SNLS045B – JULY 1999 – REVISED APRIL 2013
Figure 6. VTC of the DS90LV048A LVDS Receiver
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 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 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.
Additional information on fail-safe biasing of LVDS devices may be found in AN-1194.
PIN DESCRIPTIONS
Pin No.
2, 3, 6, 7
1, 4, 5, 8
10, 11, 14, 15
16
Name
R
IN+
R
IN−
R
OUT
EN
Non-inverting receiver input pin
Inverting receiver input pin
Receiver output pin
Receiver enable pin: When EN is low, the receiver is disabled. When EN is high and
EN* is low or open, the receiver is enabled. If both EN and EN* are open circuit,
then the receiver is disabled.
Receiver enable pin: When EN* is high, the receiver is disabled. When EN* is low or
open and EN is high, the receiver is enabled. If both EN and EN* are open circuit,
then the receiver is disabled.
Power supply pin, +3.3V ± 0.3V
Ground pin
Description
9
EN*
13
12
V
CC
GND
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