[TriEmbed] PWM to Analog 0 to 5 VDC?

Charles A chanoia33 at gmail.com
Mon Jun 29 23:08:14 CDT 2020


 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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