DS90LV031AQML_14 Datasheet, PDF (7/15 Pages) Texas Instruments – 3V LVDS Quad CMOS Differential Line Driver

DS90LV031AQML
www.ti.com
SNLS204A – NOVEMBER 2011 – REVISED APRIL 2013
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. Lab experiments show that differential signals which are 1mm apart radiate far less noise than
traces 3mm apart since magnetic field cancellation is greater with the closer traces. Plus, 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 auto-route function for differential traces. Carefully review dimensions to match
differential impedance and provide isolation for the differential lines. Minimize the number of 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 resistor which best matches the differential impedance of 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, connect a single resistor across the
pair at the receiver end.
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 (< 2pF) scope probes with a wide bandwidth (1GHz)
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.
Balanced cables (e.g. twisted pair) are usually better than unbalanced cables (ribbon cable, simple coax.) for
noise reduction and signal quality. Balanced cables tend to generate less EMI due to field canceling effects and
also tend to pick up electromagnetic radiation a common-mode (not differential mode) noise which is rejected by
the receiver. For cable distances < 0.5M, most cables can be made to work effectively. For distances 0.5M ≤ d ≤
10M, CAT 3 (category 3) twisted pair cable works well, is readily available and relatively inexpensive.
Fail-safe Feature
The LVDS receiver is a high gain, high speed device that amplifies a small differential signal (20mV) to CMOS
logic levels. Due to the high gain and tight threshold of the receiver, care should be taken to prevent noise from
appearing as a valid signal.
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