Bypass Capacitors Case Study

Best practices for SMT bypass capacitors placement The ability for the bypass capacitor to reduce the noise in a circuit and stabilize the supply depends on a combination of factors. This includes its capacitance, voltage, and temperature ratings as well as the physical placement on the board.

Usually connected between the VCC and the ground, the capacitor provides a low impedance path that allows the AC components in the DC power line to pass to the ground. It also acts as a reserve - storing the charge that helps in filling in the voltage dips arising from fluctuations in the load.

Despite the ability to resolve several issues, the wrong choice or placement of the capacitor may cause power losses, create additional noise or lead to an

Minimizing the trace length reduces inductance, resistance, and the overall impedance.

Multiple Bypass capacitors for wide bandwidth circuits

Place multiple capacitors in ascending order

A single capacitor is usually best suppressing noise over a particular frequency range but insufficient for devices operating over a wide range of frequencies. In wide bandwidth applications, the best solution is to connect several capacitors with different values in parallel. The high-value capacitors will provide a low impedance path to the low frequencies while the smaller capacitors will handle the higher frequencies.

With the proper selection and placement, a designer can provide a low impedance path for all the applicable frequencies. When it comes to the placement, the best practice is to arrange them in ascending order, starting with the smallest value capacitor closest to the power pin and then adding the larger ones in ascending order. Fig. Placement of multiple bypass capacitors in ascending

Although the device may work with one or two capacitors, it is good practice to add at least one bypass capacitor for each of the supply pins and as close as physically possible. This prevents instabilities when the device has several outputs switching simultaneously.

The ground connection

The designer should use closest ground connection or pin to minimize the inductance and make it easier for the ac nose signals to pass to the ground. The effective way of achieving this is by connecting the other ends of the bypass capacitors to low impedance ground planes. This is best done through short trace lengths or vias.

Summary of best practices for placing bypass capacitors

• Place the capacitor as close as physically possible to the power supply pin of the device. This reduces the inductive influence of the trace.

• When using multiple capacitors in parallel, place the smallest capacitor (in value) closest to the power pin and add the others in ascending order

• Connect the other terminal of the capacitor directly to the device ground pin when the distance is short enough. If not, connect it to the ground plane using the shortest trace or a via.

• Place the capacitor underneath the chip