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PWM and Analog Signal Quality

I'm converting a 0-5v PWM signal to an analog signal using an LM358D. I've connected the output to an oscilloscope to look at the signal but I'm not really sure what I'm looking at. The PWM looks ok but I'm unsure of the analog signal.

Here's some images, please let me know what you think.

Both pics are at 50% signal strength. The oscilloscope is set to defaults, simply connected the probe.

Thanks

Text

EDIT:

Here's the schematic for the PWM to analog converter. The cap is a tantalum. The dip switch changes the analog from 5v to 10v, the signal looks the same on both and is correct voltage so seems to be ok?

Text

Answer*

> The PWM looks ok but I'm unsure of the analog signal.

The “analog signal” looks like the PWM smoothed by a first-order
low-pass filter. It still has some oscillations, 584&nbsp;mV
peak-to-peak. If you can tolerate that, then it's perfectly fine. If
not, then increase the time constant of your filter.

A first-order low-pass filter behaves, in its stop band, like an
integrator. Thus, a square signal is turned into a triangle. You are
seeing exactly the expected behavior.

> The oscilloscope is set to defaults

There are no “default” settings in the oscilloscope. You probably used
the “autoset” button instead. This button tries to find some settings
that let you see _something_ on the screen. I mean, some kind of signal,
not just straight line. If the signal is almost flat, the autoset will
increase the gain until you can see it is not flat. In this case, it
increased the gain by a factor 10 (200&nbsp;mV / division instead of
2&nbsp;V used previously). It also added a negative offset in order to
avoid the trace going out of the screen. See the small pentagon on the
far left with a “1” inside? On the first picture this tells you where
the zero (the reference potential) is relative to your signal. On the
second picture, the polygon pointing at the bottom of the chart tells
you the zero is off-scale.

If you want to really appreciate the smoothing action of the filter, you
have to look at its input (the PWM signal) and its output (the “analog”
signal) using the exact same settings. Connect the input, hit “autoset”,
and look. Then connect the output, _do not_ change the settings (do not
hit “autoset”) and look at the output.

Better yet: forget the autoset button, adjust the settings manually and
mind their values.

Better yet: as you have a two channel scope, display _both_ signals
simultaneously, using the exact same settings on both channels.

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Thank-you for the detailed reply, I appreciate it! I updated the original post with schematic. What's an "acceptable" peak-to-peak? If a resistor or cap would improve that would be good. You can see I have PWM freq at 1000 Khz, is there a "magic" number? I use pca9685. I couldn't see any difference on scope, only PWM. My scope does have a "default" button and "autoset", latter was useless as it zoomed out so I manually set to view, I did use same settings for both signals just had to change voltage scale so I could view better. Thanks!

tryingHard
– tryingHard

2021年03月18日 12:34:01 +00:00
Commented Mar 18, 2021 at 12:34
@tryingHard: 1. It is up to you to decide what you would consider "acceptable". It's your application! 2. If you want less residual oscillations, increase the value of R78. 3. The PWM frequency is 1 kHz, not 1000 kHz. 4. There is no magic.

Edgar Bonet
– Edgar Bonet

2021年03月18日 13:42:16 +00:00
Commented Mar 18, 2021 at 13:42
Thanks, impossible for me to know what is acceptable when I don't know what the number means, not something you can understand after an hour of reading. Maybe 500mV isn't bad or maybe it's horrible. I will see what happens with R78 and thanks for clarifying frequency.

tryingHard
– tryingHard

2021年03月19日 00:15:01 +00:00
Commented Mar 19, 2021 at 0:15
@tryingHard: Re "acceptable": it depends on your application, on what you want to do with that signal. "Acceptable" means "acceptable by whoever will be the consumer of the signal".

Edgar Bonet
– Edgar Bonet

2021年03月19日 08:07:41 +00:00
Commented Mar 19, 2021 at 8:07
I hear you, it's for LED drivers and aquarium power heads.

tryingHard
– tryingHard

2021年03月19日 08:58:11 +00:00
Commented Mar 19, 2021 at 8:58

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