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 for point-to-point links. Multidrop

(driver in the middle) or multipoint configurations are typically

terminated at both ends. The termination value may be lower

than 100

Ω due to loading effects and in the 50Ω to 100Ω

range. Remember that the current mode outputs need the

termination resistor to generate the differential voltage.

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

(12mm MAX).

Probing LVDS Transmission Lines:

Ω), low capacitance

scope. Improper probing will give deceiving results.

FailSafe Feature:

The BLVDS receiver is a high gain, high speed device that

amplifies a small differential signal (30mV) to BLVDS ouput

drive levels. Due to the high gain and tight threshold of the

receiver, care should be taken to prevent noise from appear-

ing 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 high level output voltage ) for floating, termi-

nated or shorted receiver inputs.

1.

Terminated Input. If the driver is disconnected (cable

unplugged), or if the driver is in a power-off condition,

the BLVDS outputs will again be in a HIGH state, even

with the end of cable 100

Ω termination resistor across

the input pins. The unplugged cable can become a

floating antenna which can pick up noise. If the cable

picks up more than 10mV of differential noise, the re-

ceiver may see the noise as a valid signal and switch. To

insure that any noise is seen as common-mode and not

differential, a balanced interconnect should be used.

Twisted pair cable will offer better balance than flat

ribbon cable.

2.

Shorted Inputs. If a fault condition occurs that shorts

the receiver inputs together, thus resulting in a 0V differ-

ential input voltage, the BLVDS outputs will remain in a

HIGH state. Shorted input fail-safe voltage range is 0V

to 2.4V.

3.

External Biasing. 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

Ω

to 15k

Ω range to minimize loading and waveform distor-

tion 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. Please refer to

application note AN-1194 “Failsafe Biasing of LVDS In-

terfaces” for more information.

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