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How to read datasheets

What is the main reason a datasheet recommends "placing a 0.1 µF ceramic capacitor as close as possible to each supply pin"?

  1. ATo lower the DC resistance of the supply line and thereby reduce the IC's average current consumption
  2. BBecause longer wiring reduces the capacitance of the capacitor itself accordingly
  3. CTo reduce wiring inductance and suppress supply voltage fluctuations caused by rapid current changes
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Answer: C. To reduce wiring inductance and suppress supply voltage fluctuations caused by rapid current changes

Digital circuits draw large currents from the supply for short periods on every clock. The wiring from the supply to the IC has inductance L, which produces a voltage drop of L × di/dt in response to current changes. For example, with 2 nH of wiring and a current change of 0.1 A/ns, the supply voltage drops by as much as 0.2 V. Placing the capacitor right next to the pin lets it supply this current over a short path. Wiring length does not change the capacitance value itself; the problem is the inductance of the path.

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