19.4.3 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 multistage RC network. The output filter has three operating modes, as shown in Table 19-8.
Table 19-8. 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 19-2. Transition Mode Timer Equations

TMODTIME[9:0] = TTR * FDAC/2

TTR = TMODTIME[9:0] * 2/FDAC

Equation 19-3. Steady-State Mode Timer Equations

SSTIME[9:0] = TSS * FDAC/2

TSS = SSTIME[9:0] * 2/FDAC

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