[sdiy] linear CV to log / antilog / inverse?

Nicholas Gregorich nicksdsu at mac.com
Fri Sep 21 23:28:02 CEST 2007


Julian wrote:
> Hi Nicholas,
> 
> 
>> How about an AVR or PIC microcontroller? You would sample with the 
>> ADC, use a lookup table to apply the function, and a PWM (and filter) 
>> to go back to the analog domain.
> 
> 
> Have thought a lot about that approach, and, yes, it would work well, 
> expect for the financial cost.
> 
> Id be loath to use less than 12 bit dacs on the output, and, even with 
> some ammount of demultiplexing, the number of controlls that im dealing 
> with (i said 20, but itll probably be more like 23) will make, i think, 
> the solution somewhat pricy (?)
> 
> But, yes, if it wasnt for the cost issue, i think that would be a nice 
> solution.
> 
> Julian
> 

Doesn't noise prohibit pots from being sampled at much more than 10 bits 
(or even 8 with breadboarded circuits) anyhow? I guess more bits on the 
DAC would still be useful since the transfer function will be non-linear.

There are some 16-bit PWMs, I think the PWM frequency would be around 
100-200Hz (shouldn't be too limiting for pot CVs, should it?).

You could run 4 pots into a ATTiny24 and have 4 PWMs, but you'd be stuck 
with 8-bit PWM. It would be cheap though, $12 or so for all 20 channels.

The Tiny24 could do 2 channels of 10-bit sampling and 16-bit PWM. $24 
for 20 channels.

Nick.



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