Crystals' Shunt & Load Capacitance

The shunt capacitance is the total capacitance of the device itself across it's terminals.

The load capacitance is **normally** the loading capacitance required to operate the device nominally.

Once you know those numbers, you can calculate the required loading caps (necessary to operate the device properly).

As circuit stray capacitance and amplifier capacitance vary, the crystal mfrs simply specify what the crystal has (for shunt which is also sometimes the static capacitance, Co).

Note that different manufacturers have different notations, so in this case Cs may refer to the capacitance of the terminals when mounted on a circuit board, in which case it becomes part of the stray capacitance below, rather than being the internal static capacitance, which affects equivalent resistance and drive levels.

So if:

CL = loading capacitance from datasheet Cs = Circuit stray capacitance Cx1 and Cx2 are loading caps we have to put on the circuit, which may be the same value, but sometimes are not (see below)

Then for proper operation CL = Cs + [(Cx1 * Cx2) / (Cx1 + Cx2)]

In some cases, oscillators won't start up properly unless they are deliberately capacitively imbalanced, in which case the crystal loading caps will be different (a ratio of 1.5:1 - 2:1 is typical).

Cheers

PeteS

Wow, that's way more complicated than I thought. But thanks so much.

So, I know the load capacitance and its use. I still don't get the shunt capacitance on its usefulness. Why do we need to know this? How does it affect the purchasing decision?

Thanks.

PeteS wrote:

In this particular case, the shunt capacitance is *probably* the capacitance of the package and must be used in the Cs (above) to calculate the proper loading capacitors. It's also important when choosing an amplifier (or choosing a crystal for a particular chip that has an internal oscillator).

Cheers

PeteS