DS90LV032AQML

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SNLS205A – NOVEMBER 2011 – REVISED APRIL 2013

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

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. 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 resistor which best matches the differential impedance of 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 termination. Typically, connect a single resistor across the

pair at the receiver end.

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 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.

CABLES AND CONNECTORS, GENERAL COMMENTS

When choosing cable and connectors for LVDS it is important to remember:

Use controlled impedance media. The cables and connectors 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 reduction and signal quality. Balanced cables tend to generate less EMI due to field canceling effects and

also tend to pick up electromagnetic radiation as common-mode (not differential mode) noise which is rejected by

the receiver. For cable distances < 0.5M, most cables can be made to work effectively. For distances 0.5M

≤ d ≤

10M, CAT 3 (category 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 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 DS90LV032A is a quad receiver device, and if an application requires only 1, 2 or 3

receivers, 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 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 TRI-STATE or

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.

Copyright © 2011–2013, Texas Instruments Incorporated

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