I am using the AD1974 ADC which is sampling at 16kHz. The master clock is sourced from a 6.144MHz crystal.
The sampled audio on channel 2 and 4 contains a tone which is not noticeable at channel 1 and 3. The frequency of the tone on channel 2 is not equal to the frequency of the tone at channel 4. There is no analog circuit connected to the analog input of the ADC. There is no difference in output between leaving the analog pins floating or connecting a capacitor to it.
I already changed the power supply, crystal load capacitors, provided additional ground return paths and put some EMI blocking plates around the PCB. All these changes did not affect the tone at all. The only way to affect the tone is heating up the crystal, after heating up the tone becomes noticeable at channel 1 and 3 as well.
What could cause the tone at my output? A time-frequency plot of the output is attached.