AN3225

STM8L/STM8AL audio output using on-chip 12-bit DAC

Doc ID 17566 Rev 3

9/13

3

STM8L/STM8AL audio output using on-chip 12-bit

DAC

The hardware solution benefits from the STM8L/STM8AL 12-bit DAC with buffered output.

Only a few external components are needed to build an application with audio output. A

reconstruction low pass filter is needed to reduce the high frequency content in the DAC

output (see Figure 4). An audio amplifier is also necessary when boosted power output is

required.

Figure 4.

DAC output

1.

LP = Lowpass filter

The reconstruction filter should be set up as a lowpass filter with a cutoff frequency lower

to sufficiently attenuate the aliased and higher harmonic components present in the output

frequency spectrum of the DAC. The presence of higher harmonic components are caused

by a staircase shaped signal of the on-chip DAC. It has a standard topology output circuit

which implements the zero-order hold function. The higher the band stop attenuation, the

lower the output noise. The order of the lowpass (LP) filter determines the steepness (roll-

off) of the frequency characteristic transition from passband to stopband. The order of the

LP filter is determined by the attenuation at the Nyquist frequency (half of the sampling

frequency) and the ratio between the Nyquist frequency and the cutoff frequency. A

common requirement is to keep the noise amplitude over the Nyquist frequency (attenuated

by the filter) well below quantization noise.

Figure 5.

DAC output filtered by RC filter

MS30977V1

DAC

LOWPASS FILTER

AMPLIFIER