Hi! I'm currently designing a device around the AD9958 involving the internal oscillator. I was somewhat stunned by the unusually high recommended capacitor values (39pF versus the usual 18-22pF). If the crystal manufacturer recommends 20pF, which would be the right value to use? Or would I have to look out for crystals that are explicitly specified for higher capacitances? Sorry for the noob-ish question; I just want to avoid potential sources of error for such a delicate circuit.

Thanks,

M

Hi Matthias,

The important thing is to match the CL(load capacitance) specification of the XTAL to it's load. In this case, the recommended value for the shunt capacitance is 39 pF. The formula for this is CL = (C1*C2)/(C1 + C2) + CS; where C1 and C2 are the shunt capacitance (both 39 pF) while CS is the board capacitance which is usually 2 - 5 pF(we'll use 2 pF). So using the suggested value, we get CL which is approximately 21.5 pF. This means that you can use this configuration if you use an XTAL with a CL specification of 21.5 pF. In your case, the typical load capacitance is 20 pF which is very close to 21.5 pF, with this criteria, you can tell that the crystal you chose is usable in the configuration specified in the datasheet.

Thanks,

Louijie