AN1714 datasheet(28/46 Pages) STMICROELECTRONICS

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Demonstration board for ST7538Q

AN1714

28/46

to filter this supply pin adding an inductor (L8) in series to the capacitor (C16) or using a

specific EMC component (for example a TDK chip beads series MMZ1602C).

2.4

Crystal oscillator

The ST7538Q crystal oscillator circuitry is based on a MOS amplifier working in inverter

configuration.

This circuitry requires a crystal having a maximum load capacitance of 16 pF and a

maximum ESR of 40

Ω.

It is very important to keep the crystal oscillator and the load capacitors as close as possible

to the device.

The resonant circuit must be far away from noise sources such as:

●

Power supply circuitry

●

Burst and surge protections

●

Mains coupling circuits

●

Any PCB track or via carrying a signal

To properly shield and separate the oscillator section from the rest of the board, it is

recommended to use a ground plane, on both sides of the PCB, filling all the area below the

crystal oscillator and its load capacitors. No tracks or vias, except for the crystal

connections, should cross the ground plane.

It is also recommended to use a large clearance on the oscillator related tracks to minimize

humidity problems, see Figure 25.

Connecting the case to ground is also a good practice to reduce the effect of radiated

signals on the oscillator.

Figure 25.

A recommended oscillator section layout for noise shielding

It is possible to provide an external clock with the requested characteristics at the XOUT pin.

Probably in this case the global power consumption of the application will have a relevant

increase.

ST7538Q

SGND

XOUT

XIN

TOP Layer

BOTTOM Layer

Clearance

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