[sdiy] Driving SSI2144 freq control with PWM from a microcontroller

[Ben Gebhardt]

Interesting discussion!

I ran across this EDN article and want to try it out when I get a chance.  Seems like a good compromise between and RC and full blown active filter.  Has anyone done this before for synths?  I wonder if it’s practical for this application.

https://www.edn.com/cancel-pwm-dac-ripple-with-analog-subtraction/



> On Nov 5, 2022, at 12:02 PM, Eric Brombaugh via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Correct - range is 1/3 of the original, but much more linear and you can usually make up for that with gain elsewhere in the system. I used this approach (along with differential outputs) in my FPGA oscillator project:
> 
> https://github.com/emeb/up5k_osc
> 
> Eric
> 
>> On 11/5/22 08:28, René Schmitz wrote:
>>> On 05.11.2022 16:04, Eric Brombaugh via Synth-diy wrote:
>>> There's a way to mitigate that - ensure that you always have a high and low transition in every cycle. Basically you create a very high frequency PWM signal that's either 33% or 67% depending on the state of your PDM bit, so the speed of the high & low transitions are always part of your output and don't vary with the data value. Downside is that your PDM bitrate has to be 1/3 of that you'd get without doing this approach.
>> If I understand correctly, you also loose 2/3 of the output range. And assumes the t_on and t_off are the same.
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