So I have dug into the ADAU1966 datasheet and evaluation board manual (applies to ADAU1962 as well).

The datasheet states AVDD is 5V, ranging 4.5V to 5.5V only. The evaluation board has the option to feed AVDD 3.3V or 5V.

As per the last blog post, pin 44 also has different information on datasheet and evaluation board manual. And I don't get the expected CM voltage.

Am I smelling a silicon bug here? Just a guess. It was strange that the ADAU1966 suddenly became unobtanium last year. Analog Devices still say it is in product. Farnell at one point said no longer manufactured. Mouser stocks the "lesser" ADAU1962 (non A) saying life cycle is unclear! Which is no good. Digikey says there are 7000 factory stock of the ADAU1966.

Things are clearer looking at the new ADAU1966A and ADAU1962A. They only allow 3.3V on AVDD, so not directly compatible with the old parts which would annoy any volume manufacturers using the product. (You need us to change voltage regulators as well!? But we already have stock of 5V parts... I can hear them saying).

I shall update my design for the A parts, adjust supply voltages. Quite honestly I cannot be bothered with wasting time where part specs seem plain wrong and inconsistent. Drawback is reduced dynamic range, not sure that's an issue. With current design, it works, you just don't get peak to peak voltages as per the ADAU1966 datasheet. No idea if it effects other performance (THD+N etc).


Finally I was looking to order some AK4458VN parts for the new DAC board, and there is no stock. Digikey will let you order from 1+ quantity on 20 week lead time. Cheap as well £4.36 each up to ten, or £3.90 each for 10+ and so on. Will likely have to try this and just wait.

So the CM voltage pin on the ADAU1966 can be 1.5V or 2.25V depending on supply voltage.

It should be 1.5V with 3.3V analog supply, and 2.25V with 5V. So why do I have 1.5V with 5V supply?

The evaluation board manual states that pin 44, used for configuration in standalone mode selects 1.5V (set to 1) or 2.25V (set to 0). However the Rev E datasheet states this should always be set to 0 for standalone mode. Nothing like inconsistent data to help you.

I have tried changing this pin with no difference, the investigation will continue... 

Finally, after quite a break, I have been playing with my 2 input, 8 output box again.

Interesting times. ADAU1452 still works as expected with EEPROM writing/booting fine, I left it at this point before moving last year.

Running the XLR inputs through ADC, DSP, DAC straight to 2 of the outputs works ok. I have played with it hooked up to my Cyrus One and CM10's. With this in the signal chain, or removed, I could not hear any difference. This was driven from my Asus Xonar U7 USB sound card which is pretty good.

Playing a bit more;  sometimes on power up, or using the reset button on the DSP board, there is a lot of static/hiss on the outputs. It seems to be coming from the ADC as unplugging the I2S cable removes the noise. Music still sounds fine, so unsure what is the problem. 

The PCM4202 board has it's own reset circuit, but is not reset from the DSP when this resets. Might trying enabling this with some tacked on wire. 

Had a quick play with ARTA demo. I am using my Xonar U7 for measurements for now. 

I am seeing low level 50Hz noise. This is an artefact of my setup. Even with the DSP box switched off (i.e. doing nothing) similar 50Hz is present (maybe 3dB to 5dB lower). On my Cyrus One, which is a sensitive amp (37dB gain), with the volume at max, I could hardly hear any low level hum. 

I have run some basic THD tests with around 1Vrms on the outputs (max the Xonar U7 line input can take). However these are not accounting for the U7's distortion, although that should be low (not sure at max input level though).
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20kHz = 0.046% (THD+N)
10kHz = 0.057%
5kHz = 0.045%
1kHz = 0.031%
500Hz = 0.030%
100Hz = 0.029%

I am happy with these for now. There are several issues to resolve, such as why the ADAU1966 reference voltage was low when last measured. All inputs and outputs are single ended, the XLR input negative terminal connects to ground at the Xonar U7 single ended output (which I am lead to believe is the correct way to do it). I also have no idea what I am doing with ARTA as I have not used it before, so this was just a quick play. 

Oh and I also have a 600Hz and 700Hz test signal on two of the other outputs, generated by the DSP, at around 1.7Vrms. Not sure if crosstalk from these has any affect yet. 

Example measurement below: