• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 22 subscribers
  • Views 613 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMK00304 offset for AC-coupled inputs

Calvin Field
Prodigy 180 points
Other Parts Discussed in Thread: LMK00304

We have a system where we route a single ended sinewave through a transformer to create a differential signal that is ac-coupled into a clock input of an LMK00304.  We notice that LMK00304 inputs are offset by ~300 mV (P side DC offset from N) possibly due to a pullup/pulldown structure in the LMK00304.  The offset results in an increase in the minimum operating level requirement (Vid) from 150 mvpp to ~450 mVpp. Is there perhaps a failsafe built into the inputs to prevent oscillation when no signal is present?  Would you please confirm.  Also do you have a recommendation to eliminate this offset? Thank you. 

  • 0
    TI__Mastermind 32575 points
    Yes, there is input offset voltage of ~300 mV as a failsafe to prevent self-oscillation/chatter when inputs are left open; this ensures logic low input & output state when inputs are left open. The input offset is determined by "weak" bias resistors (>50 kohm).

    To collapse the input offset, you can apply a differential load resistor across P and N inputs (e.g. 100 ohm line termination); however, chatter can occur if the input clock goes missing or has low slew rate/amplitude. Alternatively, you could provide external biasing on P and N separately using smaller resistors (< 5kohm) that will override the weak internal input bias resistors.

    Alan
  • 0 in reply to Alan O
    Prodigy 180 points
    Thank you for the quick response. Is this information in the data sheet? I found no reference to it.