[SOLVED] How is noise induced in analog blocks due to digital blocks?

[venugopala_0202]

Hello

Although I do understand that it is common knowledge to isolate analog blocks from digital blocks because of the noise sensitivity of the analog circuitry, I want to understand how is it that digital blocks induce noise in analog blocks? I would appreciate it if someone could explain the exact mechanism behind how digital blocks can cause noise in analog blocks.

 

[prestonee]

The main contribution is through the power supplies, aka the substrate and any shared metal connections typing grounds or powers. This is typically why high precision designs use deep nwell to isolate digital from analog as well as use seperate powers which arent tied until they reach the board level if ever. The noise is due to the current surge of digital gates driving their loads ( they always drive to ground or supply due to the nature of it being digital), you will see these current spikes in any sim when the digital block is actively driving a voltage to the opposite polarity. these current surges can cause a slight changes of voltage on the ground or power line causing biases etc to change in analog, altering their behavior briefly , also the digital switch noise ( we call it switch noise because it looks like noise spikes at the switching frequencies in a FFT) will have significant effects on the analog ac behaviors, specially if the analog psrr is poor and the switching frequencies are related to the signal(pretty common if the digitial is generating the timing for the analog switching circuits).
-Pb