[sdiy] Synthex Oscillator

[Quincas Moreira]

Brian, the list has stripped the attachment...

On Mon, Jul 10, 2017 at 2:58 AM, <rsdio at audiobanshee.com> wrote:

> More notes on the Synthex digital-controlled current sink.
>
> I'm sure folks who looked at the schematic noticed that the resistors are
> not exactly twice the value of each smaller neighbor.
>
> I filled out a quick spreadsheet and learned that the 7.15 kΩ resistor is
> 4.67% low, the 470 Ω resistor is 2.08% low, and the 14.7 Ω resistor is 2%
> low. Meanwhile, the 62 Ω resistor is 3.33% high and the 1 MΩ resistor is
> 4.17% high. I picked the 30 kΩ, 120 kΩ, and 240 kΩ resistor values as the
> reference.
>
> I graphed the weights resulting from binary combinations, and noticed that
> there are a couple of points in the 256-code range where the resistance
> actually drops when the code increases, simply because the resistor values
> don't add up in a way that produces a monotonically increasing value. Of
> course, this simple analysis is beside the point, because the resistors are
> combined in parallel. This is the light gray line on the attached image.
>
> I then graphed the voltage divider output that would be produced by the
> parallel combinations of resistors across the full 256-code range, assuming
> that there is an ideal op-amp voltage follower. In other words, this
> ignores the significant current flowing out of the capacitor, but I still
> thought it would be interesting to see how well this circuit would perform
> if it actually were a Voltage output DAC rather than a digitally-controlled
> current sink. This is the blue line in the image.
>
> Here's the graph, although I'm not sure whether the mailing list will
> strip the attachment.
>
>
> Brian Willoughby
>
>
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>
>


-- 
Quincas Moreira
Test Pilot at VBrazil Modular
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