DS90LV018ATMX Datasheet, PDF (6/17 Pages) Texas Instruments – DS90LV018A 3V LVDS Single CMOS Differential Line Receiver

DS90LV018A
SNLS014D – JUNE 1998 – REVISED APRIL 2013
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Within a pair of traces, the distance between the two traces should be minimized to maintain common-mode
rejection of the receivers. On the printed circuit board, this distance should remain constant to avoid
discontinuities in differential impedance. Minor violations at connection points are allowable.
TERMINATION
Use a termination resistor which best matches the differential impedance or your transmission line. The resistor
should be between 90Ω and 130Ω. Remember that the current mode outputs need the termination resistor to
generate the differential voltage. LVDS will not work without resistor termination. Typically, connecting a single
resistor across the pair at the receiver end will suffice.
Surface mount 1% - 2% resistors are the best. PCB stubs, component lead, and the distance from the
termination to the receiver inputs should be minimized. The distance between the termination resistor and the
receiver should be < 10mm (12mm MAX).
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 DS90LV018A is a single receiver device. Do not tie the 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 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.
PROBING LVDS TRANSMISSION LINES
Always use high impedance (> 100kΩ), low capacitance (< 2 pF) scope probes with a wide bandwidth (1 GHz)
scope. Improper probing will give deceiving results.
CABLES AND CONNECTORS, GENERAL COMMENTS
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 discontinuities.
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.
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