The method of PDM is a common technique to generate a one bit serial data stream from a parallel digital vector, to represent its corresponding analog value at only one pin. This is often required to transport digital data or turn it into an analog value without a common DAC. The signal changes rapidly in between two levels drawing the average level in the long run. In fact a PDM with a subsequent filter is a kind of DAC as explained here: PDM for Audio

Delta Sigma Conversion

 As explained in the article, delta sigma conversion generates an ideal one bit output of the digital value which in theory only has to be filtered appropriately to obtain an analog voltage. In the digital domain using FIR-filters this works pretty fine because ideal mathematical algorithms are used. Practically and in the analog domain, especially when it comes to higher currents there are some problems. Analog filters add a frequency delay to the signal causing possible phase issues. Driving a loudspeaker with it, further distortion occurs.

Using a modern digital circuit like a PLD one has the opportunity to correct several issue. First it is possible to add an allpass filter with a tricky frequency behaviour (e.g. the high branch of a "cow tale") to manipulate the phase delay for higher frequencies and thus applying a linear distortion compensation.

 Furthermore it is possible to predistort the pseudo analog signal before changing it into a one bit stream that way that non linear behaviour like introduced by the amplifier circuit and the loudspeaker (magnetic <-> air response) can be preassumed and corrected in advance. See the nonlinear predistortion approach.

Generally a PLD has a high switching frequency to perform both clocked and non clocked operation when generating the PDM output. With the clocked version, one has total control over the introduced harmonics. With the self running technique most precise results can be achieved, but requires a loop back for regulation.

Here the clocked version is used to drive an output amplifier circuit.

Learn more about the function of this circuit in the former article mono amplifier.

Related Information

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