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Importance of PCB design
Among the customers who have adopted SiTime's MEMS oscillators, there must be some customers who were in charge of "PCB design".
“Oscillators”, like other semiconductor devices, must be properly “decoupled and bypassed” to take advantage of their characteristics.
In general, there is a way to place capacitors closer together.
These capacitors are essential for three important points:
1. Supply instantaneous current to the device!
2. Reduce noise on your system!
3. Release power noise to GND!
From here on, we will discuss the use of MEMS oscillators. guidelines and, Recommended layoutI will explain about.
A Guide to PCB Design for MEMS Oscillators
decoupling capacitor
The oscillator puts a fairly large load on the power supply.
In particular, it is difficult to quickly supply current from the power supply when the rise of the signal is steep at high frequencies.
A decoupling capacitor is required to supply sufficient current to the device.
For SiTime products, whether single or differential, it is recommended to use a 0.1 µF ceramic decoupling capacitor between the oscillator's VDD pin and GND.
The figure below is a layout example when a 0.6 x 0.3 size decoupling capacitor is attached to a 4-pin oscillator.
*1 Listed for each package
*2 Protect the wiring with solder resist.
*3 Note that pin 1 may be assigned a function depending on the oscillator used.
(enable, standby, spread spectrum disable, power control, etc.)
bypass capacitor
A decoupling capacitor of 0.1 μF is sufficient for bypassing SiTime MEMS oscillators for the purpose of system noise suppression.
No additional bypass capacitor is required. In addition, for differential oscillators that output high frequencies exceeding 150 MHz, reduce noise to the power supply.
Countermeasures are also effective. As an example, adding a 1nF or 10nF bypass capacitor may help. Please try it when you are in trouble.
Reduced power supply noise
By attaching a 0.1μF capacitor x 1 pcs between VDD and GND, power supply noise on VDD can escape to GND.
SiTime's MEMS oscillators have a built-in regulator, making them less susceptible to power supply noise.
Care should be taken in jitter sensitive systems such as telecom applications.
To further reduce the effect of power noise, it is also effective to use a power noise filter using RC or LC.
Recommended layout for MEMS oscillators
The details are defined by the manufacturer, but the general points are summarized below.
・ Place the decoupling capacitor between VDD and GND as close to VDD as possible (1 to 2mm)!
・ Place the oscillator as close as possible to the clock supply destination!
・ Keep the clock signal wiring short!
・ Do not place the clock signal near the edge of the PCB!
• Do not place VDD or other high-speed signal traces in the PCB area under the oscillator! (Highly recommended to place GND)
・ Do not use vias to route clock signals! (impedance changes, risk of reflection increases)
・ Do not place the clock signal on the VDD or GND layer of the PCB!
・ Do not bend the clock signal at right angles!
・ When wiring a differential clock signal, make sure that the pair lengths are the same!
At the end
What did you think.
This time, we have summarized the precautions and guidelines for PCB design of SiTime's MEMS oscillator.
Please feel free to contact us if you have any questions during the design process.