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A lowpass filter for a DAC...

P

Peter

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An update on this...

I have the synthesiser working. It looks like I will be running 64
samples per cycle. I can run way more with the hardware (up to about
500 before approaching the max DAC feed rate (400Hz sinewave out) but
I also need to able to amplitude-modulate the sine table fairly fast,
and even though I won\'t be using \"double sin(x)\" for that (will use a
64 sample lookup table) 64 samples will be a lot faster to modify.

To answer earlier comments about 12 bit values producing steps which
are invisible on a scope, that\'s only if you also have 4096 steps
horizontally as well :) With 64 steps, the steps are extremely
visible.

BTW the table is double buffered so one can regenerate one while DMA
is reading the other and feeding the DACs with it.

So I will try the Sallen-Keys 2-pole low pass filter, and what sort of
-3db rolloff frequency should I aim for? For 400Hz, 64 samples, so the
fundamental of the steps is ~25kHz, 5-10kHz would seem to be in the
right ballpark?

Some here mentioned using E92 resistors. Is this required? I see a
lowpass SK filter here

https://www.electronics-tutorials.ws/filter/sallen-key-filter.html

and it suggests two Rs the same and the two Cs the same.





Peter <nospam@nospam9876.com> wrote

Hi All,

I am trying to work out the likely phase lag for a lowpass filter
whose job is to take out the notches from a DAC.

The DAC is 12 bit and will be generating a 400Hz sinewave. It will be
driven at around 250x i.e. 100kHz. So the fundamental to filter out
will be 100kHz.

I am not good at maths but doing some digging around, it looks like a
simple RC has a 45 deg phase shift at the 1/2piRC point, and somewhere
around 1 degree when a factor of 100 away from that (40kHz).

I am happy with 1-2 degrees of lag, but more importantly it needs to
be fairly constant from 400Hz to 500Hz, or at least quantifiable,
because the table feeding the DAC can be shifted to compensate.

What about a 2nd order filter? The filter performance should be better
for a given phase lag, no?

Obviously perfection is impossible to achieve but I think a 10kHz
rolloff frequency would produce a really clean result. The Q is what
delay can be achieved at that rolloff.

There is a huge amount of stuff online but a lot of it is the audio
stuff, which is full of BS :)
 
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