About ADAU1979 signal quality

Input 1K sine wave

Hello hebing,

How are you measuring this? The units are cutoff in your screenshot but it looks like Vrms? So therefore you are going through a DAC and other circuitry?

The spike is at a curious frequency in that it is not a harmonic of 1kHz. 

What is the input level to the ADC? At the pins. 

If you look at figure 6 and 7 in the datasheet, you will see what is expected of the part. This was measured directly digitally. Figure 6 is with a lower level signal and figure 7 is with a -1dBFS signal which is just before clipping. You will see the resulting distortion products are all harmonics of the 1kHz fundamental. 

So I suspect it is something else in your system that is involved here. 

I do not think this will be a clock jitter issue but I cannot rule that out. 

I would look at power and ground carefully. Is there anything in your system that might produce a 4.5kHz signal? LED controllers? 

Change the input frequency and see if it changes with it?

Try a level that is about -20dB from full scale. 

Dave T

YES,it's Vrms.

Hello Dave:

I‘m very gland for your reply！I had found the reason，the power of ADC not reach expect 8V

tksy very much.