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THVD1400: Does THVD1400 need biasing resistors?

Dmitry Marukh
Prodigy 110 points

Part Number: THVD1400

Unlike datasheets for other devices that clearly show biasing resistors or say that they are not required, this one only mentions "Internal biasing of the receiver inputs" once. Planning on using these at 460800 baud with very short (under 2m) bus.

  • +1
    Guru 320450 points

    The THVD1400 itself has fail-safe inputs and does not need fail-safe resistors. If there are other receivers on the bus that do not have fail-safe inputs, then the bus needs fail-safe resistors.

  • 0
    TI__Guru 53772 points

    Hi Dmitry,

    So there are two common types of biasing resistors in RS-485. The first is to add a common mode level to bus (usually VCC/2) - this is never required by RS-485 but may be implemented due to system needs  and the other is fail-safe biasing resistors which set a default idle voltage to prevent idle bus communication failures - these aren't required either by RS-485 but if you want idle fail-safe they are used and are pretty common. 

    The THVD1400 does not need fail-safe biasing resistors as both its positive going and negative going differential input thresholds are less than 0V - so the device is inherently receiver fail-safe to open, idle, and short bus conditions. You can add biasing resistors to increase the noise margin - as worst case the THVD1400 has 45mV of margin during open, idle, or short - if you add fail-safe biasing resistors you can increase the noise margin. 

    That being said - as Clemens mentioned if other devices on the bus are not fail-safe then you need to add fail-safe biasing resistors (one pair - one from A to VCC and one from B to GND) to ensure that the bus is at least idle failsafe (open and short fail-safe pretty much need an integrated fail-safe to work properly ).

    Please let me know if you have any other questions!

    Best,

    Parker Dodson