[sdiy] Choosing a DAC

[Stewart Pye]

For a 6 bit (64 clocks) resolution pwm output on the ATMega644 using
fast PWM mode the frequency would also be 312500Hz. I think it's time
for some experimenting.

Regards,
Stewart.



Tom Wiltshire wrote:
>
> 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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