18.4.2 Pulse Density Modulation (PDM) DAC

Each instance of the High-Speed Analog Comparator with the Slope Compensation DAC has a dedicated DAC that is used to program the comparator threshold voltage via the DACxDAT register. The DAC comprises a digital Pulse Density Modulation (PDM) module, followed by a multistage RC filter. The PDM module generates a high-frequency output signal whose density is proportional to the DACxDAT register value.

The DACxDAT register values have limits of 0x0CD and 0xF32 and will provide a DAC output of 5% to 95% of V_DD. The DAC is implemented using a digital Pulse Density Modulation (PDM) scheme and a multistage RC output filter. Operation near min or max values results in bit stream patterns that incur higher noise on the filtered output. Noise and spikes at DAC output degrade performance capabilities such as offset and INL. The performances specification listed in the device data sheet's Electrical Specifications are only valid within the 5% to 95% range. For any intermediate value in the register, between 0xCD and 0xF32, the output voltage of the DAC will be proportional. The equation to calculate the DAC output voltage based on the V_DD voltage source is provided in DACxDAT. The DAC voltage can be varied in steps of V_DD/(2^N – 1), where N is the number of DAC bits (N = 12). The DAC modules are controlled by the DACON bit (DACCTRL1[15]). The DACON bit enables or disables all of the comparator modules instantiated on a given device or device core. The DACEN bit (DACxCON[15]) provides individual control of the DAC module. The individual DAC registers have an output enable bit, DACOEN (DACxCON[9]), which enables the DAC output voltage to be routed to an external output pin, DACOUTx. The DACOUTx pin can only be associated with a single DAC. If more than one DACOEN bit is set, the DACOUTx pin will be a combination of the signals. A configuration example to set the DAC output voltage is shown in Configuration of DAC Register.

When first enabling the DAC output (DACOEN = 1), a transient can occur as the output amplifier stabilizes. The duration is dependent on the order of operations. If DACOEN is set after the ON bit, transients and setting times are up to 10 us. If DACOEN is set before the ON bit, the duration of transient is 1 us followed by setting time of 10 us.

/* DAC Register Settings */
DAC1DATbits.DACDAT = 0x4D9;			/* DAC Output set to 1V (VDD = 3.3V)*/
DAC1CONbits.DACOEN = 1;				/* Enable DAC 1 output on pin DACOUT1 */
DAC1CONbits.DACEN = 1;				 /* Enable DAC 1 */
DACCTRL1bits.DACON = 1;				/* Turn ON all DACs */