When the CDCEx937-Q1 device is used as a crystal buffer, any parasitics across the crystal affect the pulling range of the VCXO. Thus, place the crystal units on the board. Crystals must be placed as close to the device as possible, verifying that the routing lines from the crystal terminals to Xin and Xout have the same length.

If possible, cut out both ground plane and power plane under the area where the crystal and the routing to the device are placed. In this area, always avoid routing any other signal line, as the signal line can be a source of noise coupling.

Additional discrete capacitors can be required to meet the load capacitance specification of certain crystals. For example, a 10.7pF load capacitor is not fully programmable on the chip, because the internal capacitor can range from 0pF to 20pF with steps of 1pF. Therefore, the 0.7pF capacitor can be discretely added on top of an internal 10pF.

To minimize the inductive influence of the trace, TI recommends placing this small capacitor as close to the device as possible, and symmetrically with respect to Xin and Xout.

Figure 10-10 shows a conceptual layout detailing recommended placement of power-supply bypass capacitors. For component-side mounting, use 0402 body-size capacitors to facilitate signal routing. Keep the connections between the bypass capacitors and the power supply on the device as short as possible. Ground the other side of the capacitor using a low-impedance connection to the ground plane.