The problem is, that you have a typical mixed application, mixed in terms of using analog and digital circuitry. This will need very very careful layout considerations! Not only needing proper grounding, but also proper filtering!!
First, you must think about how and where to connect all the indiviual grounds, means ground of ADC, ground of switcher (SG3524), ground of AT89C51, etc. Only remedy is the choose of one single ground star point, where the digital, analog and switcher's grounds are joined together.
You CAN choose the same Vcc for all the analog and digital chips and the switcher. But then you urgently need proper supply filtering. Otherwise noisy ground return currents will flow across the ground routing!!

So, what you apply to the supply of SG3524, means this 22µF electrolytic is not at all sufficient to suppress noise. What you need is a handful of small value resistors and/or ferrite beads to form RC- or better RLC-low pass filters in the supply lines. And, of course, a proper ground routing using a single ground star point.

Your application is a good example for, that performance of PCB can totally differ from what the schematic promises! Noisy ground return currents or supply voltage spikes can totally ruin an application...

Kai