SNLS045B – JULY 1999 – REVISED APRIL 2013
APPLICATION INFORMATION
General application guidelines and hints for LVDS drivers and receivers may be found in the following application
notes: LVDS Owner's Manual (lit #550062-002), AN-808 (SNLA028), AN-977 (SNLA166), AN-971 (SNLA165),
AN-916 (SNLA219), AN-805 (SNOA233), AN-903 (SNLA034). The latest applications material is available on the
web at:
LVDS drivers and receivers are intended to be primarily used in an uncomplicated point-to-point configuration as
is shown in
This configuration provides a clean signaling environment for the fast edge rates of the
drivers. The receiver is connected to the driver through a balanced media which may be a standard twisted pair
cable, a parallel pair cable, or simply PCB traces. Typically, the characteristic impedance of the media is in the
range of 100Ω. A termination resistor of 100Ω (selected to match the media), and is located as close to the
receiver input pins as possible. The termination resistor converts the driver output (current mode) into a voltage
that is detected by the receiver. Other configurations are possible such as a multi-receiver configuration, but the
effects of a mid-stream connector(s), cable stub(s), and other impedance discontinuities as well as ground
shifting, noise margin limits, and total termination loading must be taken into account.
The DS90LV048A differential line receiver is capable of detecting signals as low as 100mV, 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.
The DS90LV048A has a flow-through pinout that allows for easy PCB layout. The LVDS signals on one side of
the device easily allows for matching electrical lengths of the differential pair trace lines between the driver and
the receiver as well as allowing the trace lines to be close together to couple noise as common-mode. Noise
isolation is achieved with the LVDS signals on one side of the device and the TTL signals on the other side.
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.
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