[sdiy] Digital accumulator VCO core?

cheater cheater cheater00social at gmail.com
Fri Feb 12 04:22:41 CET 2021


I hope that when talking about the plan of replacing a single
capacitor with an adc and a microcontroller, when people suggest
improving this design by adding a capacitor, the humor isn't lost on
them.

Anyways, an adc by design measures charge, and a lot of the effort
behind adc design goes into making it appear like it's not measuring
charge, but to instead make it look like it's measuring voltage or
current. It must be natural for there to exist an adc architecture
that measures charge directly - so what is it?

On Thu, Feb 11, 2021 at 8:14 PM Guy McCusker <guy.mccusker at gmail.com> wrote:
>
> There won't be any "down" because the current is the output of an
> exponential converter.
>
> So the integrator will just ramp up. Why not reset it every time it
> gets to a certain value, say 5V? Then measure the frequency of the
> resets and use that as your oscillator frequency.
>
> The fact that you have already built an analog VCO at this point
> should be considered a benefit rather than a bitter irony.
>
> On Thu, Feb 11, 2021 at 1:19 PM Tom Wiltshire <tom at electricdruid.net> wrote:
> >
> > This would be the function of an integrator, wouldn’t it?
> >
> > Easy enough to build one of those with an op-amp. The problem tends to be keeping the output within reasonable limits, since a perfect integrator will just keep adding and adding and adding - that’s ok as long as you get as much “down” as you get “up” (that is to say, as much negative voltage/current as positive). Otherwise, you need a leaky integrator to help “forget” some of the past.
> >
> > Tom
> >
> > ==================
> >        Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >
> >
> > > On 11 Feb 2021, at 12:40, cheater cheater <cheater00social at gmail.com> wrote:
> > >
> > > This is true in that the anti-aliasing filter limits the bandwidth of
> > > the signal, making it constant between sampling periods before it gets
> > > to the ADC. However, that's not the same as sampling the whole charge
> > > that was transported in the time between the sampling intervals. In
> > > specific, the anti-aliasing filter will shunt a portion of that charge
> > > to ground, bypassing the ADC.
> > >
> > > On Thu, Feb 11, 2021 at 1:32 PM Mattias Rickardsson <mr at analogue.org> wrote:
> > >>
> > >> Den tors 11 feb. 2021 08:48cheater cheater <cheater00social at gmail.com> skrev:
> > >>>
> > >>> Here's one thing I've been wondering about. Is there a form of ADC
> > >>> where every sample represents the total charge that passed through a
> > >>> conductor since the last sample? Normally, an ADC will only sample
> > >>> voltage (or current) at the time of sampling, but we don't really know
> > >>> anything about what happened between sampling intervals. If we assume
> > >>> that the current was roughly constant during the sampling, that's
> > >>> fine. But if it changed during that time, then we have to do some sort
> > >>> of interpolation. Even if we do a linear interpolation, maybe the
> > >>> current going through the conductor was concave or convex. Being able
> > >>> to sample the total charge going through a cap would be much easier,
> > >>> since I'd be converting the input data into a charge amount anyways
> > >>> (to be added to the virtual capacitor).
> > >>
> > >>
> > >> This is essentially what an anti-aliasing filter does for you.
> > >>
> > >> /mr
> > >>
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