The DS90LV048A differential line receiver is capable of de-

range centered around +1.2V. This is related to the driver off-

set voltage which is typically +1.2V. The driven signal is cen-

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.

The DS90LV048A has a flow-through pinout that allows for

easy PCB layout. The LVDS signals on one side of the de-

vice easily allows for matching electrical lengths of the differ-

ential 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 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

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 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. Care-

fully review dimensions to match differential impedance and

provide isolation for the differential lines. Minimize the num-

ber 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 allow-

able.

Termination:

Use a termination resistor which best matches the differen-

tial impedance or your transmission line. The resistor should

be between 90

Ω and 130Ω. Remember that the current

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% 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

(12mm MAX)

Probing LVDS Transmission Lines:

Always

use

high

impedance

100k

Ω),

low

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 100

Ω. 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 re-

duction 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 differen-

tial mode) noise which is rejected by the receiver.

work effectively. For distances 0.5M

≤ d ≤ 10M, CAT 3 (cat-

egory 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 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.

Open Input Pins. The DS90LV048A is a quad receiver

device, and if an application requires only 1, 2 or 3 re-

ceivers, the unused channel(s) 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 internal circuitry will guarantee a

HIGH, stable output state for open inputs.

www.national.com

5