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LMK04828: CLKIN Input termination

KLN
Intellectual 320 points

Part Number: LMK04828
Other Parts Discussed in Thread: LMK04832

Hi Team,

I have a question on LMK04828/32 CLKIN Pin impedance

Does CLKINx pin of LMK has internal 50Ohm Termination internal to the chip

We are using DC Coupled CLKIN from OCXO to CLKIN, Is it required to terminate the CLKIN Pin with 50 Ohm?

Thanks

  • 0
    TI__Mastermind 35320 points

    LMK04828 and LMK04832 do not have internal termination at the CLKin pins. The inputs are nominally high-impedance in both bipolar and MOS modes. Because of the limited signal swing and the need to cross a threshold at nominal 1.65V, we generally recommend AC-coupling signals, and placing the 50Ω termination before the AC-coupling capacitor and the AC-coupling capacitor before the pin. If you DC-couple LVCMOS, ensure that the signal swing is limited to 2.4Vpp at the pin, and that the input still transitions across the 1.65V nominal threshold (a 0V to 2.4V CMOS signal is sufficient). Because LVCMOS drivers are often not equipped for 50Ω loading, you could instead use a resistive divider load like 50Ω and 100Ω to reduce the input amplitude of e.g. a 3.3V LVCMOS signal to acceptable levels. For DC-coupled sine wave, particularly at low frequencies, the slew rate limitations may prevent DC-coupling with sufficient amplitude.

    If you only use one input pin, terminate the other pin with 50Ω and a capacitor to GND in series.