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OT: Crystal ground?

 
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MikeValencia



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OT: Crystal ground?
PostPosted: Mon Aug 29, 2005 5:37 pm     Reply with quote

I usually use thru-hole HC49US footprint crystals, which only have 2 legs for soldering.

I noticed that many surface mount crystals have the two pads for the crystal terminals, and two pads for ground.

Am I supposed to be grounding the metal housing of an HC49 then?

~~~~~~~~~~~~~~~

Also, has anyone come across an HC49 crystal that has a plastic insulator layer on the bottom? I'm finding that the layout guy ran traces beneath the crystal, and the only thing that separates them is the very thin film over the traces.
newguy



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PostPosted: Mon Aug 29, 2005 5:53 pm     Reply with quote

Most commercial products I've seen with the standard two-lead HC-49 crystals actually have them mounted on their side (tipped over). There is usually a ground plane under the package, with a separate bare wire, also grounded on both ends, wrapped over the crystal. Sort of like a tie-wrap.

Short answer: technically you should ground the case to avoid EMI.

Digikey carries crystal insulators. Search for "crystal insulator", 5 matches pop up.
Mark



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Re: OT: Crystal ground?
PostPosted: Mon Aug 29, 2005 7:33 pm     Reply with quote

MikeValencia wrote:
I usually use thru-hole HC49US footprint crystals, which only have 2 legs for soldering.

I noticed that many surface mount crystals have the two pads for the crystal terminals, and two pads for ground.

Am I supposed to be grounding the metal housing of an HC49 then?

~~~~~~~~~~~~~~~

Also, has anyone come across an HC49 crystal that has a plastic insulator layer on the bottom? I'm finding that the layout guy ran traces beneath the crystal, and the only thing that separates them is the very thin film over the traces.


That depends on the crystal. Many say NC or no connect. Yeah there's a pad there but you shouldn't ground it.
asmallri



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Re: OT: Crystal ground?
PostPosted: Mon Aug 29, 2005 9:53 pm     Reply with quote

MikeValencia wrote:
I noticed that many surface mount crystals have the two pads for the crystal terminals, and two pads for ground.

Am I supposed to be grounding the metal housing of an HC49 then?


The ground mounts also give the surface mounte crystals some mechanical strength that was lost in the move from through hole to surface mount.
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SherpaDoug



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Re: OT: Crystal ground?
PostPosted: Tue Aug 30, 2005 6:25 am     Reply with quote

Mark wrote:

That depends on the crystal. Many say NC or no connect. Yeah there's a pad there but you shouldn't ground it.


Actually it does no harm to ground the "ground" pads. For SMD packages they give mechanical support. For metal can packages a third connection is good mechanically, and adds some EMI protection if it is grounded.
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PostPosted: Tue Aug 30, 2005 6:57 am     Reply with quote

I follow the manufacturers datasheets. Here something from Epson

http://www.eea.epson.com/go/go/Resources/Catalogs/qd_e_0109.pdf

Look at page 7 of the catalog (14 of the pdf) and the installation example. See the note
Quote:
Direct soldering to the case may deteriorate the characteristics


That means I probably ought not do it.

The go look at page 15 (22 of pdf) in the figure, do you see the note
Quote:
Do not connect #2 and #3 to external device


Well one would have to argue whether or not ground should be included in this. I say you should ground it. No problem putting a pad there as the recommend that. I just don't think those pads should be grounded.

Still not convinced? Lets look at this datasheet for a DS1307

http://rocky.digikey.com/WebLib/Dallas/Dallas%20Web%20Data/DS1307.pdf

Take a look at the recommended layout for the crystal on page 3. Well I see a guard ring around the leads and a ground plane around the crystal but they don't show connecting it to ground.

Like I said, I like to follow the recommended layout since I am less likely to have problems or if I do, then I have a "scape goat". Does it hurt to ground them, probably not.
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