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LMK1C1103: Output Impedance and Transmission Line Matching

Part Number: LMK1C1103
Other Parts Discussed in Thread: LMK1C1102, LMK1C1104,

Tool/software:

The output impedance of the LMK1C1102 / LMK1C1103 / LMK1C1104 is shown in Section 7.5. This changes with voltage (Ro = 50Ω @ 3.3V, Ro = 52.5Ω @ 2.5V, Ro = 60 @ 1.8V) but later on in the datasheet in section 10.2 Typical Application it shows matching to a 50Ω transmission line via series termination and says that "a series resistor is placed near to the IC to closely match the characteristic impedance of the trace to minimize reflections". If the output impedance of the buffer is already 50Ω then surely an additional explicit series resistor is not required for matching to the 50Ω transmission line. Is this typical Application wrong and if so then what is the actual output source impedance of the buffer?

  • Hi Joel, 

    Output impedance of the LVCMOS driver is around 50 ohm itself. So, you would not require external Rs to match the trace impedance. But sometimes you can use this Rs to smooth out any overshoot problem if the receiver is sensitive to that.

    Regards, 
    Vicente