converting PWM to DC voltages using RC - Page 2 - Electronic Circuits Projects Diagrams Free

ikalogic · 31st October 2007, 08:49 PM

don't worry guys it does not confuse me

i am familiar with the energy storing/dissipation of electronic components. i was trying to connect theory and practice..

by the way here is another question. still in the application note provided by microship, the authr chose to use a pwm frequency 5 times higher than the one of the orriginal signal.. could i use a frequency 20 times higher? or more? on what basis do we chose the frequency of the PWM signals ?

As far as i understand, the higher the better.. right?

thx

**Nigel Goodwin** · 31st October 2007, 10:24 PM

It really depends what you're wanting to do?, if you're only wanting a DC voltage, without any fast changes, you can have a cutoff frequency MUCH lower than 1/5th of your PWM frequency. But assuming you're using PWM for an amplifier, and you're wanting the filtered output to have a good frequency response (say 20KHz for audio), then using 100KHz for the PWM makes sense, as running it higher may require more expensive transistors, more complicated circuitry, and bigger heatsinks (higher frequency, higher losses).

But if the filter is entirely removing the PWM frequency, then increasing the PWM frequency won't have any helpful effect.

OutToLunch · 3rd November 2007, 12:03 AM

The accuracy of the output signal will be dependent upon the speed at which the original signal is sampled. A sampling rate 5x that of the original signal frequency will yield good results on the output. If there is any higher frequency content in the original signal, then this information will likely be lost.

Just out of curiosity, what is the purpose of using pwm to recreate the signal? Is this a buffer of some type? If so, then for low power, an op-amp would easily suffice and have low output impedance. If you're taking a signal and looking to significantly boost the power of the signal, then you may want to look into an LC filter to recreate the original signal from the pwm version.

Using an RC filter will require that the output be tied to a high impedance block so that the output impedance is not altered.

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31st October 2007, 08:49 PM	(permalink)
ikalogic	don't worry guys it does not confuse me i am familiar with the energy storing/dissipation of electronic components. i was trying to connect theory and practice.. by the way here is another question. still in the application note provided by microship, the authr chose to use a pwm frequency 5 times higher than the one of the orriginal signal.. could i use a frequency 20 times higher? or more? on what basis do we chose the frequency of the PWM signals ? As far as i understand, the higher the better.. right? thx __________________ Ibrahim Kamal check my electronics and robotics page: http://www.ikalogic.com/

31st October 2007, 10:24 PM	(permalink)
Nigel Goodwin	It really depends what you're wanting to do?, if you're only wanting a DC voltage, without any fast changes, you can have a cutoff frequency MUCH lower than 1/5th of your PWM frequency. But assuming you're using PWM for an amplifier, and you're wanting the filtered output to have a good frequency response (say 20KHz for audio), then using 100KHz for the PWM makes sense, as running it higher may require more expensive transistors, more complicated circuitry, and bigger heatsinks (higher frequency, higher losses). But if the filter is entirely removing the PWM frequency, then increasing the PWM frequency won't have any helpful effect. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

3rd November 2007, 12:03 AM	(permalink)
OutToLunch	The accuracy of the output signal will be dependent upon the speed at which the original signal is sampled. A sampling rate 5x that of the original signal frequency will yield good results on the output. If there is any higher frequency content in the original signal, then this information will likely be lost. Just out of curiosity, what is the purpose of using pwm to recreate the signal? Is this a buffer of some type? If so, then for low power, an op-amp would easily suffice and have low output impedance. If you're taking a signal and looking to significantly boost the power of the signal, then you may want to look into an LC filter to recreate the original signal from the pwm version. Using an RC filter will require that the output be tied to a high impedance block so that the output impedance is not altered.