Do I need to ground a 4 pad crystal?

Discussion in 'General Electronics Chat' started by Tim Fernandez-Hart, Apr 18, 2019.

  1. Tim Fernandez-Hart

    Thread Starter New Member

    Nov 30, 2018
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    I used a crystal in my design from the SparkFun library without properly checking it. The schematic has two wires, but the footprint has 4 pads.

    Unfortunately I have already had my PCB manufactured and only two of these pads are connected to my IC, so I'm hoping that this crystal will do the trick:
    https://abracon.com/Resonators/abm3b.pdf

    However, the datasheet says that the two pins that are not connected to the crystal should be grounded. This is going to be tricky unless I start hacking at my PCB's with a knife to fudge some sort of connection to a ground plane.

    Old crystals had only two pins by design. What will happen if I don't connect the two ground pins for this crystal?
     
  2. Wolframore

    Well-Known Member

    Jan 21, 2019
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    You should be ok most crystals work fine with 2 connections... it may be case grounding or just extra pads to strengthen the hold... have you tried it yet?
     
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  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    The grounding pads, other than also strengthening the hold as wolframore's just said, connect the crystal's casing to ground so as to turn it into a shield against EMI, increasing its reliability.
     
  4. Wolframore

    Well-Known Member

    Jan 21, 2019
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    absolutely... that's what case grounding is for... also blocks unintentional RF emissions
     
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  5. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Of course, if the item is indeed only a crystal AND if you have the correct pads connected, no problem. BUT if the device is a crystal oscillator module then you also need a power supply connection. So the concern is which do you have?
     
  6. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    From the information he posted, it's a crystal. I would properly install it, or use a different crystal.
     
  7. Tim Fernandez-Hart

    Thread Starter New Member

    Nov 30, 2018
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    Well I am in the midst of a steep learning curve (the most productive kind) but thankfully the schematic is for just a crystal.

    I didn't thoroughly check the design before buying, and I just jumped in and bought a crystal oscillator which required a VCC and GND. Reading that datasheet made me realise my mistake. I now know the difference between an oscillator, and a plain crystal, and the different form factors that these may come in.

    In answer to Wolframore, no I haven't tested my plain crystal yet, its in the post and should arrive next week. I'm hopeful that it just works.
    Will I need to shield against EMI? Could I do that with the whole case? i.e. could I solder a wire to it and attach that to a GND via? This is a DSP board for audio based on the ADAU1701 from analogue devices.

    Thank you to everyone who has replied.

    Tim
     
  8. Wolframore

    Well-Known Member

    Jan 21, 2019
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    Many folks fix their first version PCB using wires. Grounding it with a wire should work. Try without it first. It might be fine depending on what the end use is and what other components are around it.

    Example if your crystal is a fundamental (or harmonic) freq for a RF circuit it may have an effect... it really depends... reminds me of the discussions about superheterodyne mixing in RF class... it can get complex. The grounded case avoids much of that. In your situation it might not matter.
     
  9. Tim Fernandez-Hart

    Thread Starter New Member

    Nov 30, 2018
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    Thanks Wolframore. Just so its clear in my mind, when you say "grounding it with a wire should work" your suggesting I solder a wire to the crystal case and attach that to a GND somewhere?

    I will try without it first, with my fingers crossed.
     
  10. Wolframore

    Well-Known Member

    Jan 21, 2019
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    that is correct - repairs happen like this:
    upload_2019-4-18_15-44-7.png

    This guy forgot to run traces for SPI - my OCD is bothered by the components not being very straight.

    upload_2019-4-18_15-50-39.png

    and yet more boo boos - eek there's way too much solder for my liking...
    upload_2019-4-18_15-52-1.png
     
    Last edited: Apr 18, 2019
  11. Tim Fernandez-Hart

    Thread Starter New Member

    Nov 30, 2018
    12
    1
    Fantastic!

    So I'll hope for the best, but be ready to fudge as required.

    Many thanks to everyone who has taken the time to reply. Especially Wolframore. Those pictures are great!

    Have a good easter everyone.

    Tim
     
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  12. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    I have seen circuit boards with a wire soldered across the crystal to hold it to the board. But those were through-hole mount crystals with wire leads through holes. And years ago I did buy some oscillators, only to discover that they were VERY small and so I had to add wires to all 4 holes to mount them But it worked perfectly and the customer was totally pleased and they are still working now ten years later.
     
  13. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    And if needed and you’ve soldered a ground wire, a couple drops of epoxy under the unsoldered pads can help stabilize the crystal case.
     
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