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LMK1C1104: Termination and output impedance

Ella KIM
Expert 3742 points
Part Number: LMK1C1104
Other Parts Discussed in Thread: CDCLVC1104,

Hi,

I'm looking at LMK1C1104 to replace CDCLVC1104. I have two basic questions regarding termination.

1. The same receiver can be terminated with 50ohm like Figure 8-2 while it can be un-terminated like Figure 8-3. What's the difference in operation between two?

>> LMK1C1104 datasheet

2. When I'm comparing Figure 8-3 in LMK1C1104 and Figure 5 in CDCLVC1104, it looks the former one does not require Rs. May I know why?

>> CDCLVC1104 datasheet

3. Output impedance of CDCLVC1104 is 45ohm at 3.3. I expected Rs should be 5ohm but it's recommended to be 10ohm in Figure 5. Can you kindly explain why?

Thanks,

Ella

  • 0
    TI__Genius 9170 points

    Hi Ella,

    Ella KIM said:

    The same receiver can be terminated with 50ohm like Figure 8-2 while it can be un-terminated like Figure 8-3. What's the difference in operation between two?

    >> LMK1C1104 datasheet

    Figure 8-2 is usually used to drive 50 ohm load in this configuration to see the drive current of the part.  Note that it would reduce your swing by half.

    Figure 8-3 termination is usually LVCMOS to LVCMOS common termination which would have proper LVCMOS swing at the receiving end.

    Ella KIM said:

    2. When I'm comparing Figure 8-3 in LMK1C1104 and Figure 5 in CDCLVC1104, it looks the former one does not require Rs. May I know why?

    >> CDCLVC1104 datasheet

    Output impedance of both driver are 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.

    Ella KIM said:
    3. Output impedance of CDCLVC1104 is 45ohm at 3.3. I expected Rs should be 5ohm but it's recommended to be 10ohm in Figure 5. Can you kindly explain why?

    Rs = 5 ohm would be ideal case here as well. I agree with you but datasheet number is typical so I am not sure if it vary some ohm over PT.

    Best,

    Asim