Above is the circuit diagram of my power supply. it converts input 12V DC into 5V DC and then 3.3V DC with linear regulator. i have highlighted it in red color in the block diagram.
So current situation is, whatever noise is there on 12V input voltage shows up on 5V and 3.3V. how do i filter this ? how do i improve my design to get clean 5V and 3.3V even with a noisy 12V input. If i use clean power supply 12v, i am getting best results.But i cant give 12V clean power supply to everyone. Customers can pick up any noisy power supply and it should work with my design.
below i am attaching two waveforms , Yellow is 12V , Blue is 5V and Pink is 3.3V.
If you look at the waveforms, the noise appearing on 12V is carried on to the 3.3V. i was assuming LDO will at least attenuate it more but, it is very high.on the PCB the ground is properly stitched with multiple vias from top to bottom.
how do i get rid of this, what ill have to change or add in my design to improve it?
Note: Input 12V Dc power supply used is a 220V AC to 12V DC adapter.