A lowpass filter for a DAC

On 11/12/2021 05:47 PM, Peter wrote:
> 
>  Johann Klammer <klammerj@NOSPAM.a1.net> wrote:
> 
>> On 11/12/2021 03:06 PM, Johann Klammer wrote:
>>> Altho, if it's slow stuff like a 400Hz sine, slowly varying, you may just have to measure the zero crossing 
>>> and guess the delay from that. 
>>>
>>>
>> Also, 1 deg of 40kHz is around 70 nS. 
>> I don't think this is relevant at all as it's well below your sampling period.
>> you could not compensate this with your 100kHz.
> 
> 1 degree of *400Hz* (which is what I am generating) is 7us, which is
> about 1 DAC sample period.
> 
> So despite the misunderstanding, you are still very right; there won't
> be any realistic way to compensate for the phase shift by shifting the
> table.
> 
> I could have 360 samples per cycle and then 1 sample shift would be
> exactly 1 degree. Or the sample rate could be tweaked to exactly equal
> the filter delay, and then I could just delay by one sample :)
> 
you said your lowpass has 1 deg at 40kHz.

On Fri, 12 Nov 2021 07:03:45 +0000, Peter <nospam@nospam9876.com>
wrote:

>
>John Larkin <jlarkin@highland_atwork_technology.com> wrote
>
>>On Thu, 11 Nov 2021 15:12:58 -0800 (PST), Phil Allison
>><pallison49@gmail.com> wrote:
>>
>>> John Larkin wrote:
>>>===================
>>>> 
>>>> 
>>>> Given a 12-bit DAC making a 500 Hz sine wave at 100K samples/second, 
>>>> it will look perfect on a scope, with no filter.
>>>
>>>** Not on good analog one, with a sharp trace. 
>>
>>I'd expect that if the sine wave fits on the screen, you wouldn't see
>>the stairsteps. 
>>
>>How many spot diameters fit on one screen? Surely not 4000.
>
>Indeed - the 12 bits is plenty. But I won't have 4k samples per cycle.
>So there will be steps larger steps than 4k/cycle.
>
>I can see that removing the steps is not difficult, because of the
>huge "oversampling". They will need to be removed however, as far as
>possible, for EMC reasons.

Each dac step will be tiny, so the waveform won't make much EMI.

These days, one doesn't have to do all the math; just Spice it.



-- 

Father Brown's figure remained quite dark and still; 
but in that instant he had lost his head. His head was
always most valuable when he had lost it.



  

On Fri, 12 Nov 2021 16:47:49 +0000, Peter <nospam@nospam9876.com>
wrote:

>
> Johann Klammer <klammerj@NOSPAM.a1.net> wrote:
>
>>On 11/12/2021 03:06 PM, Johann Klammer wrote:
>>> Altho, if it's slow stuff like a 400Hz sine, slowly varying, you may just have to measure the zero crossing 
>>> and guess the delay from that. 
>>> 
>>> 
>>Also, 1 deg of 40kHz is around 70 nS. 
>>I don't think this is relevant at all as it's well below your sampling period.
>>you could not compensate this with your 100kHz.
>
>1 degree of *400Hz* (which is what I am generating) is 7us, which is
>about 1 DAC sample period.
>
>So despite the misunderstanding, you are still very right; there won't
>be any realistic way to compensate for the phase shift by shifting the
>table.
>
>I could have 360 samples per cycle and then 1 sample shift would be
>exactly 1 degree. Or the sample rate could be tweaked to exactly equal
>the filter delay, and then I could just delay by one sample :)

Are you rolling your own DDS in an FPGA? It would be easy to tweak the
phase to arc-seconds.



-- 

Father Brown's figure remained quite dark and still; 
but in that instant he had lost his head. His head was
always most valuable when he had lost it.



  

jlarkin@highlandsniptechnology.com wrote

>Are you rolling your own DDS in an FPGA? It would be easy to tweak the
>phase to arc-seconds.
>
I am using DMA to stuff data into a 12 bit DAC (internal to the 32F4
CPU). The DAC sample period is determined by a timer value.

The DAC spec is a bit vague but it seems to want to see at least 1us
between samples, if you want a nice output. It sounds like if you
loaded it at 10MHz then it wouldn't work. But also I would need lots
of RAM for the table, whih I don't want.

I realise the fact that I am generating a sinewave makes other
approaches possible but a) I want to do it all in the CPU hardware and
b) an arbitrary waveform capability is handy.

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 :)