# [sdiy] Digital accumulator VCO core?

cheater cheater cheater00social at gmail.com
Thu Feb 11 13:49:30 CET 2021

```Right, but it still removes some of the charge that you do want to measure

On Thu, Feb 11, 2021 at 1:47 PM Mattias Rickardsson <mr at analogue.org> wrote:
>
> 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
>> >

```