Layout about the crystal

[will_fung]

Hi All,
I have a question about the layout of Crystal, do i need to add more GND vias on the yellow round mark to GND layer? or keep one via on the left side to GND layer? which one is better?
Thank you so much.

 

[andre_luis]

I see no specific reason for all this concern in spreading the ground plane to the opposite face so close to the crystal. One or other vias suffice.

 

[KlausST]

Hi,

Read the Xtal's datasheet or application note.

For usual Xtals I don't connect the unused pins to GND nor do I place a GND ring around the XTAL.
Mind: your layout increases stray capacitance, maybe you need to adjust the values of the capacitors.

Klaus

- - - Updated - - -

Added:

But usually I add a resistor in parallel to the XTAL. Leave it nc when not used. But it can help to ensure start up and improve start up time.

Klaus

 

[FvM]

But usually I add a resistor in parallel to the XTAL. Leave it nc when not used. But it can help to ensure start up and improve start up time.

A resistor is required by some crystal oscillators. If so it's specified in the IC data sheet. If not, it's useless, I think. I presume it's not required here.

For usual Xtals I don't connect the unused pins to GND nor do I place a GND ring around the XTAL.
Mind: your layout increases stray capacitance, maybe you need to adjust the values of the capacitors.

The pins are specified as ground pins for most 4-pin SMD packages and connected to metal cover. They should be connected to ground, but there's no particular need for more than one ground via.

 

[KlausST]

Hi,

The pins are specified as ground pins for most 4-pin SMD packages and connected to metal cover.

Interesting. After reading this I did check all of the datasheets of my XTALs in stock.
And indeed one type mentioned the connection to the metal cover in the text, but in the connection diagram they are marked as "nc".
Another datasheet clearly says: "Do not connect these pins to external device"....what ever this means.

Klaus

 

[FvM]

I see the ground connection e.g. for Abracon ABM3b, 8, 11 and 12 (ceramic cases 3.2x5 mm and smaller), also for many Epson crystals.

 

[c_mitra]

do i need to add more GND vias on the yellow round mark to GND layer?

In general, not needed. Crystals mechanical devices and electrically they act like inductor and capacitor combinations. They have high Q and low capacitance and in some applications you may need to reduce the Q and increase the capacitance before it will work. The positioning of the via is not critical.

 

[will_fung]

A resistor is required by some crystal oscillators. If so it's specified in the IC data sheet. If not, it's useless, I think. I presume it's not required here.
yes, from the datasheet and AN of the IC, the resistor is not needed.

The pins are specified as ground pins for most 4-pin SMD packages and connected to metal cover. They should be connected to ground, but there's no particular need for more than one ground via.

I'm using NDK crystal, in the datasheet, it specified other 2 pins are GND (connection cover).