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#5.7 PWM vs. Sigma-Delta
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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.



  You may adjust the Clk-frequency between 25, 50 and 100 MHz in order to observe the PWM and PPM outputs.

SW6..SW0 = the 7- bit PCM value for the PWM and PPM duty cycles.
SW7          = Selection between High and Low Clk frequency - Calculate the exact number yourself

LD3           = The PWM output
LD4           = The PPM output

  •  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.

  •  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)


Please note!
In order to avoid clock-skew are all F/F's driven by the same Clk-signal.

The Scaled_Clk will only be high for one Clk-period and hence can it serve as CE (Clock Enable) signal in the processes for PWM and PPM generation.





  Simulation with N=4 and Max=16
Please note - The circuit above only a suggestion which not optimized for size/speed.
It appears that the PPM value first will be 1 after 16 clock-pulses - this fact shouldn't be a problem in real-life applications, but its possible fix "the bug" with minor changes (how?)