Active filter for DAC reconstruction

[Stephen Boulet]

A while back someone posted that a Sallen-key active op amp filter might
not be the best topology for preserving the SNR of a signal coming from
a DAC.

What is the preferred filter type to get rid of alias products for an
audio signal after a DAC?

Stephen

 

[tlbs]

The Sallen-Key topology has no DC path from its input to ground, so if
you have a current-output DAC it will not "play" into a Sallen-Key. A
Multiple FeedBack (MFB) topology allows for DC current to flow. If
your DAC is a voltage-output DAC, then the Sallen-Key will probably do
OK -- just make sure it can drive capacitive loads.

As for the preferred characteristic type -- for audio, generally a
Butterworth is used because people want that "flat" audio response.
But, it all depends on your sampling rate, and the audio bandwidth. If
your sampling rate is 44 kHz and you want that "20-20kHz" bandwidth, a
Butterworth will require many poles (=many OpAmp stages). If you can't
afford too many OpAmp stages, then you might consider a medium-ripple
Chebychev characteristic.

 

[PeteS]

A while back someone posted that a Sallen-key active op amp filter might
not be the best topology for preserving the SNR of a signal coming from
a DAC.

What is the preferred filter type to get rid of alias products for an
audio signal after a DAC?

Stephen

Apart from the other notes already here, the filter function should be
considered. If you are reconstructing previously encoded analog
signals, then a raised cosine filter is the most common solution.
(That's because the encoding process induces a SinX/x Power
distribution on the signal).

A good paper on this may be found at
http://www.filter-solutions.com/raised.html

(Note : I have nothing whatever to do with the site)

Cheers

PeteS

 

[Pooh Bear]

A while back someone posted that a Sallen-key active op amp filter might
not be the best topology for preserving the SNR of a signal coming from
a DAC.

What is the preferred filter type to get rid of alias products for an
audio signal after a DAC?

I use the one recommended by the DAC maker !

Graham

 

[John Larkin]

Apart from the other notes already here, the filter function should be
considered. If you are reconstructing previously encoded analog
signals, then a raised cosine filter is the most common solution.
(That's because the encoding process induces a SinX/x Power
distribution on the signal).

A good paper on this may be found at
http://www.filter-solutions.com/raised.html

(Note : I have nothing whatever to do with the site)

Cheers

PeteS

"total attenuation at thigh frequencies"

Skinny legs?


John