Bypass Capacitors – Size and number
There is a fair amount of discussion and disagreement in the EMC community about the subject of bypass capacitors. What size should they be? Should you use more than one in a given location to cover a wider frequency range? My position is simple and here is the basis behind it.
I recommend that a single size bypass capacitor be selected and used everywhere a bypass capacitor is needed. Past experience shows that .1 uF is a good value to use. It provides a low impedance path and lower frequencies and the inductance of the loop formed by the capacitor and connecting traces drives the impedance of the circuit at higher frequencies. You will hear arguments that say that if you need to decouple a wide range of frequencies, the spectrum of a trapezoidal waveform for example, you should use a pair of capacitors, separated in value by a couple orders of magnitude. My argument is that this does not help and only serves to increase parts count, required board real estate and costs.
Why is this so? A capacitor, as installed, is part of a series LC circuit. The inductance (L) of the circuit is caused by the traces connecting the capacitor to the device it is decoupling. You cannot avoid this, it is basic circuit theory. For typical surface mount components a value of 3 nH is a good approximation of the inductance of the circuit. Some proposed installation practices might decrease this value a bit, but it will serve for this example.
Take a look at the graph following this paragraph. It shows the impedance of 3 different capacitors, each assuming a loop inductance of 3 nH. The values used are .1 uF, 470 pF and 47 pF. Note that the cutoff frequency rises as the value of the capacitor decreases, but the impedance below the cutoff frequency is higher for the smaller capacitors and the impedance above the cutoff frequency continues to be driven by the inductance in the circuit. Nothing is gained by adding smaller value capacitors.
I hope this graph makes it clear why I recommend using a single capacitor for bypassing purposes. If you have questions about EMC, please feel free to contact me at Ghery@PettitEMCConsulting.com or (360) 790-9672.