[sdiy] Digital accumulator VCO core?

Brian Willoughby brianw at audiobanshee.com
Fri Feb 12 08:22:36 CET 2021

On Feb 11, 2021, at 22:54, Brian Willoughby wrote:
> On Feb 11, 2021, at 21:48, cheater cheater wrote:
>> On Fri, Feb 12, 2021 at 6:41 AM Brian Willoughby wrote:
>>> On Feb 11, 2021, at 19:22, cheater cheater wrote:
>>>> 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:
>> https://en.wikipedia.org/wiki/Successive-approximation_ADC#Charge-redistribution_successive-approximation_ADC
> The description in that section about how this works uses voltage several times in the text. The relationship between charge and current and voltage is always in play.
> Since SAR ADC includes a DAC as part of the circuit, all that architecture is showing is the charge-scaling DAC. The ADC part works the same no matter what kind of DAC is used.
> Thanks for that link, though. I didn't realize that folks were using charge-scaling DAC elements in SAR these days.

It might be fun to look at the SAR from the point of view of a vintage polysynth. Because the charge-scaling DAC had not yet been invented, those synths built their SAR ADC using DAC chips that don't have capacitors.

The ADC half of a SAR design uses a comparator between the unknown input voltage and the known DAC voltage. When the DAC is a switched resistor or an R-2R, the ADC works entirely without capacitors. It's just a process of stepping one bit at a time, from most significant to least significant, and comparing the input voltage to the DAC voltage.

If you were to "upgrade" the DAC from R-2R to charge-scaling, the front end of the ADC would still work the same, based on voltages and a comparator.


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