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PWM to get steady DC voltage - Ringing

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atferrari
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Username: atferrari

Post Number: 712
Registered: 05-2005


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Posted on Thursday, 18 June, 2009 - 01:00 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Much in line with this thread http://www.electro-tech-online.com/micro-controllers/88132-digital-analogue-conversions-via-pulse-width-modulation.html#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.
Agustín Tomás - Buenos Aires - Argentina
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epithumia
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Username: epithumia

Post Number: 661
Registered: 06-2006

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Posted on Thursday, 18 June, 2009 - 08:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Agustín,

Received wisdom is that electrolytic capacitors are not great at high frequencies.

I would add a stage of pre-filtering using say a 10nF capacitor.

If you want to keep the current filter time constant then I suggest splitting the first 1k resistor into 2x510 ohm in series, then add the 10nF capacitor from between them to ground.

I would also be tempted to increase the resistor values - say use 10k/1uF instead of 1k/10uF - just to reduce the amount of current flowing and hence reduce resistive noise in the ground wire.

Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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atferrari
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Username: atferrari

Post Number: 718
Registered: 05-2005


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Posted on Friday, 19 June, 2009 - 08:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola Epi,

Would you mind showing a sketch of it?

Yes I am aware of electrolytic so I have tantalum or ceramic, right?
Agustín Tomás - Buenos Aires - Argentina
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epithumia
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Username: epithumia

Post Number: 662
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Posted on Friday, 19 June, 2009 - 11:31 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

myimage

Also, check that the ringing isn't caused by an earth loop or a long earth wire.

(Message edited by epithumia on 19 June, 2009)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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atferrari
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Username: atferrari

Post Number: 721
Registered: 05-2005


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Posted on Saturday, 20 June, 2009 - 08:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ground loop. Yes! I have to rework the board.

Thanks for attaching the suggestion.
Agustín Tomás - Buenos Aires - Argentina

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