Understanding the TDA1541(A) DEM decoupling capacitors
This old subject gets rehashed periodically, so it might be time to shed some light on what these capacitors really do and how they do it. Also, this topic is kind of a follow-up to two previous topics on TDA1541(A) DEM, so I would advise reading them, too.
AYA 5 decoupling capacitors?
TDA1541A DEM clocking
The problem we start with, i.e., the problem that has to be solved with these capacitors, is shown in the waveform shots I've taken at all the unfiltered DEM pins of one TDA1541A chip (here it is series HSH9314). Also, since the idea was to see not only the waveforms at these pins, but also the spectral content and levels of these spurious signals breaking through to the DAC output, and since the audio measurement setup I normally use is limited (by the software) to Fs=192 kHz, I used 1 nF at the Cosc pins, which sets the TDA1541A DEM frequency to 75-80 kHz, so the results are representative within the resulting 96 kHz (Fs/2) bandwidth.
So, I ditched the DEM decoupling capacitors to capture the spurious signals at the unfiltered DEM pins. You will note that the spurious signals vary considerably between the different pins. In the order shown below, the pins are MSB, MSB-1, MSB-2, MSB-3, MSB-4, MSB-5, and MSB-6.
The next part will show how these spurious signals add up and make their way through to the audio output. Stay tuned.