DS90LT012A, DS90LV012A

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SNLS141D – AUGUST 2002 – REVISED APRIL 2013

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 DS90LV012A and DS90LT012A are single receiver devices. It is not required to tie

the receiver inputs to ground or any supply voltage. Internal failsafe 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.

The DS90LV012A and DS90LT012A are compliant to the original ANSI EIA/TIA-644 specification and is also

compliant to the new ANSI EIA/TIA-644-A specification with the exception the newly added

Due to the internal fail-safe circuitry,

relevant unless more than 10 receivers are used.

Additional information on fail-safe biasing of LVDS devices may be found in AN-1194 (SNLA051).

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 a 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.

Copyright © 2002–2013, Texas Instruments Incorporated

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