Fully Isolated RS-485 bus - LTM2881-3

Hi,

I am interested in the LTM2881-3 isolated RS-485 transceiver.  The typical application circuit on page 24 of the datasheet (also attached) shows the isolated RS-485 bus being completely isolated on all nodes.  What keeps the isolated part of the bus from floating beyond the maximum 560V working voltage of the device due to coupling on long distance cables?  Is it the 1Gohm input to output resistance for example?

What use is connecting the screen to the fully isolated 0V?

If this configuration works it is of interest to my application as I have floating 12V d.c low voltage supplies with no earth connection on any nodes of the bus.  Any advice on additional surge protection devices and connectivity is also welcome:  

Surges EN 61000-4-5 due to wires >10m.  Open circuit voltage waveform Tr/Th of 1.2/50us, short circuit current waveform of 8/20us:

    • Line to ground voltage ±1kV
    • Line to line voltage ±0.5kV
  • 0
    •  Analog Employees 
    on May 10, 2018 12:19 AM

    Hi Louis,

    LTM2881-3 has built in coupled inductors( for signal isolation) and transformer( for power isolation) to keep the logic side and bus side isolated, and the galvanic isolation barrier can sustain the maximum 560Vpk working voltage. Connecting the screen to the isolated 0V may help reduce the noise induced by external environment.

    If you want additional ESD/EFT/surge protection skills, we have an application note for your reference: EMC-Compliant RS-485 Communication Networks 

    Regards,

    Neil

  • Thanks Neil,
    Could you look into the main query I had which was:
    What keeps the isolated part of the bus from floating beyond the maximum 560V working voltage of the device due to coupling on long distance cables?
     
    Regards,
     
    Louis
  • 0
    •  Analog Employees 
    on May 15, 2018 4:41 AM

    Hi Louis,

    I think we need to know the difference between "working voltage" and "transient voltage", the "working voltage" is voltage that can be applied to the input and output of isolated part continuously, and the "transient voltage" is short-period higher energy signal like ESD/EFT/Surgethat injected into the bus pins. 

    The isolation barrier would have some issue if you keep the bus side floating beyond the maximum working voltage for a long time, so you should look into your application and make sure the working voltage of the isolated RS-485 could meet the system requirement. For transient protection, we do not have such application notes like inserting a 1M Ohm resistor in parallel with capacitor between the isolated 0V and earth ground, but you can add additional protection skills and leave the isolated bus side ground floating directly.

    The resistor should comply with creepage/clearance/working voltage requirements for reinforced insulation.

    Regards,

    Neil

  • What keeps the isolated part of the RS-485 bus from floating beyond the maximum 560V working voltage of the device due to coupling on long distance cables?

    For example the datasheet for the Renesas giant magneto resistive device ISL32745E suggests a 1MΩ resistor such as the HVC1206-1M0FT3 between the isolated 0V and the local 0V to prevent build up of static charge on floating grounds and cable shields.  Albeit with a capacitor in parallel to discharge surges to earth which would not be suitable in this SELV application due to there being no earth.  Does the resistor compromise the reinforced insulation?