[sdiy] Digital accumulator VCO core?

Tom Wiltshire tom at electricdruid.net
Thu Feb 11 14:18:11 CET 2021

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.


       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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