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CDCM61001 Impact cristal with ESR 80Ohm

yannick
Prodigy 10 points
Other Parts Discussed in Thread: CDCM61001

Hi,

I made a mistake in the design of my board by using  a crystal osccillator with an ESR of 80Ohm max instead of 50 as specified in the datasheet of CDCM61001.

What would be the impact if the ESR is out of spec ?

Thanks,

Yannick

  • 0
    TI__Expert 4150 points

    Hi Yannick,

    you might possibly find the XTAL not starting up at all. HOwever, we find that XTAL manufacturer put a very big guardband around their spec, meaning when they spec 80 Ohm, most likely their real ESR will be a lot lower than 50 Ohm. I recommend you to have one of your boards tested for negative resistance, and check that you have sufficient negative resistance. Your XTAL vendor should be able to help you with this. Also, be sure to delay the PowerDown pin sufficiently long, as a higher ESR could also cause a bit longer startup time I suppose.

     

    Best regards. Falk Alicke

  • 0 in reply to Fritz
    Prodigy 130 points

    Yes ESR is always given as worst case max, which is more important at lower freq. since reactance of xtal is lower. and Q is related to Leff / Resr of Xtal.

    Since 30MHz is considered high for a fundamental Xtal, It should be ok. Overtone Xtals have even higher ESR.  Normally it will oscillate with extra gain margin in chip.   It is always advisable to test many chips min max Vdd and Temp for margin. Adding a 25ohm series R in test cct will confirm you have sufficient margin for device variation. Then verify V+/-, Temp+/- tolerances. with spectrum analyzer for spurious outputs or phase noise.

    high Q will take longer to startup and marginal gain will take long to start up.

    lower Q may have more phase noise. higher ESR reduces Q in series mode but may be acceptable for your application.

    Tony in Toronto