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Old 30th August 2007, 12:40 AM   #1
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Question Not quite Class D but many ideas taken from

Hello I want to make a digital intercom system I have some ideas on some novel ways (at least to me) to do the ADC and DAC. I just want to run them by people here to see if I am not totaly barking up the wrong tree!

ADC:
signal and triangle wave at 10x the singals max freq fed into comparator and output sampled at 100x audio freq (not sure if I need to sample to PCM signal this often) by PIC 16F84.

DAC:
take same signal and put it through a 2nd order low pass filter with butterworth response and f0=40khz. Then put into simple class AB mossfet amplifier.

I would use a class D amp for the output but they seem very hard to make without using expensive components and the design seems too much effort for the limited gain. (although I would love to design one as a seperate project - can't start too many at once!)

thanks for your replies
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Old 7th September 2007, 07:02 AM   #2
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Default Re: Not quite Class D but many ideas taken from

[QUOTE]Originally posted by kipman725
[B]Hello I want to make a digital intercom system I have some ideas on some novel ways (at least to me) to do the ADC and DAC. I just want to run them by people here to see if I am not totaly barking up the wrong tree!

ADC:
signal and triangle wave at 10x the singals max freq fed into comparator and output sampled at 100x audio freq (not sure if I need to sample to PCM signal this often) by PIC 16F84.

------------------
u cant use a 16f84 to do this job, u need a dsp .



DAC:
take same signal and put it through a 2nd order low pass filter with butterworth response and f0=40khz. Then put into simple class AB mossfet amplifier.
------------------------------------
yes, u can do this , simple but not good
u can drive a pwm class-d amp ,


I would use a class D amp for the output but they seem very hard to make without using expensive components and the design seems too much effort for the limited gain. (although I would love to design one as a seperate project - can't start too many at once!)
------------------------
neednt a dac, just output the pcm signal , the drive a pare mosfets
but hard to fix the thd from mosfet and output filter
the big problom is the digital af err feelback
not ennogth speed .

rg

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Old 7th September 2007, 08:34 PM   #3
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hmm the pic sample rate only needs to be 10x the audio freq which is perfectly posible. they can be clocked upto 10mhz and non branching instructions take up 1 clock cycle and branching ones 2 clock cycles. So basicly what were doing is sensing the state of one one pin and setting another pin to the same value which is hmm I think 3 clock cycles.

the driving issue was why I thought no class D.. if you have a simple way to drive mosfets with such a high frequancy pwm signal I would love to hear it the peak drive currents will probobly upto 1A ish?
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