I am currently investigating the ADAU1442 for sine wave (more generally signal) generation and signal analysis.
I have the ADAU1442 EVAL board.
I built a very simple project with the tone generator.
The measured THD+N (through SPDIF OUT) is about -93 dB. Really not impressive...
Looking at the FFT, I saw an awful amount of pikes in the frequency spectrum.
I could drastically improve things by adding a narrow band pass filter at the output of the generator that tracks the frequency of the tone.
I have now a THD+N ranging from -133 dB to -144 dB. But I am not sure it is valid for all frequencies in the audio bands.
I made 10s of measurements with frequencies that would not relate to the sampling rate (just to get a worst case).
But this is not a exhaustive audio band scan, obviously.
Is there a better way to generate a clean sine wave?
I tried to build an oscillator in an "analog" manner, just by applying positive feedback in a system and a controllable phase to define unambiguously the oscillation frequency. This is a trick that worked perfectly on a TI DSP TAS3101A.
But here, I cannot manage to get any oscillation starting. Are there special tricks to apply for that?
Note & other questions:
1. To get SPDIF OUT operational, I had to modify the board. For any reasons, the transformer was shunting the signal. I bypassed it, used a capacitive coupling and it worked. Being obliged to do that on such an expensive board is a bit of a pity.
2. When I proceed to read out from the DSP (level analysis), the USBi interface crashes all the time. I can transmit (configure blocks) but not receive. This is very annoying as I also need to use the DSP for signal analysis and sample capture. I just can't do half of the evaluation of that chip. Is there a way to solve that problem?
3. Where can I find a detailed documentation about the DSP blocks. I would like to know which registers are used for what. For the moment, I have to guess from what I see in the SigmaStudio. Not very efficient. This kind of documentation should be somewhere but I cannot find it.
Thanks for your support,