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Based on the datasheet of XMEGA controllers, it has sample rate up to 2Msps (2MHz sampling speed).

But it is also written that the conversion time for 12-bit need 3.5µs which gives ca. 300 KHz real sampling rate.

I really want to know what is the real sampling rate for XMEGA controllers.

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You will find that it has "fast mode" options for the ADC peripheral, and that you will need to use lower resolution, like 8 bit, to get the maximum speed listed. The higher the resolution of the ADC the slower and more power used during conversion.

There is a whole application note on the use of ADC in the XMEGA series by Atmel by the way. AVR1300

From reading that briefly, it seems you need to have a XMEGA A series chip to get the maximum 2Mhz ADC clock rate, and even then to clock get 8 bit resolution samples at 2Msps the source impedance needs to be under 1k Ohms. See graph on page 14.

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