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Receiver Hysteresis SN65hvd

Siddharth Gupta84
Intellectual 450 points


Hi,

I want to select a sigle to differential converter for RS485 bus.

I am unable to decide what should be hysteresis requirement for my differential receiver. As transreceivers are available for different receiver hysteresis.

Throughput < 1Mbps

Kindly guide how to calculate requirement of receiver hysteresis for my application.

Siddharth Gupta

  • 0
    TI__Guru* 98135 points
    Siddharth,


    I've moved your post to the Industrial Interface Forum where you should be able to find an answer. Your original post was in Etc...Data Converters Forum, where people usually post questions about analog-to-digital or digital-to-analog questions.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru 62920 points
    Hi Siddharth,

    Receiver hysteresis can help to prevent a receiver from toggling on its output when its differential input level is close to the switching threshold and there is some noise present. So, the amount of hysteresis that is needed depends on the amount of differential noise you expect to be present. One potential drawback to receiver hysteresis, though, is that it can introduce a little bit of additional jitter (duty cycle distortion) due to using different switching thresholds for rising and falling edges. This can be more of a problem when signal amplitudes are low (i.e., at the receiving end of a long cable) or when signal switching times make up a significant portion of the bit period.

    Max
  • 0 in reply to Miguel Robertson
    Intellectual 450 points

    Hi Robertson,

    Thanks for the reply. I have no idea on how could I estimate differential noise of my system and therefore select a suitable transceiver. I have a 1.5m communication distance (with no multidroping). 

    Data Rate: 19200 bps

    As receiver hysteresis range in TI Ics is 25mV - 650mV. 

    Q1: How could I estimate differential noise of the system?

    Q2: What is the relation between differential receiver hysteresis and differential noise.?

    Thanking you,

    Siddharth Gupta

  • 0 in reply to Siddharth Gupta84
    Intellectual 450 points

    Hi,

    Kindly suggest:

    Q1: How could I estimate differential noise of the system?

    Q2: What is the relation between differential receiver hysteresis and differential noise.?

    Thanking you,

    Siddharth Gupta

  • 0 in reply to Siddharth Gupta84
    TI__Guru 62920 points
    Hi Siddharth,

    Typically you would want the amount of hysteresis to be larger than the expected differential noise level. It is difficult to estimate noise levels very precisely unless you have some past experience with similar systems to use a reference, though. In your case I would just choose a transceiver with "moderate" level of hysteresis (~50 mV) and then revisit the decision if problems arise. If you do expect extreme levels of noise (e.g., if you are using differential cabling that is not twisted together and is operating near potential noise sources like switched-mode power supplies, motors, etc.) then I would start with a device with a high level of hysteresis.

    Best regards,
    Max