• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 637 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.

lm4675 pop

Kevin Braswell
Intellectual 295 points

Other Parts Discussed in Thread: LM4675, LMC6035

I'm debugging an audio quality issue in a design using the LM4675.  The LM4675 is implemented (almost!) as per Figure 28 (single-ended input) in the datasheet (rev 1.3).

Component Values...

Ri = 75k

Ci = 3.3nF

Cs = 4.7uF + 22pF

The notable difference from Figure 28 is that the AC ground is tied to IN- (not IN+).  Correspondingly, the single-ended input is tied to IN+ (not IN-).

Could this be the cause of pop and click noises we observe?  This design is implemented in very small mechanics with limited options to re-route the signals through rework.

Thanks!

  • 0
    Intellectual 295 points

    I did find a way to cut traces and solder in some 36 AWG wire to route the single-ended input to IN- and the AC GND to IN+.

    It did not immediately fix the problem.  All of the audio testing is subjective, so there could have been some improvement (i.e. reduction in the pop) but it is certainly still present.  It also seemed that the SPL might have been higher, but no definitive measurements.

    As for the single-ended input signal, it starts as ~32kHz PWM and goes through two second-order Butterworth filters, Sallen-Key topology (i.e. 4 poles).  The op-amps we are using are LMC6035.  Each second-order filter cut-off is at 3.3kHz.  For reference, Q = 0.347.

  • 0 in reply to Kevin Braswell
    Intellectual 295 points

    Just as a test, I disconnected the filter input from the PWM signal and tied the filter input to GND instead.  The pop is still there.

  • 0 in reply to Kevin Braswell
    TI__Guru**** 189401 points

    Hi, Kevin,

    I highly doubt the issue is due to the routing of the single-ended signal.

    Most likely, your pop is caused by mismatches of the input impedances seen by the part. If there is a differential voltage applied to the inputs at start-up due to different time constants, a pop will result.

    -d2