[sdiy] digitally controlled potentiometers

[ASSI]

On Sonntag, 6. Mai 2007 19:01, Heiko van der Linden wrote:
> This will make it possible to make real analog synths that have
> patch memory and will be able to dump the movements of the knobs
> as automation tracks via MIDI. 

This has been achieved before (and more than once) without the use of a 
digital potentiometer.

> Another option would be the digital
> potentiometer ICs that are prevalent on the market. But most of these
> ICs have standard values and a much too low resolution. I am looking
> at 12 bits or higher and would like to go to 16 bits to allow very
> accurate tracking of the movements of a potentiometer.

There is a reason why these aren't of any higher resolution.  You don't 
really need a potentiometer in many cases, often you can use an OTA or 
multiplying DAC instead.

> So... I have started to take my own approach to these potentiometers.
> The approach is pretty standard I guess. Basically I have a number of
> resistors in series or parallel and their values increase in a binary
> fashion. The lowest resistor is R, than 2R, 4R, 8R etc. I want to use
> digitally controlled switches to bypass the resistors and thus
> increase or decrease the resistance according to a digital value.

You will find that it is nigh impossible to switch a binary weighted 
ladder without glitches and to match them to 10 or more bits of 
precision is another problem of it's own.  Also, if I follow you 
correctly, you still just have a adjustable resistor, not a 
potentiometer. For these and other reasons you'll find linear resitor 
strings in certain types of DAC.

> The first method will result in a voltage loss due to the forward
> diode drop over the substrate diode and probably also in deterioration
> of the sound passed through this circuit.

No.  The substrate diode is bypassed by the channel when the switch is 
on.


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

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