3.2 Decoupling Capacitors

The use of decoupling capacitors on every pair of power supply pins, such as VDD, VSS, AVDD and AVSS, is required.

Consider the following criteria when using decoupling capacitors:

  • Value and type of capacitor: Recommendation of two 0.1 µF (100 nF) in parallel rated at 10-20V. These capacitors should be low-ESR and have a resonance frequency in the range of 20 MHz and higher. Ceramic capacitors are recommended.
  • Placement on the Printed Circuit Board: The decoupling capacitors should be placed as close to the pins as possible. It is recommended to place the capacitors on the same side of the board as the device. If space is constricted, the capacitor can be placed on the opposite side of the PCB connected through a via; however, ensure that the trace length from the pin to the capacitor is within one-quarter inch (6 mm) in length.
  • Handling high-frequency noise: If the board is experiencing high-frequency noise above tens of MHz, add an additional ceramic-type capacitor in parallel to the decoupling capacitors. The value can be in the range of 0.01 µF to 0.001 µF. Place this capacitor next to the primary decoupling capacitors. In high-speed circuit designs, consider implementing a set of capacitances as close to the power and ground pins as possible. For example, 0.1 µF in parallel with 0.01 µF and 0.001 µF.
  • Maximizing performance: On the board layout from the power supply circuit, run the power and return traces to the decoupling capacitors first and then to the device pins. This ensures that the decoupling capacitors are first in the power chain. It is equally important to keep the trace length between the capacitor and the power pins to a minimum, thereby reducing PCB track inductance.