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SN65HVD21A spec check

Shu-Cheng LIN
Guru 11785 points

Hi Sirs,

Sorry to bother you.

We will use HVD21A, some spec need confirm.

End customer spec inquire as below, does HVD21A could meet?

especial at signal current  max 2mA @ differential input of 2V. 

Thanks for your reply.

  • 0
    TI__Mastermind 20820 points

    Hi Shu-Cheng,

    The SN65HVD21A will support the input requirement that you highlighted above. The input voltage on the A and B pins can can range from -20 to 25V and the input current will be limited by the input stage of the transceiver to the hundreds of uA range depending on your input voltage. Below is a screen capture from the receiver specs in the SN65HVD21A datasheet:

    In order for the input current to reach 2mA the input resistance of the transceiver would have to be 1k ohm and you can see in the table above that the input resistance is much higher which is the limiting factor. Please let em know if you have any more questions?

  • 0 in reply to Michael Peffers
    Guru 11785 points

    Hi Michael,

    Thanks for your reply.

    Looks HVD21A couldn't meet 2mA because the input resistance the min is 96K. Right?

    So does TI have another solution can do it?

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 20820 points
    Hi Shu-Cheng,

    The spec you pointed out is a maximum spec and is supported by the SN65HVD21A. The 96k that the driver is seeing when transmitting a message is typical across all RS-485 transceivers. Your spec says that you need to support 2V differential and a maximum of 2mA. What the SN65HVD21A receiver will see from the driver is a 2V differential voltage and 20.8uA (2V/96k). I hope this is clear to you now, I may be misunderstanding your question so please clarify further if need be.