# DAC Speed and Glitches

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B) Asynchronous Switching

Suppose we want to increment the setting of a DAC from 00001111 to 00010000. Five switches have to be thrown: one goes from 0 to 1, while the last four go from 1 to 0. What happens if they don't all switch at the same moment (to a fraction of an attosecond, faster than any currently-available transistor)? We get settings that are NEITHER the initial nor final state! If the switches reset from least significant to most significant, the states will be

00001111 start

00001110

00001100

00001000

00000000

00010000 end

So instead of smoothly going from 15/256 of the reference voltage to 16/256, we go through 14/256, 12/256, 8/256, 8/256, and 0 on the way to 16/256. That puts out a downward voltage spike.

Exercise: if the bits reset from most significant to least significant, what happens?

In reality, the bit resetting isn't necessarily in a specific order. There is thus a "jumping around" of the output potential for a brief period while settings are in transition.