# [sdiy] Digital accumulator VCO core?

Mattias Rickardsson mr at analogue.org
Thu Feb 11 13:47:41 CET 2021

```The filter doesn't make the signal constant between samples. It doesn't
even make its slope constant between samples.

What it does do is to average the fastest wiggles in a signal in a way
similar to what you are asking for. :-)

/mr

Den tors 11 feb. 2021 13:41cheater cheater <cheater00social at gmail.com>
skrev:

> 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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