3
Receiver Fail-Safe
The UT54LVDS032 receiver is a high gain, high speed device
that amplifies a small differential signal (20mV) to TTL 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 UT54LVDS032 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 three-state 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 offers 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 remains in
a HIGH state. Shorted input fail-safe is not supported
across the common-mode range of the device (V
SS
to
2.4V). It is only supported with inputs shorted and no
external common-mode voltage applied.
