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DAC Speed and Glitches

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A) Switch Capacitance and Inductance

The transistors that do the switching don't go from off to on (or switched left/ground switched right/virtual ground as in the diagrams in Ladder Networks) instantaneously. Because there are junctions between dissimilar materials inside the transistors, there is charge storage at the junctions. These act as capacitors, charge storage devices. When a digital 0 changes to a 1 or vice versa, the output current or potential response is slowed down by the charging and by the resistance of wires or other components in the external circuitry. It takes a time RC (resistance in ohms times capacitance in farads has units of time in seconds) for the response to get to 63% of its final behavior and 5 RC to get to 99.3%. Even if the capacitance is small, a few picofarads (10-12 F) and the resistance is small, say 100 ohms, RC = 10-10 s and the time to complete switching is 0.5 ns. That sounds so small as to be negligible, but in reality settling time for the ladder network involves much larger resistances. R in the network is typically 1 kilohm to 10 kilohm so that the currents are small and the power dissipiation reasonable. At 10 kilohm and 10 pF (quite realistic values), RC = 100 ns, and the ladder network settles in 0.5 μs. If one only wishes to make measurements a few thousand times a second, this still seems fast, but for many high-speed measurements in optical or mass spectroscopy, one wishes to make measurements at megahertz frequencies. In this case, the slowing of the DAC by RC dissipation limits the accuracy of the DAC.






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