AD1938 55 Hz noise

• Jitter: Ensure that the clock signals (MCLK, BCLK, etc.) are clean and stable. Jitter in the clock can cause artifacts that might manifest as noise in the audio output.
• Synchronization: Check that the clocks for the AD1938 and ADAU1462 are properly synchronized. Any mismatch in clock rates can cause glitches or unexpected DC offsets in the audio output.

•  Improper grounding can lead to noise issues, all the ground pins on the Codec are internally connected It is best to use a ground plane and connect the parts to this plane               with traces as short as possible

• please refer the below circuit which is used in our Eval Board design

Thank you for the detailed response Harish, much appreciated!

I believe all the pulses coming from the DSP into the AD1938 are clean and stable, from what I recall. I have not checked the MCLK pins though, I will do that sometime this week.

But your mention of synchronization is making me think - on my board, AD1938 and ADAU1462 are both powered from two separate crystal oscillators. They're both the same frequency, same load capacitances, but they're independent of one another. Do you think this may be a potential source of problems? I did leave provisions on the PCB to connect the MCLK OUT of the DSP to the DAC, but left them unconnected, and turned off MCLK OUT on the DSP.

The grounding could be a potential issue as well, I made a mistake on the PCB, and one of the analog grounds was left unconnected - I jumped with a short piece of copper wire to a nearby ground. The rest of the grounds are going to a dedicated ground plane. I did expect noise from the layout, but this issues seems to be closer to your first point about mismatching clocks, it feels like the DAC just drops out every now and again.

As for the output filters, I think I followed Figure 31 from the AD1938 datasheet (I pasted it below) It looks a little bit different from the Eval board setup you show. Do you think this could be a potential culprit?

Thank You,

-Ernest

Hello Ernest,

The master clocking is the problem. You are seeing where the drift of the frequencies are such where occasionally there are too many or too few master clock cycles per frame so the DAC will mute. If the frequency was way off you would get nothing but the two crystals are close enough where it is right for a short time but wrong at about a 55Ha rate.  

You MUST send out a bit clock and frame (LR) clock that is divided down from the master clock that is being fed into the codec. 

Regarding the filter. The difference is that the AD1937 is a differential output and the AD1938 is single ended so the filters are slightly different. Your filter circuit is fine. 

Dave T

Hello Dave,

Thank you for the fix! This did it, I deleted the oscillator circuit on AD1938, and activated the MCLK_OUT pin on the ADAU1462, sounds like butter now!

Many thanks,

-Ernest