Re: [SI-LIST] : Capacitor dielectric type

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From: Dennis Schmitz (den@positron.net)
Date: Wed Dec 06 2000 - 12:51:32 PST


The problem was that it coupled to the signal ground plane, which wouldn't ordinarily be a problem except for that single-ended signals used for static stuff like board insert detection were picking it up as well, causing false triggers.

I'm convinced we have to detune the resonance to a lower frequency so that the ping is broader and of a lower amplitude. Pre-charging the planes of the boards being inserted (to eliminate the excitation event) also seems prudent, but I wouldn't want to rely on that for system integrity.

I was hoping for some solid research references to bear this out.

  ----- Original Message -----
  From: Mark Gill
  To: 'Michael Nudelman'
  Cc: 'Dennis Schmitz' ; si-list
  Sent: Wednesday, December 06, 2000 2:10 PM
  Subject: RE: [SI-LIST] : Capacitor dielectric type

  Hey Mike -

  Decoupling the backplane is not required if the boards are correctly filtered/decoupled. None of the systems I have worked on require decaps on the backplane (reliability problem), and none of them have ever exhibited this type of problem. But we do have good filtering/decoupling on the -48 V input to each card. Smart powering (which Joseph mentions) is a slicker approach, and very reasonable when considering switching power systems. But depending on the actual parasitics (inductances, resistances, and capacitances), you certainly can get high frequency ringing upon insertion - try simulating with SPICE to get a feel. The toughest parasitic should be the inductance. There are some ferrites which offer good resistive losses at higher frequencies. I would still call this an inrush phenomenon.

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