<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"></head><body dir="auto">I was hoping an EE would join in. :-)<div dir="auto"><br><div dir="auto">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?</div><div dir="auto">Pete</div></div><div><br></div><div style="font-size:100%;color:#000000" dir="auto"><!-- originalMessage --><div>-------- Original message --------</div><div>From: Kevin Schilf via TriEmbed <triembed@triembed.org> </div><div>Date: 6/30/20 3:14 AM (GMT-05:00) </div><div>To: Jon Wolfe <jonjwolfe@anibit.com>, Charles A <chanoia33@gmail.com> </div><div>Cc: triembed@triembed.org </div><div>Subject: Re: [TriEmbed] PWM to Analog 0 to 5 VDC? </div><div><br></div></div><div class="ydp81202e6eyahoo-style-wrap" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:16px;"><div></div>
<div dir="ltr" data-setdir="false">Hi Chuck,</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">A few thoughts sight unseen. :-)<br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">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.</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">Faster switching implies more power. No free lunch.<br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">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.<br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">Good luck,</div><div dir="ltr" data-setdir="false">Kevin Schilf<br></div><br><div dir="ltr" data-setdir="false"><br></div><div><br></div>
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On Tuesday, June 30, 2020, 12:08:32 AM EDT, Charles A via TriEmbed <triembed@triembed.org> wrote:
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<div><div id="ydpc2377ac1yiv5987644365"><div><div dir="ltr"> 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. <br clear="none"><div><br clear="none"></div><div>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. <br clear="none"></div></div><br clear="none"><div class="ydpc2377ac1yiv5987644365yqt2847000422" id="ydpc2377ac1yiv5987644365yqt78964"><div class="ydpc2377ac1yiv5987644365gmail_quote"><div class="ydpc2377ac1yiv5987644365gmail_attr" dir="ltr">On Mon, Jun 29, 2020 at 11:57 PM Jon Wolfe <<a shape="rect" href="mailto:jonjwolfe@anibit.com" rel="nofollow" target="_blank">jonjwolfe@anibit.com</a>> wrote:<br clear="none"></div><blockquote class="ydpc2377ac1yiv5987644365gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex;"><u></u>
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<div>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.</div><div><br clear="none"></div>
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<p style="color:black;font-size:10pt;font-family:sans-serif;margin:8pt 0px;">On June 29, 2020 10:32:55 PM Pete Soper via TriEmbed <<a shape="rect" href="mailto:triembed@triembed.org" rel="nofollow" target="_blank">triembed@triembed.org</a>> wrote:</p>
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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.<div>Pete</div><div><br clear="none"></div><div style="font-size:100%;color:rgb(0,0,0);"><div>-------- Original message --------</div><div>From: Charles A via TriEmbed <<a shape="rect" href="mailto:triembed@triembed.org" rel="nofollow" target="_blank">triembed@triembed.org</a>> </div><div>Date: 6/29/20 10:23 PM (GMT-05:00) </div><div>To: <a shape="rect" href="mailto:triembed@triembed.org" rel="nofollow" target="_blank">triembed@triembed.org</a> </div><div>Subject: [TriEmbed] PWM to Analog 0 to 5 VDC? </div><div><br clear="none"></div></div><div dir="ltr">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.<div><br clear="none"></div><div>Thanks,</div><div>Chuck</div></div>
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