[sdiy] Digi-Mod schematic

[Tom Wiltshire]

On 23 Nov 2008, at 16:08, Neil Johnson wrote:
>
> - page 12: not sure what you're trying to achieve here with your  
> funky dual feedback circuit.  This looks like an output buffer, for  
> driving long capacitive lines.  Are you trying to make a LPF?

This goes back to an earlier discussion about the best way to buffer  
CV for feeding to an ADC.

Quoting from Eric Brombaugh's post of 18th Nov:

> Seb Francis wrote:
>
>
>> I've been considering the input buffer circuit for the CVs in my  
>> digital module and have been reading this article:
>> http://www.audiodesignline.com/210605596?printableArticle=true
>> Read about the "Compensated buffer topology", shown in figure 6.   
>> This seems like an ideal buffer circuit and has advantages over  
>> the one shown in figure 5 (which is what I previously suggested in  
>> reply to Tom, and the one Eric is using on his CV processing board).
>>
>
> That's very similar to the driver topology often found in the line  
> outputs of modular synths - a bit of drive impedance to protect the  
> amp from short circuits, and DC feedback wrapped around that  
> impedance to preserve accuracy with varying loads. AC feedback is  
> direct from the amplifier to keep HF stability.
>
> Would certainly work and provides a bit of direct observability of  
> the ADC input node to reduce lag caused by the output RC time  
> constant. This improves tracking accuracy, but I'm not sure if the  
> resolution of the dsPIC 12-bit ADC would actually benefit from it.  
> Pearls before swine as it were. No harm though, so why not?
>
> Eric