[TriEmbed] PWM to Analog 0 to 5 VDC?

Pete Soper pete at soper.us
Tue Jun 30 06:49:34 CDT 2020


Jon and Charles: If you have EE degrees and I forgot, my apologies! But 
Kevin is one of the guys on this list who could write at least parts of 
"The Art of Electronics" while many of us, like me, can only read, 
hoping some day to master the equations hiding under the 
practical/intuitive presentation of that book. I spent a summer a few 
years back getting my math back to where i was when I graduated from 
high school. I still haven't gotten my act together to get the math to 
where it was when I bombed out of Auburn. The inconvenient detail of 
math exams is that  they're frequently like programming settings: 
"close" is a fail. :-)

Regards,
Pete
On 6/30/20 7:35 AM, Pete Soper wrote:
> I was hoping an EE would join in. :-)
>
> Your point is excellent. By the time you have the usable bits on the 
> ADC side you've either got PWM frequencies that don't happen to be 
> available w many hobby MPUs or enough complexity that only in an 
> industrial setting can you say you're saving X pennies by avoiding a 
> DAC chip. But the extra headache is that most (all?) DAC chips require 
> at least two pins for control. Anybody know of an exception?
> Pete
>
> I was hoping an EE would join in. :-)Your point is excellent. By the time you have the usable bits on the ADC side you've either got PWM frequencies that don't happen to be available w many hobby MPUs or enough complexity that only in an industrial setting can you say you're saving X pennies by avoiding a DAC chip. But the extra headache is that most (all?) DAC chips require at least two pins for control. Anybody know of an exception?Pete
>
>
> -------- Original message --------
> From: Kevin Schilf via TriEmbed <triembed at triembed.org>
> Date: 6/30/20 3:14 AM (GMT-05:00)
> To: Jon Wolfe <jonjwolfe at anibit.com>, Charles A <chanoia33 at gmail.com>
> Cc: triembed at triembed.org
> Subject: Re: [TriEmbed] PWM to Analog 0 to 5 VDC?
>
> Hi Chuck,
>
> A few thoughts sight unseen. :-)
>
> Increasing switching speed pushes the switching artifacts out to the 
> right on a frequency plot giving you greater frequency "space" between 
> the DC value you want and the switching noise you don't.  This gives 
> you flexibility on the pole frequency/ses and room for the low pass 
> filter to roll off without having to use a higher-order filter to 
> shorten the tail.  Basically, you want the highest pole frequency with 
> sufficient roll off using the simplest acceptable filter topology 
> (single pole RC (simplest), double pole LRC, active filter).  You can 
> use a Spice simulator (LTSpice, etc.) to do trade-offs.
>
> Faster switching implies more power.  No free lunch.
>
> Playing devil's advocate, if you control the head end another option 
> is a simple D2A converter instead of PWM?  Many micros now provide 
> D2A's.  You may also be able to digitally filter the A2D readings in 
> the receiver.
>
> Good luck,
> Kevin Schilf
>
>
>
> On Tuesday, June 30, 2020, 12:08:32 AM EDT, Charles A via TriEmbed 
> <triembed at triembed.org> wrote:
>
>
>  Well I have the low pass RLC filter into the OpAmp.  I get less 
> ripple at the higher PWM frequencies however at those frequencies I 
> have much less granularity on the duty cycle.  Trying to deal with the 
> ripple.  Guess I should look at different RLC filter values next.
>
> The ADC input is on another board and I have no control over it.  I 
> need to supply a stable DC voltage to it. The current device (obsolete 
> now) only deviates by 10 mV or less according to its raw ADC reads.  I 
> need to match that or the SW reading the ADC is not satisfied.  My low 
> pass filter attempt so far deviates at best by 50 mV.
>
> On Mon, Jun 29, 2020 at 11:57 PM Jon Wolfe <jonjwolfe at anibit.com 
> <mailto:jonjwolfe at anibit.com>> wrote:
>
>     Yeah an rc low pass filter is the way to go.  I've done it that
>     way many times. The unity gain op amp could give you a buffer on
>     the filter output, but I think ADCs are usually high impedance
>     inputs. Depending on how fast you need the signal to change could
>     impact what op amp you would need to use. You could play around in
>     LT spice with different frequencies and component values to find
>     something that works.  Pete would know better than me, but I think
>     the downside to higher frequency might be higher power draw, since
>     more of the signal is going through the cap in the filter.
>
>     On June 29, 2020 10:32:55 PM Pete Soper via TriEmbed
>     <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
>
>>     The classic way to do this is with a low pass filter. If you
>>     google "PWM DAC" you'll find what you need. But the performance
>>     is going to be a function of the PWM frequency and how precisely
>>     you can change the duty cycle.
>>     Pete
>>
>>     -------- Original message --------
>>     From: Charles A via TriEmbed <triembed at triembed.org
>>     <mailto:triembed at triembed.org>>
