# [sdiy] Digital accumulator VCO core?

cheater cheater cheater00social at gmail.com
Fri Feb 12 06:48:27 CET 2021

```On Fri, Feb 12, 2021 at 6:41 AM Brian Willoughby
<brianw at audiobanshee.com> wrote:
>
>
>
> On Feb 11, 2021, at 19:22, cheater cheater wrote:
> > 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.
>
> Well, it is you who started chasing his own tail, so you get what you ask for. :)
>
>
> > 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?
>
> No, an ADC by design measures voltage, using a comparator.
>
> There are perhaps dozens of different ways to perform ADC - successive approximation, flash conversion, delta-sigma - but I can't recall any that work directly on current.
>
> I'm gonna drag you screaming back to the courtroom and ask for a citation here ... again.
>
> If someone can point out an analog to digital conversion circuit topology based on current, I'd be very curious to see it.

I didn't say current, I said charge.

Take the Charge-redistribution successive-approximation ADC for
example. Look at this:

what the ADC does is it captures /charge/ into capacitors. That charge
is then later measured.

The capacitors, however, cannot "capture voltage". A capacitor can
only capture charge. It's impossible for a capacitor to capture
voltage. For this reason, any ADC that first captures signal into a
capacitor of some sort or an S&H (which is capacitor based) by default
measures /charge/.

> You might be thinking of DAC architectures. There are some DAC designs that directly produce current, and that poses a problem. Some include an internal current to voltage converter, but sometimes that has more issues than the DAC itself, and some DAC designs might work better without the I-to-V stage. I've designed at least one product with a super-fast 125 MHz DAC (only 14-bit, though) with differential current output.

No - again, I said charge, not current. I hope you're not mixing those
two up. Charge is like an amount of water. Like a glass of water.
Current is like the amount of water that passes through a pipe per
second. Like, one liter of water per second. Two different things.

```