[sdiy] Control voltages for digital modules

[Elain Klopke]

Thank you all! That's a lot of information to chew through. Yay research!

On Fri, Jul 14, 2017 at 8:14 AM, <paula at synth.net> wrote:

> Elaine,
>
> Here's the problem: how do I get a pitch CV from a sequencer or other
>> module converted to the 0-5 volt input necessary for the microcontroller
>> ADC.
>>
>
> so, for this you need to understand what the resolution of the micro's ADC
> is, and also the error (in number of bits).
> for example, you could have a 14 bit ADC (awesome) but it could have 5
> bits of error (booo), giving you a "reliable" 9 bit ADC.
>
> If you're doing Pitch CV from a modular this is typically 1v/oct. Let's
> assume you have a 10 octave range (same as MIDI), then you'll need 7 bits
> to get every semitone.
> But these need to be a REALLY accurate 7 bits. Typically I'd look to use a
> minimum of a 12 bit ADC with an error no greater than 1 bit to give an
> accurate CV.
>
> Now, to go back a step, 10 octaves at 1V/oct will mean a maximum input of
> 10V to your ADC, which runs the risk of frying your micro if it runs from
> 5V.
> You can use a simple resistor divider to drop 10V down to 5V (two equal
> value resistors), again for this it's worth getting high precision (0.1%)
> resistors.
>
> You'll also need a filter (can be a simple one) to help reduce aliasing.
>
> Here is a good paper on ADCs - http://cache.nxp.com/docs/en/a
> pplication-note/AN5250.pdf
>
>
>
>> And what sort of DAC should be used to make proper CVs out of the digital
>> signals?
>>
>>
> For GorF-XL and the 002 I used the MCP4822, same as Tom suggested.
> It's a good little DAC at a good price, odds on your micro will also
> support SPI, so you can control it from that without needing to write your
> own 'bit banged' spi function.
>
> In general, I'd go for as a higher quality ADC and DAC as you can afford :)
>
> Good luck
> Paula
>
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