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Are the 27ohm resistors and 47pF capacitors really really necessary? — Parallax Forums

Are the 27ohm resistors and 47pF capacitors really really necessary?

william chanwilliam chan Posts: 1,326
edited 2014-09-10 11:37 in General Discussion
I am referring to the components usually placed near the USB connectors of course.
I am running out of PCB space and are wondering if I can "get away with it" by excluding these 4 components.

Of course everyone knows the safest answer would be "They Are Required", but what is the not-so-safe answer?

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-06-03 03:05
    The context of each component is important to deciding to eliminate, and how to eliminate.
  • tonyp12tonyp12 Posts: 1,951
    edited 2014-06-03 11:42
    I guess you are referring to fdti default schematic.
    I attached it after I modified for better visual.
    The 47pf caps are there to filter out noise on the line, and the 27ohm resistors will help with signal bounce and impedance of trace/wire.

    The 47pf caps and also the 10nF cap and ferrite-bead I would eliminate first.
    FT230X.jpg
    731 x 422 - 44K
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-06-03 12:48
    William, I hope your customers don't look at this forum when you come here asking how you can cut corners in a design. Here are a couple DigiKey part numbers that will help you fit everything on your board without compromising performance:

    -Phil
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2014-06-03 14:37
    Phil Pilgrim (PhiPi) wrote: »
    William, I hope your customers don't look at this forum when you come here asking how you can cut corners in a design. Here are a couple DigiKey part numbers that will help you fit everything on your board without compromising performance:

    -Phil

    Some EE at some point made a choice to put these in there for stability in some for or another .

    I would not cut out any parts that are for things of time sensitive nature . Be it RF or digital at more then a 100 k Baud.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2014-06-03 15:05
    "Some EE at some point made a choice to put these in there...." That's kind of a busting rocks with hammer attitude....

    The resistor/capacitor form a low pass filter with a cut-off near 125MHz ... the exact reason for targeting 125MHz I am not exactly clear on except it's well below any PC or computer interference that might be likely to potentially cause problems.
  • Steve AndruesSteve Andrues Posts: 29
    edited 2014-06-03 16:55
    The 125 MHz frequency is right in the middle of the aviation voice communication band, for what it's worth. Everything should work fine as long as you cannot see an aircraft.
  • oPossumoPossum Posts: 3
    edited 2014-06-03 18:17
    The impedance matching will be off without the resistors and it will be unreliable. It may work for you in a simple test, but others will have problems. The caps are less important, but unwise to omit them.
  • TubularTubular Posts: 4,702
    edited 2014-06-03 20:47
    Phil Pilgrim (PhiPi) wrote: »
    William, I hope your customers don't look at this forum when you come here asking how you can cut corners in a design. Here are a couple DigiKey part numbers that will help you fit everything on your board without compromising performance:
    -Phil

    You can even get a combination in a single package, eg
    http://www.digikey.com/product-detail/en/EZA-DLU01AAJ/PDLU01CT-ND/385399
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2014-06-03 21:49
    Might I suggest some night time lite reading on USB http://www.usb.org/developers/docs/whitepapers/

    Loads of info on USB impedance and all its qwerks .
  • tonyp12tonyp12 Posts: 1,951
    edited 2014-06-04 07:57
    Here are other all-in-one's, some with ESD protection:
    http://www.mouser.com/Passive-Components/EMI-RFI-Components/EMI-Filter-Beads-Chips-Arrays/_/N-bw7paZscv7?Keyword=USB&Ns=Pricing|0
    If they have built-in 15K Ohm pull-down resistor, then it's for downstream (eg this chip goes in your computer) and is not what you're looking for.
    You want a upstream without built-in 1.5K pull-up resistor , as the fdti (and all other) have this resistor in their IC's nowadays.

    This one seems really good at 20cents with 0.5mm pin-spacing small but not to small.
    NUF2042XV6T1G
    http://www.mouser.com/ProductDetail/ON-Semiconductor/NUF2042XV6T1G/?qs=%2fha2pyFaduhA8A9SoVEwnuNfvXo%2fHfo948NpzPQqNL0%3d
  • TubularTubular Posts: 4,702
    edited 2014-06-04 17:11
    Those look very nice, thanks tonyp12
  • bill190bill190 Posts: 769
    edited 2014-06-05 08:36
    Everything is there for a reason!

    I've worked with engineers in the past who have spent weeks or months testing and tracking down intermittent problems with electronics. Then the fix to those problems many times is an extra resistor here or an extra capacitor there. Then end of problem - the customer is happy with the "trouble free" gizmo.

    They are happy many months and many years after the sale. When they purchase something again, they will go back to your company because your products are trouble free.

    On the other hand, people who buy products which have intermittent problems each time you run the vacuum cleaner, turn on the blender, or a police officer nearby uses his radio, will return those products to the store and NEVER buy another product from your company again!

    Having satisfied customers is a good thing for the future of a company!
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-06-05 08:53
    From what I gather, many times the actual inclusion of capacitors, resistors, and coils as filters can only be adequately determined after a printed circuit board is made and populated. The schematic may show exactly what works with that particular board, but when you create your own design based on the schematic.. such values might require re-evaluation.

    The need for filtering might in some cases be due to quirks of how the board routed its wiring. And one engineer might prefer items such as ferrite beads, while another prefers coils. There are some real hazards with presuming you can download a schematic from the internet provided by the manufacturer and expecting to always get a perfect solution for a small run of boards for personal use.
  • denizdeniz Posts: 1
    edited 2014-09-10 11:37
    william chan, the not-so-safe answer is not, they are not. But considering the fact that is a not-so-safe answer, I assume you won’t go ahead with it. As you start operating on higher frequencies, these capacitors and resistors form certain filters that are sometimes absolutely necessary. So you can only draw a conclusion here after studying your application and whether or not you’re operating at a frequency where this becomes absolutely necessary.
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