DS90LV028 NSC [National Semiconductor], DS90LV028 Datasheet - Page 4

• Manufacturer Part Number: DS90LV028
• Description: 3V LVDS Dual CMOS Differential Line Receiver
• Manufacturer: NSC [National Semiconductor]

www.national.com
Applications Information
set voltage which is typically +1.2V. The driven signal is cen-
tered around this voltage and may shift
ter point. The
potential difference between the driver’s ground reference
and the receiver’s ground reference, the common-mode ef-
fects 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
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 0.001µF 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 board layers (top to bottom): LVDS sig-
nals, 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
flections 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 can-
cellation benefits of differential signals and EMI will result!
(Note that the velocity of propagation, v = c/E
speed of light) = 0.2997mm/ps or 0.0118 in/ps). Do not rely
solely on the autoroute function for differential traces. Care-
fully review dimensions to match differential impedance and
provide isolation for the differential lines. Minimize the num-
ber 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
±
1V shifting may be the result of a ground
<
10mm long). This will help eliminate re-
CC
, but exceeding V
±
1V around this cen-
(Continued)
r
where c (the
CC
will
4
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 90
mode outputs need the termination resistor to generate the
differential voltage. LVDS will not work without resistor termi-
nation. 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
(12mm MAX).
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 re-
ceiver, care should be taken to prevent noise from appearing
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 stable known state of HIGH output voltage) for
floating, terminated or shorted receiver inputs.
1.
2.
3.
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 5k
waveform distortion 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.
Probing LVDS Transmission Lines:
Always use high impedance (
(
scope. Improper probing will give deceiving results.
Cables and Connectors, General Comments:
<
2 pF) scope probes with a wide bandwidth (1 GHz)
device, and if an application requires only 1 receiver, the
unused channel inputs should be left OPEN. Do not tie
unused receiver inputs to ground or any other voltages.
The input is biased by internal high value pull up and pull
down resistors to set the output to a HIGH state. This in-
ternal circuitry will guarantee a HIGH, stable output state
for open inputs.
unplugged), or if the driver is in a power-off condition,
the receiver output will again be in a HIGH state, even
with the end of cable 100
the input pins. The unplugged cable can become a float-
ing antenna which can pick up noise. If the cable picks
up more than 10mV of differential noise, the receiver
may see the noise as a valid signal and switch. To insure
that any noise is seen as common-mode and not differ-
ential, a balanced interconnect should be used. Twisted
pair cable will offer better balance than flat ribbon cable.
the receiver inputs together, thus resulting in a 0V differ-
ential input voltage, the receiver output will remain in a
HIGH state. Shorted input fail-safe is not supported
across the common-mode range of the device (GND to
2.4V). It is only supported with inputs shorted and no ex-
ternal common-mode voltage applied.
Open Input Pins. The DS90LV028A is a dual receiver
Terminated Input. If the driver is disconnected (cable
Shorted Inputs. If a fault condition occurs that shorts
and 130 . Remember that the current
to 15k range to minimize loading and
>
termination resistor across
100k ), low capacitance
<
10mm