SNAS252S October 2005 – December 2014 LMX2531
For the layout of the LMX2531, perhaps the most important factor is to be aware of the package footprint. The asymmetrical land pattern can cause issues if not correctly done.
These pins are inputs to voltage regulators. Because the LMX2531 contains internal regulators, the power supply noise rejection is very good and capacitors at this pin are not critical. An RC filter can be used to reduce supply noise, but if the capacitor is too large and is placed too close to these pins, they can sometimes cause phase noise degradation in the 100 — 300 kHz offset range. Recommended values are from open to 1 μF. The 10 Ω series resistors serve to filter power supply noise and isolate these pins from large capacitances.
If the VrefVCO capacitor is changed, it is recommended to keep this capacitor between 1/100 and 1/1000 of the value of the VregVCO capacitor.
Because this pin is the output of a regulator, there are stability concerns if there is not sufficient series resistance. For ceramic capacitors, the ESR (Equivalent Series Resistance) is too low, and it is recommended that a series resistance of 1 — 3.3 Ω is necessary. If there is insufficient ESR, then there may be degradation in the phase noise, especially in the 100 — 300 kHz offset. Recommended values are from 1 μF to 10 μF.
The choice of the capacitor value at this pin involves a trade-off between integer spurs and phase noise in the 100 — 300 kHz offset range. Using a series resistor of about 220 mΩ in series with a capacitance that has an impedance of about 150 mΩ at the phase detector frequency seems to give an optimal trade-off. For instance, if the phase detector frequency is 2.5 MHz, then make this series capacitor 470 nF. If the phase detector frequency is 10 MHz, make this capacitance about 100 nF.