047ATMX Datasheet, PDF (7/21 Pages) Texas Instruments – DS90LV047A 3V LVDS Quad CMOS Differential Line Driver

DS90LV047A
www.ti.com
SNLS044C – MAY 2000 – REVISED APRIL 2013
POWER DECOUPLING RECOMMENDATIONS
Bypass capacitors must be used on power pins. Use high frequency ceramic (surface mount is recommended)
0.1μF and 0.001μF capacitors in parallel at the power supply pin with the smallest value capacitor closest to the
device supply pin. Additional scattered capacitors over the printed circuit board will improve decoupling. Multiple
vias should be used to connect the decoupling capacitors to the power planes. A 10μF (35V) or greater solid
tantalum capacitor should be connected at the power entry point on the printed circuit board between the supply
and ground.
PC BOARD CONSIDERATIONS
Use at least 4 PCB layers (top to bottom); LVDS signals, ground, power, TTL signals.
Isolate TTL signals from LVDS signals, otherwise the TTL may couple onto the LVDS lines. It is best to put TTL
and LVDS signals on different layers which are isolated by a power/ground plane(s).
Keep drivers and receivers as close to the (LVDS port side) connectors as possible.
DIFFERENTIAL TRACES
Use controlled impedance traces which match the differential impedance of your transmission medium (ie. cable)
and termination resistor. Run the differential pair trace lines as close together as possible as soon as they leave
the IC (stubs should be < 10mm long). This will help eliminate reflections and ensure noise is coupled as
common-mode. In fact, we have seen that differential signals which are 1mm apart radiate far less noise than
traces 3mm apart since magnetic field cancellation is much better with the closer traces. In addition, noise
induced on the differential lines is much more likely to appear as common-mode which is rejected by the
receiver.
Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase
difference between signals which destroys the magnetic field cancellation benefits of differential signals and EMI
will result. (Note the velocity of propagation, v = c/Er where c (the speed of light) = 0.2997mm/ps or 0.0118
in/ps). Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match
differential impedance and provide isolation for the differential lines. Minimize the number or vias and other
discontinuities on the line.
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 allowable.
TERMINATION
Use a termination resistor which best matches the differential impedance or your transmission line. The resistor
should be between 90Ω and 130Ω. Remember that the current mode outputs need the termination resistor to
generate the differential voltage. LVDS will not work without resistor termination. Typically, connecting a single
resistor across the pair at the receiver end will suffice.
Surface mount 1% to 2% resistors are 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.
CABLES AND CONNECTORS, GENERAL COMMENTS
When choosing cable and connectors for LVDS it is important to remember:
Use controlled impedance media. The cables and connectors you use should have a matched differential
impedance of about 100Ω. They should not introduce major impedance discontinuities.
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