USB251xB/xBi - What is the required drive level of the 24 MHz crystal?

For USB251xB/xBi, the drive level must be between 200 µW and 300 µW — the higher, the better. In order to obtain the best performance from a crystal oscillator, it is necessary to ensure that the crystal is driven at the correct level. If the drive level for the crystal is too high, then the parasitic resonances of the crystal may be excited. The crystal oscillator may even run at an incorrect frequency. Additionally, if the drive level is too high then the phase noise performance of the crystal oscillator will degrade.

The crystal can be damaged if the drive level is too high; in particular, the miniature types are susceptible to damage. Even if permanent damage is not incurred, the high level of drive within the crystal oscillator increases the rate of aging and can cause a frequency shift. It is important to ensure that the level of drive within the crystal oscillator circuit is approximately correct.

To ensure the correct operating conditions for the crystal oscillator itself, it is necessary to optimize the circuit for stability, gain, and drive level. Be sure that it is accurate on the crystal as well. The USB251xB/xBi requires a 24 MHz clock accuracy of better than +/- 350 PPM over the entire temperature range of your application.

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