>>     Date: 6/29/20 10:23 PM (GMT-05:00)
>>     To: triembed at triembed.org <mailto:triembed at triembed.org>
>>     Subject: [TriEmbed] PWM to Analog 0 to 5 VDC?
>>
>>     Anyone have a favorite circuit or chip to convert a PWM signal to
>>     a 0 to 5 VDC signal?  The resulting voltage needs to be very
>>     stable.  It feeds an ADC input. I've looked at an RL circuit into
>>     an OpAmp that also has a cap to ground at the OpAmp input.  The
>>     DVM says it's stable but the ADC reading the voltage says it's
>>     not. I'm measuring 100 mV deviations.  Would like to get to a 10
>>     mV deviation. I've tried changing cap values on the input as well
>>     as adding caps on the output side of the OpAmp.  Made
>>     improvements but still not good enough.  So looking for
>>     suggestions please.
>>
>>     Thanks,
>>     Chuck
>>     _______________________________________________
>>     Triangle, NC Embedded Computing mailing list
>>
>>     To post message: TriEmbed at triembed.org
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>>
>
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> -------- Original message --------From: Kevin Schilf via TriEmbed <triembed at triembed.org> Date: 6/30/20  3:14 AM  (GMT-05:00) To: Jon Wolfe <jonjwolfe at anibit.com>, Charles A <chanoia33 at gmail.com> Cc: triembed at triembed.org Subject: Re: [TriEmbed] PWM to Analog 0 to 5 VDC?
>          Hi Chuck,A few thoughts sight unseen.  :-)Increasing switching speed pushes the switching artifacts out to the right on a frequency plot giving you greater frequency "space" between the DC value you want and the switching noise you don't.  This gives you flexibility on the pole frequency/ses and room for the low pass filter to roll off without having to use a higher-order filter to shorten the tail.  Basically, you want the highest pole frequency with sufficient roll off using the simplest acceptable filter topology (single pole RC (simplest), double pole LRC, active filter).  You can use a Spice simulator (LTSpice, etc.) to do trade-offs.Faster switching implies more power.  No free lunch.Playing devil's advocate, if you control the head end another option is a simple D2A converter instead of PWM?  Many micros now provide D2A's.  You may also be able to digitally filter the A2D readings in the receiver.Good luck,Kevin Schilf
>          
>          
>              
>                  
>                  
>                      On Tuesday, June 30, 2020, 12:08:32 AM EDT, Charles A via TriEmbed <triembed at triembed.org> wrote:
>                  
>                  
>                  
>                   Well I have the low pass RLC filter into the OpAmp.  I get less ripple at the higher PWM frequencies however at those frequencies I have much less granularity on the duty cycle.  Trying to deal with the ripple.  Guess I should look at different RLC filter values next.    The ADC input is on another board and I have no control over it.  I need to supply a stable DC voltage to it. The current device (obsolete now) only deviates by 10 mV or less according to its raw ADC reads.  I need to match that or the SW reading the ADC is not satisfied.  My low pass filter attempt so far deviates at best by 50 mV.  On Mon, Jun 29, 2020 at 11:57 PM Jon Wolfe <jonjwolfe at anibit.com> wrote:
>
>
>
> Yeah an rc low pass filter is the way to go.  I've done it that way many times. The unity gain op amp could give you a buffer on the filter output, but I think ADCs are usually high impedance inputs. Depending on how fast you need the signal to change could impact what op amp you would need to use. You could play around in LT spice with different frequencies and component values to find something that works.  Pete would know better than me, but I think the downside to higher frequency might be higher power draw, since more of the signal is going through the cap in the filter.
>
>
>
> On June 29, 2020 10:32:55 PM Pete Soper via TriEmbed <triembed at triembed.org> wrote:
>
> The classic way to do this is with a low pass filter. If you google "PWM DAC" you'll find what you need. But the performance is going to be a function of the PWM frequency and how precisely you can change the duty cycle.Pete-------- Original message --------From: Charles A via TriEmbed <triembed at triembed.org> Date: 6/29/20  10:23 PM  (GMT-05:00) To: triembed at triembed.org Subject: [TriEmbed] PWM to Analog 0 to 5 VDC? Anyone have a favorite circuit or chip to convert a PWM signal to a 0 to 5 VDC signal?  The resulting voltage needs to be very stable.  It feeds an ADC input.  I've looked at an RL circuit into an OpAmp that also has a cap to ground at the OpAmp input.  The DVM says it's stable but the ADC reading the voltage says it's not. I'm measuring 100 mV deviations.  Would like to get to a 10 mV deviation. I've tried changing cap values on the input as well as adding caps on the output side of the OpAmp.  Made improvements but still not good enough.  So looking for suggestions please.Thanks,Chuck
>
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