DS90LV031AQML Datasheet, PDF (8/14 Pages) Texas Instruments – DS90LV031AQML 3V LVDS Quad CMOS Differential Line Driver

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 100Ω. They should not introduce major impedance dis-
continuities.
Balanced cables (e.g. twisted pair) are usually better than
unbalanced cables (ribbon cable, simple coax.) for noise re-
duction 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 dis-
tances < 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 inex-
pensive.
Fail-safe Feature:
The LVDS receiver is a high gain, high speed device that am-
plifies 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 guarantee 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 100Ω 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 5kΩ 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 5. Driver Output Levels
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