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DS90LV018A: Irregular output levels SW, Relationship to the fall-safe feature

Hiromu Susami
Prodigy 750 points
Part Number: DS90LV018A


Tool/software:

Hi exparts,

In the circuit I designed, I did not follow the precaution that "the differential input terminals should be opened when not in use", and the output level changed irregularly from "H to L" to "H" to "L". Since an isolation transformer was connected to the IC, the differential input terminals were connected through the transformer.

However, not all ICs experienced this kind of output change, and some ICs had a fixed output level of "H".
Could you tell me what makes this defference? I suspect that the variation in offset voltage may be involved.

Let me know if you need more info.

Regards,
Hiromu

  • 0
    Guru 317730 points

    The section "fail-safe feature" in the datasheet describes when the output is guaranteed to be constant.

    What exactly is connected to the inputs of the isolation transformer? Is there a termination resistor? Are there signals (or noise) larger than 10 mV?

  • +1
    TI__Expert 3170 points

    Hi Hiromu,

    It sounds like there could be some noise coupling into the transformer and causing the receiver to change output state. The following is taken form the Fail-safe Feature section of the datasheet

    The unplugged cable can become a floating antenna which can pick up noise. If the cable picks up more than 10mV of differential noise, the receiver may see the noise as a valid signal and switch.

    All devices will have some process variation so it is not unexpected that some may be more susceptible to oscillating in this situation. I would recommend following the instruction of the datasheet and leaving the receiver inputs floating when not in use if possible. Additionally, as mentioned in the datasheet, resistors can be added to the inputs to increase the failsafe bias which may help avoid oscillations in this scenario. Please see this App Note on failsafe biasing.

    Regards,

    Jack

  • 0 in reply to Jack Campbell
    TI__Prodigy 750 points

    Hi Jack,

    I understood the situation. If the fail-safe offset voltage is large, it will be relatively less affected by the noise picked up by the antenna.

    What is the variation in offset voltage depending on the device? Is there any data we can disclose?

    Regards,
    Hiromu

  • 0 in reply to Hiromu Susami
    TI__Expert 3170 points

    Hi Hiromu,

    That is not data that we have unfortunately.

    Regards,

    Jack

  • 0 in reply to Jack Campbell
    TI__Prodigy 750 points

    Hi Jack,

    - Regarding the "If the cable picks up more than 10mV of differential noise", are these inspections carried out during pre-shipment testing?
    - Or, since the ”10 mV” is not specified, is this just a reference value?

    Regards,
    Hiromu

  • +1 in reply to Hiromu Susami
    TI__Expert 3170 points

    Hi Hiromu,

    10mV is just a reference. The real switching point can be anywhere between +/- 100mV as specified in the VTH, VTL specs.

    Regards,

    Jack