DS90LV028A

SNLS013E – JUNE 1998 – REVISED APRIL 2013

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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.

Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase

difference between signals which destroys the magnetic field cancellation benefits of differential signals and EMI

in/ps). Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match

differential impedance and provide isolation for the differential lines. Minimize the number 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 should remain constant to avoid

discontinuities in differential impedance. Minor violations at connection points are allowable.

TERMINATION

Use a termination resistor which best matches the differential 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 correctly without resistor termination. 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 < 10mm (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 receiver, 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 DS90LV028A is a dual receiver 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 internal circuitry will ensure a HIGH, stable output state for open inputs.

2. Terminated Input. If the driver is disconnected (cable 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

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 receiver 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.

3. Shorted Inputs. If a fault condition occurs that shorts the receiver inputs together, thus resulting in a 0V

differential 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 external common-mode voltage applied.

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 15kΩ range to

minimize loading and 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. Please refer to application note

AN-1194 “Failsafe Biasing of LVDS Interfaces” (SNLA051) for more information.

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