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LMK1C1108: Output impedance of LMK1C1108

Jose Jimenez
Prodigy 150 points
Other Parts Discussed in Thread: LMK1C1108

Hello,

After reading carefully the datasheet of the LMK1C1108, twice, I still don't know if I can drive directly a 50 ohm transmission line (without series termination in the near end) and a 50 ohm parallel termination (in far end).

On the one side, page 5 of the DS indicates a typical Rout (output impedance) of 50 ohm for a 3.3V supply. So far so good. However, in the same page, it also indicates "VOH (Output high voltage) minimum 2.8V for a IOH of 1 mA, which implies a maximum Rout of 500 ohm! Can you, please, clarify this?

Best regards,

Jose Jimenez

  • +1
    TI__Genius 12240 points

    Hi Jose, 

    For your use case, I would consider the LMK1C output impedance to be close to 50 Ohms. The VOH minimum spec has a lot of margin and is probably not the most accurate way to calculate output impedance. 

    LMK1C can drive a 50 Ohm trace with a 50 Ohm parallel termination, the output swing will just be divided in half since the output driver and termination resistor act like a voltage divider. The output driver for this device was designed to drive LVCMOS receivers which are usually modeled as a high-impedance, purely capacitive loads (usually in the range of around 2pF-10pF). Most specs in the datasheet also have this kind of use case in mind. Another concern with using a LVCMOS device to drive a resistive load is power consumption. Assuming you're using a 3.3V supply each output will need to provide a DC current of around 16.5mA which is quite a bit more than typical. If you can AC-couple and re-bias the signal at the receiver then that would be preferred. 

    Regards, 

    Connor 

  • 0 in reply to Connor Lewis
    Prodigy 150 points

    Thank you Connor, you are absolutely right about the VOH minimum spec being a not-very-good way to calculate output impedance. I use it as a last resort when having no other data, but here the datasheet clearly specifies Rout as 50 ohm. My concern remains that no min/max values are indicated for Rout. Except for that, the part looks good for our intended application.

    Your clarifications about the part being designed for standard LVCMOS receivers is also very helpful. However, in this particular application I have to drive some lengthy transmission lines and properly terminate to help with potential EMI issues (signal integrity would not really need 50 ohm parallel termination in the receivers). I take note your recommendations for AC coupling.

    Best regards,

    Jose