[sdiy] Butterworth design

[Antti Huovilainen]

On Tue, 17 Jun 2008, Csaba Zvekan wrote:

> I am gonna have to design a Butterworth 2nd Order Low Pass Filter for the 
> Audio Output after a 24-bit Burr&Brown DAC PCM1753.
> Now my little software for filter design from Texas Instrument is is 
> suggesting a 5.23K resistor. If my cutoff frequency is supposed to be 24.1 
> kHz -60dB. Of course I don't have that value handy.

PCM1753 is an 8 times oversampling DAC. This means that your lowpass 
filter stopband starts at 7 x samplerate/2 (typically 7x22 = 154 kHz). 
The datasheet says that the digital oversampling filter attenuates 50dB, 
so there is not much point in aiming for much higher attenuation.

Then you just need to design a filter that doesn't have significant 
attenuation in the passband (0-20 kHz) and that provides > 50 dB 
attenuation in the stopband. With suitable margin (say, setting the corner 
frequency at 38 kHz, giving you two octaves during which to get 50dB 
attenuation), the exact corner frequency is not critical.

Antti

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