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PWM-ing the LSB of a DAC

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Circlotron:
Say we have an 8-bit DAC that we want to squeeze a bit more resolution out of. We could PWM the LSB at one of 16 different pulse widths, and provided we do it rapidly enough and then low pass filter it we have greater analog resolution. What would it be though? 12 bits or only 11?

fcb:
Can't see why it wouldn't work. Probably DAC dependent to some extent.

As for theoretical resolution, look at it as a 7bit DAC plus a 4bit DAC = 11 bits.

Upsides:
Settling time for large transistions reduced considerably due to hybrid nature.
Much faster than a pure PWM DAC.
Uses an 8bit DAC which *might* be cheaper/more available than a 12bit DAC.

Downsides:
Slows down throughput by at least 16x (11bit) to 32x (12bit)
Device dependent - might be some anomalies in the rise/fall times of the DAC leading to non-linearaties.
Filtering will be more complex, probably get best performance with a 3rd+ order filter - (opamps may be cheap and you might already be filtering elsewhere so perhaps not a downside).

artag:
You can do it. It's called dithering.

tszaboo:
Depending on the DNL and INL of the DAC, you might get better accuracy and resolution, or worse accuracy and better resolution.

Psi:
It gets even more complex when you have 3 DACs and you use two to dynamically set the Vref min and max of the 3rd.  ;)

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