Electronic – Crystals and load capacitance


If a crystal has a rated load capacitance of 6 pF, is the right thing to do to put a 6 pF capacitor to GND on either leg of the crystal? I'm using it as the clock source (TOSC) for an XMEGA and it's got a max ESR of 50 kOhm (which is within recommendations).

Best Answer

Check out http://en.wikipedia.org/wiki/Pierce_oscillator#Load_capacitance

The load capacitance is the total capacitance seen from the two pins of the crystal looking into the circuit. So if you have a 6 pF from each leg to ground, that's 3 pF total (capacitors in series halve, capacitors in parallel add).

It's really rated for 6 pF? That capacitance could be provided by the traces and PCB layout alone, I would think. Typically I've seen more like 30 pF.