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LM4675: Reducing EMI

Derek
Mastermind 9752 points
Part Number: LM4675

Hello,

   I'm doing some emergency triage work on a a LM4675 design that is not passing EMI. It's used in a single-ended input:

IN+: has 51k ohm resistor + 0.1uF cap all in series to GND.

IN- has a 12.4k ohm resistor + 100uf Tantalum (yes, that's right) all in series from the source.

Source is CM119B CODEC, line level output. According to the datasheet, output voltage (rms) is 0.995 ohms into 10kOhm loading.

The output of the LM4675 is driving an 8ohm nominal impedance speaker. Each output (Vo1 and Vo2) has 1 100pF cap to GND plus 1 ESD diode to GND, then the connector for the speaker.

Decoupling power pins is (total) one 4.7uF cap, and two 0.1uF caps.

See attached.

I noticed that they've cut up the ground plane - I'm thinking that one change to make would be to use one continuous ground plane for the LM4675 and the CM119B CODEC, right? 

  • 0
    TI__Mastermind 20085 points

    Hi Derek,

    Some advice about the design:
    The input components should be of the same value, I mean both input capacitors should be of the same value and input resistors should the same value as well. This would avoid getting pop noise.
    The analog ground should be in one continuous ground plane. The digital ground of the CODEC should be connected in one part of the PCB layout to the analog ground.

    You should add an LC output filter to meet the EMI requirements. You can refer to the LC Filter Design application report to find further information about how you should design the LC output filter.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer