embedded system STM32F4 ADC sampling rate

Question

I am new to embedded systems. I done a bit of analogue circuit design but nothing connecting analogue circuits to a STM32 via ADC. I am using STM32F4 microcontroller and had couple of questions.

• I know that you need an antialiasing filter for analog signals to an ADC, but can't workout what sampling rate is needed? Like can I pick it? Can someone please explain it to me?

• Neil_UK: What signal are you sampling? Sample rate and AA filter depend on the information in the signal that you want to access. Thermocouple or battery level, seconds. Load cell, hundreds of Hz to kHz. Audio, tens of kHz.
• CircuitFreak: A audio signal from an auduo sensor, so from 0 to 10khz. I don't know how to based the AA filter and how much it has to attenuate by using a stm32F4. Thanks for your help
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Answer 1

If you need to sample frequencies up to 10 kHz, you need a sampling rate above double that, so more than 20 kHz. This is the Nyquist sampling theorem.

If you sample at 20 kHz, the sampled data must not have any frequencies above 10 kHz. So you need a filter that passes everything below 10 kHz and nothing above 10 kHz. In real life this kind of ideal brick wall filter is unrealizable so you need to decide how good/expensive/narrow you want your filter to be, how much aliasing is allowed, and how much higher you want your sampling rate to go to allow simpler filter and less aliasing.

So there is not really possible to say what numbers/values/filters you should use as you don't know your requirements.

• CircuitFreak: Thanks for the reply, that's is helpful. My main problem is knowing the sample rate of a board such as stm32f4. Is there a way to know what sampling rate does stm32f4 sample at?
• Justme: @CircuitFreak You read the data sheet and see the maximum sampling rate it can achieve, and you can sample as fast or slow as you want as long as you sample below the maximun rate.
• CircuitFreak: Thanks so much. I didn't think that adc on stm32f4 can sample at 42Mhz. I thought that would be way to fast.
• CircuitFreak: Thanks so much. I think I understand now. So MSPS is the maximum sampling frequency of the board. Depending on the application you chose the sampling frequency. In my case with 10kHz AA filter, and let's say that I want the signal to fully attenuate by 20kHz I have to used Nyquist theory and set the sampling frequency at 40kHz. Is this correct ? And of course depending on the filter order causes a steeper rolloff of the signal.
• Justme: That is spot on, if you want to sample something. Now in real life, if you want to sample audio, you would just connect an audio ADC chip that inegrates everything to the STM32 via I2S audio interface, which will be light years easier and better compared to be limited to using the internal 12-bit ADC and making the analog filters yourself. But that's another topic.