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An animation of a two-stage weighted CDAC

Lately the well-forgotten gif format images have resurrected again with social media and the popular website 9gag in particular. Well, this afternoon I thought, hmmm there are so many idiotic gifs online, why not make something electronics-related. Voila here it is:

A two-stage weighted cap network often used for D/A A/D conversion.

My initial intention was to try and draw moving electrons (charge) on the gif, but it somehow meant a lot more work than I initially thought. The animation here linearly increments bit switches, a bit boring I admit. But hey, I don't need to draw electrons moving, the principle is quite simple (and intelligent). With the current schematic/drawing the idea is very simple (unlike some more sophisticated charge redistribution DACs). We basically form a capacitive division, if we have a look at case 1 (LSB switch connected to Vref), then we have:

Capacitive division formed between the LSB, MSB part and the split capacitor.

We can then simply calculate the output voltage of the DAC with only LSB switch connected to $V_{ref}$ as $$V_{out} = 2V_{ref}\frac{\frac{C}{8}}{2C}$$ This split capacitance technique basically allows for total capacitor size reduction of the whole DAC as the "total weighting factor" in e.g. a tradidional CDAC here is split into two parts, which if you do the simple maths reduces cap size/area. Here is a link to the original paper from 1979.

Sadly with this my sunny day off is over!

Date:Sun Jun 06 18:37:00 CEST 2014

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