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Douglas C. Smith

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Technical Tidbit - February 2013
LVDS, Be Careful of EMI Induced Signal Corruption
(ESD and EFT Induced Errors)

LVDS diagram

Figure 1.
LVDS Diagram
http://en.wikipedia.org/wiki/File:Basic_LVDS_circuit_operation.png#filelinks
http://creativecommons.org/licenses/by-sa/3.0/deed.en

Abstract: LVDS, Low Voltage Differential Signaling, offers improvement in signal quality and EMC emissions, however it is not always effective against external pulsed stresses such as ESD and EFT. Limitations of LVDS are discussed and recommendations made.

LVDS offers substantial benefits from the differential mode signaling utilized. However, there are serious limitations with LVDS regarding pulsed EMI such as ESD and EFT.

I have noticed a trend of problems with systems using LVDS over the last several years. The problems result from the approximately two Volt common mode range of LVDS. Once this limit is exceeded, the receiver input saturates and the data is lost. Such a common mode voltage may be generated by ESD and EFT under certain conditions including:
  • Signals that leave the circuit board on which they are generated
  • Signals that travel over long shielded cables (one meter or more, for instance) or over any length of unshielded cables outside of a shielded enclosure
  • Single point grounding is attempted in the system design
  • Some PCB stack-ups and layouts, such as traversing from an upper layer referenced to a ground plane of a board with many layers, for instance 16, to a lower layer referenced to a power plane unless several other conditions are met
  • The signals are generated or used on a four layer PCB, unless great care is taken in routing and grounding
If the receiver is likely to be driven into saturation by any cause, it is necessary for the system to be tolerant of the errors that will be generated in the data. If not, the system will likely experience problems, especially when ESD and/or EFT is present either for an intended test or in the operating environment of the equipment. Meeting one or more of the conditions above does not guarantee a problem, but there is a substantial risk of serious system problems if the system is not error tolerant. A good rule of thumb for LVDS is to consider it as two unbalanced signals with no common mode rejection as far as pulsed EMI from sources like ESD and EFT are concerned.

Summary: Differential signaling, such as LVDS, does not necessarily confer immunity to EMI generated by pulsed sources like ESD and EFT. In fact, problems are likely to occur under a set of PCB, cable, and enclosure conditions that occur in many systems.

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