Electronics > Projects, Designs, and Technical Stuff

DAC function generator filtering

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Jon86:
I'm thinking about designing a simple low-frequency AWG for audio work, but I'm struggling to understand what I've read about output filtering when using a DAC to generate waveforms.

If I generate a 20KHz sine wave with a 200KHz sample rate, I know I could have a sharp rolloff reconstruction filter above 20KHz that would remove almost all of the stepping distortion created by the DAC, but if I then want to generate say, 20Hz, how do I remove these steps using the same filter? Depending on the bit depth used, won't the samples take place at different intervals, and produce different frequencies in the output?

I guess another question would be how important is it for general purposes to actually filter this output? Would a simple RC filter with a -3dB at 1/2 the sampling frequency suffice? Especially if I'm using a 24bit DAC anyway.

Hopefully someone has done this before and has some insight, apologies if this has been discussed here already.

tszaboo:
What kind of DAC are you using? Is it a delta-sigma? The output of that doesnt look anything like the steps. It looks like a PWM signal.

And no, a simple -3db rc filter is not suitable, there should be several order filtering.

Kleinstein:
The filter is there mainly to remove the signal beyond the Nyquist limit. So with 200 kSPS from the DAC this would be the signal beyond 100 kHz. So it is the sampling frequency and not the signal frequency that set the limit. So the filter can be fixed at some 30-70 kHz, depending on how steep. The steeper the filter the high one could go for a given sampling rate.

The usual way with DSS generators is to have a sampling frequency of some 50 MHz or higher and than a higher order (e.g. 7 th order) elliptical filter, that is a low pass combined with some extra notches just in the stop band. With such a steep filter one could use up to about 1/4 to 1/3 the sampling frequency. For the high frequencies one can use a passive LC filter for this. However the inductors may get bulky much below 1 MHz.

A 24 bit DAC would be a sigma delta DAC and would thus already include some filtering / oversampling. The filter needs would be different from a normal DAC and there should be some info from the DAC used.

MarkF:
I found this online LC filter program very useful in designing a filter:
   https://www-users.cs.york.ac.uk/~fisher/lcfilter/

Just enter the filter order, frequency, configuration and impedance to get the needed component values.

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