Education Library

Frequently Asked Question

Is it possible to impedance match a clock output if it is heavily loaded? (e.g. CL=100pF)

In the classical (microwave) sense of the phrase "impedance match," the answer is no. Generally impedance matching is viable over relatively narrow bandwidths. The rich (odd) harmonic spectrum of a nearly ideal square wave, which is highly desirable in most clocking applications due to the high slew-rate edges, occupies a wide bandwidth. A narrowband impedance match would effectively filter the square wave, causing the final clock output to become more sinusoidal, and consequently reduce the slew-rate.

Heavy loads, such as CL=100pF, are best avoided. Devices with high input capacitance should be discarded and careful PCB layout is necessary to reduce board parasitics. Clock distribution parts such as the AD9510 can be used to drive several devices from a single reference, thereby solving the fan-out issue.