We've a strange issue related to ADAU1961. In an AV product we have 4-4 ADAU1961 on an add-on board. The first 4 are used as inputs (analog to digital), they work all right. The 4 outputs (I2S to analog) have the same settings in their registers.
The IC’s are configured once at startup, and only accessed when the user explicitly changes settings (e. g. output volume). Initialization steps:
- The clock source is always the MCLK input, we do not use the PLL in the chip.
- The output chips are I2S slaves, all other bits in Serial Port 0 register are 0.
- The ADC is disabled. DAC is enabled on both channels, other bits are 0 (deemphasis, polarity, mono mode).
- Playback power management enabled on both channels.
- All other record/input regs are at their default values.
- Playback mixer in enabled and unmuted.
After a random amount of time (between a few hours and a few days), one of the output IC’s starts outputting additive noise (~150 mV). According to our tests, only the one that has a speaker connected to it. The others are still working. The register maps match between them. The noise is still present when the I2S data is a const 0 (while the clock signals are still active). If we remove the clock signals, the noise disappears. We measured all of the I2S pins, all 4 IC’s receive the same signals, they have the same settings but one of them is noisy.
We were able to remove the noise, and also our signal, by disabling the core clock, or disabling the playback power management channels, or disabling DAC. Also, setting the DAC volume affects the volume of our noise as well. We tried to disable the IOVDD voltage, but that did not help at all.
We were Not able to remove the noise permanently without a hard-reset of the part, both IOVDD and AVDD disabled. Rewriting most registers in the init sequence of the soft restart did not fix the problem.
Please let me know if you have any idea or if you have experienced similar symptoms before. As it is really hard to investigate this issue, any hint is highly appreciated.