20.4.3 DAC Output Filter Modes

The PDM DAC requires an output filter to convert the digital pulse stream into an analog signal. The output filter is implemented as a configurable multi-stage RC network. The output filter has three operating modes.

Table 20-8. Output Filter Modes
Output Filter ModeResponse TimeFilter Bandwidth
Transition modeFastestWider
Fast modeMediumMedium
Steady-State modeSlowestNarrower

Each mode has more filter stages. Transitions are controlled with a set of programmable timers, TMODTIME[9:0] (DACCTRL2L[9:0]) and SSTIME[9:0] (DACCTRL2H[9:0]). TMODTIME defines the duration of Transition mode and SSTIME defines the hold-off time until Steady-State mode. The delta between them is effectively the duration of Fast mode. A write to the data registers DACDAT (and DACLOW, if used) will invoke Transition mode and start both TMODTIME and SSTIME timers. Transition and Fast modes can be omitted by setting the timers to zero.

Equation 20-2. Transition Mode Timer Equations

Equation 20-3. Steady-State Mode Timer Equations

The control scheme is dependent on the operating mode of the DAC. The operating modes are:

  • Static (fixed DC voltage)
  • Slope
  • Ramp (triangle wave)
  • Hysteretic