Audible noise when using ADC-DAC MSP430F2619

Well, when you sample an analog signal you lose fidelity, both because of quantization resolution in bits and because the signal is no longer continuous in time.

You don't mention which device (and thus whether ADC10 or ADC12) and you don't mention the sampling frequency and bandwidth of your analog input, so it's hard to start pinpointing exact source of the noise. You also don't mention what the full-scale signal swing is of the input. That can affect which choice you make for VREF so that you can maximize the ADC bits that you do have.

Hi!

Thx for the reply.

Device is mentioned in the title of the call: msp430f2619, thus, it has an ADC12.

Signal voltage swing is from 0 to 3.3V. Sampling frequency is 20 KHz (BW of 10 KHz). Signal bw is 280 Hz (20 to 300). MSP is supplied by 3.3V, so swings from 0 to 3.3V. ADC Vref is config. to be GND and Vcc (Also, it's decoupled as datasheet advice to).

Could it be a DAC question?

Thx

What kind of noise it is? Please characterize it.

Hi!

Low freq (when i filter my samples from 20 to 200 Hz it vanishes), looks like a sizzle. Again, when full analog, system does not introduces this noise.

Thank you!

F���vero Santos said:

Low freq (when i filter my samples from 20 to 200 Hz it vanishes), looks like a sizzle.

You shall find the source. 1) Digital source. You said: " ADC is triggered by xtal timmered flag - timmer + comparator". Maybe this is source of some clicks or something? Try to make continuous ADC/DAC conversion. 2) Analog source like supply noise. Connect your audio amp through cap to digital, then analog supply to "hear" supply ripple. Maybe this is it.

Hi!

Thx for the comments.

"Connect your audio amp through cap to digital, then analog supply to "hear" supply ripple." -> I do this, and no chirps in the output. So, analog is Ok. ( i can amp my 0 to 3.3V signal to 0 to 9V and there's no noise in the output).

I do believe it's an ADC noise, but i can't find the problem. " Digital source. You said: " ADC is triggered by xtal timmered flag - timmer + comparator". Maybe this is source of some clicks or something?"->  The process of sending data from ADC to DAC is continious. In a block diagram, i'm doing this:

1 )Xtal 8MHz -> Timmer -> 1/20K = fsampling -> ADC -> DMA1 -> DAC. 

When i start to save data to my SD, i works like this:

 2) Xtal 8MHz -> Timmer -> 1/20K = fsampling -> ADC -> DMA1 -> DAC. 

ADC-> DMA2 -> int buffer -> SPI -> SD

But SPi is not root, i can hear the noise even without saving data to SD (Doing only cycle 1).

Thx!!!

F���vero Santos said:

I do this, and no chirps in the output.

Usually there's no such thing as "clean supply", especially digital one. So question is how big is noise and is it audible (< 16kHz). Either amplify more until you hear something or check with scope. Other option would be to add some low ESR capacitor to both DVCC and AVCC and check how does it change noise in output.

F���vero Santos said:

ADC-> DMA2 -> int buffer -> SPI -> SD

Did you check for noise in PCM audio data saved on SD, did you play it on computer? Same?

F���vero Santos said:

But SPi is not root, i can hear the noise even without saving data to SD (Doing only cycle 1).

Then you shall check how does noise changes when you change: 1) sampling rate 2) timing of your program by changing: 2.1) period of timer(s) 2.2) changing CPU clock, like enabling /2 divider and running CPU 2x slower.