12.1 Layout Guidelines

When the CDCEx937 is used as a crystal buffer, any parasitics across the crystal affects the pulling range of the VCXO. Therefore, take care in placing the crystal units on the board. Crystals must be placed as close to the device as possible, ensuring 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 it could be a source of noise coupling.

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

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 24 shows a conceptual layout detailing recommended placement of power supply bypass capacitors on the basis of CDCEx937. 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.

12.2 Layout Example

Figure 24. Annotated Layout