DS92001TM datasheet(10/12 Pages) NSC

10 / 12 page

Application Information (Continued)

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

the termination resistor and the receiver should be < 10mm

(12mm MAX).

Probing LVDS Transmission Lines:

Always use high impedance (> 100k

Ω), low capacitance

(< 2 pF) scope probes with a wide bandwidth (1 GHz)

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.

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.

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.

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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