8.3.3 Fail-Safe Feature
The LVDS receiver is a high-gain, high-speed device that amplifies a small differential signal (20 mV) to LVCMOS/LVTTL logic levels. Due to the high gain and tight threshold of the receiver, take care to prevent noise from appearing as a valid signal. The internal fail-safe circuitry of the receiver 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.
- Open Input Pins: It is not required to tie the receiver inputs to ground or any supply voltage. Internal fail-safe circuitry will ensure a HIGH, stable output state for open inputs.
- 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 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 10 mV of differential noise, the receiver may see the noise as a valid signal and switch. To ensure that any noise is seen as common-mode and not differential, a balanced interconnect should be used. A twisted-pair cable will offer better balance than a flat ribbon cable.
- Shorted Inputs: If a fault condition occurs that shorts the receiver inputs together, thus resulting in a 0-V 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 2.4 V). It is only supported with inputs shorted and no external common-mode voltage applied.
External lower value pullup and pulldown resistors (for a stronger bias) may be used to boost fail-safe in the presence of higher noise levels. The pullup and pulldown resistors should be in the 5-kΩ to 15-kΩ range to minimize loading and waveform distortion to the driver. The common-mode bias point should be set to approximately 1.2 V (less than 1.75 V) to be compatible with the internal circuitry. The DS90LV049H is 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 of the newly added ΔIIN specification. Due to the internal fail-safe circuitry, ΔIIN cannot meet the 6-µA maximum specified. This exception will not be relevant unless more than 10 receivers are used.
Additional information on the fail-safe biasing of LVDS devices may be found in AN-1194 Fail-Safe Biasing of LVDS Interfaces (SNLA051).