# help with low pass filter for pwm

Discussion in 'General Electronics Chat' started by frto_nr1, Nov 8, 2008.

1. ### frto_nr1 Thread Starter New Member

Nov 4, 2008
2
0
Hi,
i need to design a low pass filter to filter the pwm signal into DC voltage. Pwm is generated by ATmega64 @ 9MHz (or 16MHz when it's necessary). The pwm should be phase and frequency correct (if it's necessary fast pwm) with resolution 10bit. I calculated the frequency of pwm about 1,1kHz(@9MHz and ph.&fr.co.) but output voltage should be correct at < 5ms. At first time i tried passive low pass filter, but it's too slow. then i tried active lowpassfilter values calculated at http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html but it hasn't good result for me.
My simulation circuit is down...
thanks
excuse my english

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2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
How fast response do you need? 1ms/V? I'd use a LC filter, but several together. I think active filters will require really high speed op amps for your application, and their not really needed. Active filters were invented primarily to replace low frequency circuits that had huge inductors, which the op amp replaced. You could easily go with a LC low pass filter in that frequency range.

3. ### frto_nr1 Thread Starter New Member

Nov 4, 2008
2
0
I must generate voltage so that on the 0R1(or som kind of very small) resistor is from 0A - 10A and the max value should be there in less then 5ms. Can you explain me something about LC filter?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
If I understand our OP, his PWM frequency is 1.1kHz, or about 2kHz if he uses a 16MHz clock. An active filter will work fine at these frequencies. I think the 5mS settling time to 10 bit accuracy will be difficult to achieve.