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ISO1176: Switching Characteristics of tpLH, tpHL (100Ω line impedance)

Dice-K
Mastermind 6330 points
Part Number: ISO1176

Hello.

A 54Ω resistor is used in the test environment of the data sheet characteristics.
In the case of 100Ω line impedance, how will "tpLH, tpHL" change?

Regards,
Dice-K

  • 0
    TI__Expert 6195 points
    Hi Dice-K,

    Off the top of my head I'm not quite sure what the effect on tpLH, tpHL will be if you changed the resistor from 54Ω to 100Ω.

    I'll put together a bench test in the lab and report the results back to you as soon as I get them. I should have something to show you by the end of the week.

    Best regards,
    Dan
  • 0 in reply to Dan Kisling
    TI__Expert 6195 points

    Hi Dice-K,

    I tried this in the lab and have confirmed the results with design.

    The resistor will affect VOD and hence rise time will be the main factor here. Typical rise time is 3ns with a 54 ohm resistor. VOD would increase by roughly 1.3x for 100 ohms. 

    1.3 x 1.5ns = 1.95ns

    This is an increase in about 450ns from the 54 ohm case. So we would not expect propagation delay to increase by more than 500ps. 

    500ps is a quite small change so it can be lost in the noise in a scope shot (this seems the case in my scope shots). Nonetheless I have included them for your reference. Measurements are in the bottom banner. 

    ISO1176 with 54 Ohm resistor: 

    ISO1176 with 100 Ohm resistor:

    Please let me know if I've answered your question or need me to clarify anything!

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Mastermind 6330 points

    Hello Dan,
    Thank you for your reply.

    I have a question about your answer.

    I understand that "tr" and "tf" will be 1.3 times from your answer.
    Typical "tr" and "tf" are 3ns with a 54 ohm resistor.
    Will not the calculation be like this?

    1.3 x 3.0ns = 3.9ns

    This is an increase in about 0.9ns from the 54 ohm case.

    Best regards,
    Dice-K

  • 0 in reply to Dice-K
    TI__Expert 6195 points
    Hi Dice-K,

    Sorry for the confusion. I divided 3ns by 2 due to the definition of tPLH and tPHL. If you look at how they are defined in Figure 17 of the datasheet it's in reference to VOD crossing 0V, which is half tr / tf.

    Best regards,
    Dan