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DS16F95: Receiver output impedance

Gregory Duperon
Expert 7670 points
Part Number: DS16F95


I have a customer attempting to interface the DS16F95 with an MCU.

Customer has an interest in the output impedance of the receiver output.

In addition, how close to supply rail does receiver output swing for H/L output?

Thanks.

  • 0
    TI__Guru 62930 points
    Gregory, the impedance can be estimated from the IBIS model.
    I you look at the pullup and pulldown parameters, they show the output relationship when a voltage is forced. This can be used to estimate the impedance. For the pulldown, I estimate ~12ohms. For the pullup (drive high), I estimate 100ohms.

    The customer can also use the IBIS model to evaluate signal integrity.

    If this answers your question, please click "Verify it as the answer"
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Guru 62930 points
    I missed the second question. If they are lightly loaded, the driver will driver very close to rails.
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    Prodigy 60 points
    In this application, we're sourcing approximately 170uA from the output. Is your lightly-loaded assumption still valid in this case and if not, how would we be able to best approximate the typical VHigh output for this loading?
  • 0 in reply to Colin Andrew
    Prodigy 60 points

    Furthermore, it looks like there was some confusion, as our chip in question is the DS16F95QML RAD-Hardened version which doesn't appear to have any IBIS data associated with it. Can we be guaranteed that this device's receiver output will perform the same as the normal DS16F95? 

  • 0 in reply to Colin Andrew
    TI__Guru 62930 points
    The IBIS model for the commercial device will be representative of the QML version. Keep in mind, these models are not sign-off quality, but are still useful.

    After looking at this again closer, this technology may not respond as I had indicated with respect to lightly loaded outputs.
    The datasheet shows specification for fairly lightly (max) load results for receiver. For VOH with only 400uA of load, the minimum spec output is 2.5V.

    I was able to check with co-worker who supported these devices in the past and he provided me some additional data that will help.

    The output for driving to VCC is essentially an 80ohm resistor to VCC through the output transistor. So, the output would be a diode drop plus the drop through the 80ohm resistor. The specification is very conservative, and likely related to meeting legacy specifications.

    So, with lightly loaded, you should see a diode drop + very small amount for 80 ohm resistance.

    I also obtained some characterization data:
    With very lightly loaded (4uA) the output should be approximately 1.4V below VCC at room temp.
    At elevated temperatures, the drop is less, and is approx 1.05 below VCC at 125c.

    I did perform an IBIS simulation on your conditions.
    A simple 20kohm resistor to ground resulted in 175uA current with an output of 3.49V.

    Regards,
    Wade