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

AN3225

10/13

Doc ID 17566 Rev 3

The simplest form of the reconstruction filter is the resistor-capacitor (RC) lowpass filter. The

cutoff frequency of the RC circuit, fc, is shown in Equation 2.

Equation 2

The amplitude drop over the cutoff frequency is approximately -6 dB per octave (or -20 dB

per decade). The frequency response of the speaker can bring an additional attenuation of

the high frequency content. However, for high quality audio output it is better to implement a

second order active LP filter providing steeper attenuation of higher frequencies. One

example of an active LP filter design is the ‘Sallen-Key configuration’ design which has a

unity gain operational amplifier (op-amp). For more information about active LP filter

designs, see the LS204 datasheet.

The output of STM8L15xxx/ STM8AL31xx/STM8AL3Lxx DAC can be buffered by on-chip

output amplifier to reduce the output impedance. It can be

enabled by setting the BOFF bit

in the DAC_CR1 register. An external audio amplifier should buffer (or be integrated as a

part of) the reconstruction filter to supply a low impedance speaker or to achieve a higher

output power.

An example of a reconstruction filter with an output buffer can be found on the STM8L1528-

EVAL evaluation board. Details including schematics can be found in the UM0784.

fc

1

2

πRC

()

⁄

=