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MikeValencia
Joined: 04 Aug 2004
Posts: 238
Location: Chicago
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| OT: Crystal ground? |
 Posted: Mon Aug 29, 2005 5:37 pm |
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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?
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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. |
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newguy
Joined: 24 Jun 2004
Posts: 1907
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| Posted: Mon Aug 29, 2005 5:53 pm |
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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. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Re: OT: Crystal ground? |
| Posted: Mon Aug 29, 2005 7:33 pm |
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| 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. |
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asmallri
Joined: 12 Aug 2004
Posts: 1634
Location: Perth, Australia
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| Re: OT: Crystal ground? |
| Posted: Mon Aug 29, 2005 9:53 pm |
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| 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?
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The ground mounts also give the surface mounte crystals some mechanical strength that was lost in the move from through hole to surface mount.
_________________
Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Re: OT: Crystal ground? |
| Posted: Tue Aug 30, 2005 6:25 am |
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| 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.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Tue Aug 30, 2005 6:57 am |
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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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