[sdiy] Choosing a DAC

[Tom Wiltshire]

All Jerry's points are true, but might not be fatal to the idea.

I don't know about AVRs, but on the PIC, the PWM is driven from the  
system clock. This can be up to 20MHz. If we're counting sets of 64  
clocks to derive our PWM  output, we have a PWM frequency of 20MHz/64  
= 312500Hz. PWM accuracy should be good since the timing is crystal  
based.

The filtering will have to integrate a number of these PWM pulses to  
get the required analogue voltage, but response times in the tenths  
of a millisecond would be possible with a PWM frequency this high.  
Whether 100uS sounds like a long time or not depends on your frame of  
reference!

Still, I'm not convinced it's the best idea. Other types of DAC do  
seem rather better suited for this job.

T.



On 29 Jun 2009, at 20:37, Jerry Gray-Eskue wrote:

> Good accuracy requires a long count period to adjust the duty cycle  
> in. By
> the time you add a filter ( and its time constants) to get a solid  
> DC level
> PWM is very slow compared to other DAC types. Depending on your PWM  
> clock
> speed and what you are using it for, it MAY be fast enough for  
> satisfaction
> performance.
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of David  
> Anderson
> Sent: Tuesday, July 07, 2009 12:12 PM
> To: Synth DIY
> Subject: Re: [sdiy] Choosing a DAC
>
>
>
>
> Ingo Debus wrote:
>> Can you use a PWM-based DAC? This would be very accurate, and  
>> since you
>> only need 8 bits (or even less) it would surely be fast enough.
>>
>> I'm not an AVR guy, but many microcontrollers have some PWM output
>> already on chip.
>
> And here's an example of converting PWM output to a 0-10v range.  
> There's
> a circuit Melanie's reply:
> http://www.picbasic.co.uk/forum/showthread.php?t=245
>
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