DS90LV028AH Datasheet, PDF (4/8 Pages) National Semiconductor (TI) – High Temperature 3V LVDS Dual Differential Line Receiver

Applications Information (Continued)
stub(s), and other impedance discontinuities as well as
ground shifting, noise margin limits, and total termination
loading must be taken into account.
The DS90LV028AH differential line receiver is capable of
detecting signals as low as 100 mV, over a ±1V common-
mode range centered around +1.2V. This is related to the
driver offset voltage which is typically +1.2V. The driven
signal is centered around this voltage and may shift ±1V
around this center point. The ±1V shifting may be the result
of a ground potential difference between the driver’s ground
reference and the receiver’s ground reference, the common-
mode effects of coupled noise, or a combination of the two.
The AC parameters of both receiver input pins are optimized
for a recommended operating input voltage range of 0V to
+2.4V (measured from each pin to ground). The device will
operate for receiver input voltages up to V_CC, but exceeding
V_CC will turn on the ESD protection circuitry which will clamp
the bus voltages.
POWER DECOUPLING RECOMMENDATIONS
Bypass capacitors must be used on power pins. Use high
frequency ceramic (surface mount is recommended) 0.1µF
and 0.01µF 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 board layers (top to bottom): LVDS
signals, ground, power, TTL signals.
Isolate TTL signals from LVDS signals, otherwise the TTL
signals 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 differen-
tial 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 re-
jected by the receiver.
Match electrical lengths between traces to reduce skew.
Skew between the signals of a pair means a phase differ-
ence between signals which destroys the magnetic field
cancellation benefits of differential signals and EMI will re-
sult! (Note that the velocity of propagation, v = c/E _r 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 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 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. Remember that the current
mode outputs need the termination resistor to generate the
differential voltage. LVDS will not work correctly without re-
sistor termination. Typically, connecting a single resistor
across the pair at the receiver end will suffice.
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).
INPUT FAILSAFE BIASING
External pull up and pull down resistors may be used to
provide enough of an offset to enable an input failsafe under
open-circuit conditions. This configuration ties the positive
LVDS input pin to VDD thru a pull up resistor and the
negative LVDS input pin is tied to GND by a pull down
resistor. The pull up and pull down resistors should be in the
5kΩ to 15kΩ range to minimize loading and waveform dis-
tortion to the driver. The common-mode bias point ideally
should be set to approximately 1.2V (less than 1.75V) to be
compatible with the internal circuitry. Please refer to applica-
tion note AN-1194, “Failsafe Biasing of LVDS Interfaces” for
more information.
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 impor-
tant to remember:
Use controlled impedance media. The cables and connec-
tors 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 gener-
ate less EMI due to field canceling effects and also tend to
pick up electromagnetic radiation a common-mode (not dif-
ferential 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.
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