Electronic – STM32F103 ADC Noise

I am new to this forum. I have read the questions and answers in this forum on the similar topics but that one was for STM32F2 CPU. I am using STM32F103 and I have similar problem.

Similar to the last topic I am using a two layers board. I have separate the Analog and Digital grounds and they joined at power supply input. I have separate analog and digital power supply, using voltage regulator TLV70033. The reference voltage is 3V and is ref by REF3030. All the digital CPU supply are bypassed by 100nF 0805 ceramic Caps. Similarly the Analog supply and Ref voltage are bypassed by 100nF Caps. The Analog supply and Ref voltage bypass caps' ground is connected to the analog ground.

My CPU is using 8MHz crystal and the PLL is set to 6, so the CPU is running at 48MHz( because I want to use the USB port )

My problem is I found the signal collected by the ADC is noisy. My ADC sampling clock is 1MHz and the "sample and hold" is set at 239.5 Clk cycle ( fairly slow ). Sampling is trigger by a timer that is set at 3968Hz and the result of the ADC is DMA to a buffer. When 32 ADC results is collected, it causes an interrupt. In the interrupt routine I send out( Via USB ) those ADC results that are not Zero. I can see the results on my PC. The USB record shows that in most DMA cycle only 4 or 5 are none zero, and are fairly small. Most are less than 5 which I can accept. However, occasionally ( 1 or 2 percent ) the ADC result is bigger than 16 and some time it could be as big as 68count.

Can somebody give me some advices on the likely cause of the noise and how I could eliminate them.

Thank you in advance.

Tommy

Edit> I should have spell out in my initial posting that my board power supply input is 30V and I uses LM2842( 500KHz buck converter) to step down to 5V and then the 5V is further step down to 3.3V ( for both digital and analog circuit). My analog CRO shows that the switching noise is getting into my analog supply and VRef. When I remove the buck converter and use a 78M05 instead, I can get a very stable ADC readings ( maximum deviation 4LSB per 32 DMA tranfer ), but the 78M05 is getting very hot. I am think of using the Buck converter to step down to 8 or 9 volt, then use 78L05 or similar to step down to 5V. Hopefully the PSRR of two stages of linear power regulator will remove most of then switching noise. My worry is will the radiated noise still get into my analog circuit or should I use a lower switching frequency (20KHz) buck converter? ( I only now realize that the PSRR of TLV70033 at 500KHz is very low). Thank you in advance.