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Background

In order to convert digital values (n-bit vectors etc) to the equivalent analogue value (for instance 0 to 5 volt) can it be necessary connect hardware DAC to the system. Those usually based on a 2R-ladder network and will be quite expensive when a higher resolution needed (12-bit or more).

A cheaper alternative could a Pulse Modulation based DAC - like the PWM and Sigma-Delta types.

Lots of documentation can be found on the net - concerning the theories behind and the frequency spectres etc. The exercises below mostly about the VHDL implementations of a PWM and a PPM Sigma-Delta conversion.

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 http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html

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 http://en.wikipedia.org/wiki/Delta-sigma_modulation

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 APR8 - Principles of Sigma-Delta Modulation for Analog-to-Digital ...

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 http://www.cs.tut.fi/sgn/arg/rosti/1-bit/   - 1-bit A/D and D/A Converters

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 http://www.maxim-ic.com/appnotes.cfm/an_pk/1870
 
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 Block diagram for a PWM Digital to Analogue Converter

   
 
The PWM generator basically a Counter and a Comparator.

If the Max counter value a power of 2 minus 1(like 127, 255 etc) will this circuit normally work alright.

If the Max counter value below will there be a chance (read - risk) of mal-functionality due to the fact that not all bit changes simultaneously - Consider an extra register between the counter and the comparator.
 
 
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Block diagram for a Sigma-Delta PPM Digital to Analogue Converter
   
 
The PPM signal will have a "more equal" contribution compared with the similar PWM solution.

Hence will the requirements for filtering be less compared with the similar PWM solution.

In case of power output will the power-lost be equal to the number of switches (on/off and off/on)
 
 
 
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PWM versus Sigma Delta PPM outputs
 
Simulation with N=4 and Max=16