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DS9638: DS 9638 Output Impedance?

Dan Stanley
Prodigy 20 points
Part Number: DS9638
Other Parts Discussed in Thread: THVD1551

Hello to All....

Does anyone know what the actual output impedance of the DS 9638 RS-422 line driver chip is in ohms?

Thanks!

Dan

  • 0
    TI__Guru 62920 points
    Hi Dan,

    We'll take a look into this on Monday.

    Max
  • 0 in reply to Miguel Robertson
    TI__Guru 62920 points
    Hi Dan,

    Unfortunately the data readily available on this device is a bit sparse given it was originally developed and characterized over 20 years ago. We can infer a few things from the datasheet, though. Taking a look at the VOL spec, we see that it's maximum value is 500 mV for an input current of 40 mA; that would suggest a maximum output impedance of just 12.5 Ohms. A similar calculation for the high-side impedance using the minimum VOH of 2.0 V (meaning a 2.5-V drop from the 4.5-V VCC level to the driver output) yields a maximum output impedance of 62.5 Ohms.

    Keep in mind these should be considered maximum values. In reality the output structures of these devices are generally not ideal resistances with linear I/V characteristics; the voltage drops encompassed by the VOH/VOL specifications generally include both resistive components (such as the on-state resistances of the transistors used) as well as relatively "fixed" drops (e.g., due to diode elements).

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 20 points

    Max:

    Thanks for the response!

    I was thinking of using the DS9638 as an output buffer for a SPDIF digital audio

    signal. I am sure that you are aware that a 75 ohm value is required for an SPDIF

    output impedance, and that 75 ohm BNC jacks are used with a 75 ohm coax cable.

    It sounds like the DS9638 has an output impedance that varies in a way that would

    make it very difficult to get a steady 75 ohms output impedance with a series output

    resistor.  Would you agree?

    Thanks again!

    Dan

  • 0 in reply to Dan Stanley
    TI__Guru 62920 points
    Dan,

    Yes, if a well-controlled output impedance is needed then I wouldn't recommend DS9638 - there is too much difference between the pull-up and pull-down strength. You may want to look at the newer (but pin-to-pin) THVD1551. It has a pull-up strength in the 12-13 Ohm range and a pull-down strength in the 6-7 Ohm range. If you added a 66.5-Ohm series resistance on each it would get you closer to what you need. To be conservative, though, I'd expect up to 50% variation in this value when accounting for process variation and operation across the recommended operating voltage/temperature ranges, so still if you need something that is very precise in impedance you may need to look for devices specific to SPDIF.

    Max