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ADM3066E doesn't meet the RS485 standard?

Category: Hardware
Product Number: ADM3066E

Hello,

As far as I know, the RS485 standard requires a +/-200mV of differential voltage.

Table 14 in the datasheet states different voltage levels:

In my tests, I see that it is indeed under the +200mV,

Why isn't it similar to the RS485 standard?

Thanks 



wrong category
[edited by: avihai at 8:38 AM (GMT -4) on 31 May 2022]
  • Hello, 

    Could you please share your measurement setup for the receiver differential input voltage ? In my experience it can be difficult to get an accurate reading of the receiver threshold of the device without filtering. Ambient noise which couples into the cables can convert to a differential voltage which affects the measurement result. 

    The RS-485 specification requires that for a AB voltage of +200mV or greater, the receiver output a logic 1, and for an AB voltage of -200mV or below, the receiver outputs a logic 0. The ADM3066E meets this requirement per the table you have shared above, 

    The setup can be improved by connecting to the AB pins with a matched twisted pair cable, to ensure any ambient noise is equally coupled into both the A and B lines, and does not appear as a differential signal. I would typically recommend placing 100nF capacitors on A and B to ground, and a 1uF capacitor between A and B, when measuring this, to filter out any differential noise that manages to make it's way through to the input pins. 

    Regards,

    Neil 

  • Hello Neil,

    The problem is that it doesn't make sense to me that in this way the protocol doesn't have symmetry around 0V.

  • The asymmetry is intentionally introduced to provide the failsafe function. There some more detail on this in the RECEIVER FAIL-SAFE section of the ADM3066E datasheet. As the receiver logic low threshold is still no greater than -200mV, this still is designed to the TIA/EIA-485-B specification.

    For timing critical applications, the asymmetry can introduce some receiver skew for very slow changing input signals. The ADM3066E has fast output rise/fall time of 6ns, so on a network of purely ADM3066E devices, this will not have an impact. 

    Regards,

    Neil

  • OK, I understand.

    Thank you for the explanation.