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PWM to get steady DC voltage - Ringing

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atferrari

Well-Known Member
Much in line with this thread http://www.electro-tech-online.com/...-via-pulse-width-modulation.88132/#post732054 I am trying to obtain a reasonable clean voltage using the power control PWM module from the PIC 18F4431.

With a PWM frequency of 42 KHz, I get less than 2 mV of nosie but ringing, coincident with leading / trailing edges is quite pronounced, with peaks of +10 / -6 mV.

The (two pole) filter has R = 1K and C = 10 uF.

My questions:

a) How could I reduce if not eliminate the ringing? My intention was to use it as a variable voltage reference.

b) Tried capacitors of all kinds I have handy but result stays the same.

c) Should I consider using a DAC instead?

Gracias for any replies.
 

kchriste

New Member
Forum Supporter
1st thought:
Assuming the capacitors you are using have very low ESR and inductance, I would assume that the ringing is due to lead inductance in the circuit. So use thick ground traces on the PCB and a single point ground type layout for the LPF.

2nd thought:
The ringing you see on the scope could be due simply to the ground lead of the probe. A way to check for this is to scope the ground point where the probe alligator clip is attached to. If you still see the ringing waveform, you know what the culprit is. There are special attachments for your probes that help eliminate the ground loop on the probe. They look like a spring with a small wire tail. The "spring" slips over the metal ring at the end of the probe providing a low inductance ground point.
 
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smanches

New Member
Try a very small cap, like 100pF or so. 10uF is way to large to catch the ringing on a PWM signal.
 

atferrari

Well-Known Member
Gracias to both for replying. Reworking the PCB now.

Still have to find real low ESR capacitors.
 

atferrari

Well-Known Member
I quit after some time not getting any improvement. Went the DAC way with success.
 
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