47.6.2.4 Digital to Analog Conversion

Each DAC converts a digital value (stored in DATAx register) into an analog voltage. The conversion range is between GND and the selected DAC voltage reference VREF. The default source for VREF is the internal reference voltage VREF. Other voltage reference options are the analog supply voltage (VDDANA) and the external voltage reference (VREFA). The voltage reference is selected by writing to the Reference Selection bits in the Control B register (CTRLB.REFSEL).

The output voltage from the DAC can be calculated using the following formula:

V_{OUTx} = \frac{DATAx}{4095} \times VREF

A new conversion starts as soon as a new value is loaded into DATAx. DATAx can either be loaded via the APB bus during a CPU write operation, using DMA, or from the DATABUFx register when a STARTx event occurs.

Refer to Events for details. Even if both DAC use the same GCLK, each data conversion can be started independently.

The conversion time is given by the period T_GCLK of the generic clock GCLK_DAC and the number of bits:

T_{CONV} = 12 \times 2 \times T_{GCLK}

The End Of Conversion bit in the Status register indicates that a conversion is completed (STATUS.EOCx=1). This means that VOUTx is stable.

Since the DAC conversion is implemented as pipelined procedure, a new conversion can be started after only 12 GCLK_DAC periods. Therefore if DATAx is written while a conversion is ongoing, start of conversion is postponed until DACx is ready to start next conversion.

The maximum conversion rate (samples per second) is therefore:

{CR}_{max} = \frac{2}{T_{conv}}