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PCM2903C: PCM2903CDB_Humming & hissing noise

Part Number: PCM2903C
Other Parts Discussed in Thread: PCM2903

We've followed the layout of PCM2903 reference design & made the design but we're facing noise issue. 

We're unable to isolate whether it's grounding or Out of band noise or power supply coupling noise. 

1. Out of band noise: If we put a LPF with a cut off at 30khz then will it make any impact as the audio range is upto 20khz only! Are we talking about subharmonics over here?

2. Grounding issue: We've made the ground layout as per the reference design. As per the standard layout, every ground needs to be seperated & connected via short ground but reference hasn't followed that. So we don't know whether to rule out this or not! as it's very complicated to analyse the ground noise/bounce. 

3. Power supply noise: I did check the frequency spectrum via my smartphone app & there was a peak at 5khz & 10khz so I was suspecting the power supply noise but when we're transmitting a recorded message via USB, there's no noise coupling so getting confused on this. 

Please guide on this as the simulation done w.r.t filter design on TINA isn't matching with the practical implementation at all. 

  • Hi,

    -. The LPF is to remove out of audio band signals due to sigma delta over sampling architecture. If the signal is passed to amplifier without LPF, the out of band signal will be amplified as well.

    -. Yes even with separate gnd plane, you will need to tie them together at a point.

    -. 5KHz/10KHz looks like some interference from the surrounding that was picked up by your app. 

    Is the noise dominant when audio is playing? Is this only on playback? Have you tried with other USB streaming device?