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
<[email protected]> 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 <[email protected]> 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 <[email protected]>
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
<[email protected]> Date: 6/29/20 10:23 PM (GMT-05:00) To:
[email protected] 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
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