DS90LV047A

SNLS044C – MAY 2000 – REVISED APRIL 2013

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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.

LVDS FAIL-SAFE

This section addresses the common concern of fail-safe biasing of LVDS interconnects, specifically looking at the

DS90LV047A driver outputs and the DS90LV048A receiver inputs.

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 DS90LV048A 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 DS90LV047A driver is disconnected (cable unplugged), or if the DS90LV047A

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.

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.

Figure 8. Driver Output Levels

8

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