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I have a circuit diagram that I am trying to replicate (actually the Teensy 3.0/3.1 circuit diagram shown here https://www.pjrc.com/teensy/schematic.html) and there is a component called simply 'ferrite'. On the board it is a SMB component, maybe in a 1206 package but with no markings.

Is this a common component such that there is a 'standard' value for it? (e.g. 'pullup resistors' are usually around 10kOhm or specified according to the internal resistance of the driving pin)

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    \$\begingroup\$ I looked at the schematic. Just put in 0-Ohm resistors. If you have problems you can experiment with ferrites later. In particular, the ferrite connecting analog GND (VSSA and VREFL) to GND is ill-advised. I would suggest you omit that one altogether and merge analog GND to GND, especially if the board will have a GND plane. \$\endgroup\$ – mkeith Jan 2 '16 at 17:56
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No, there isn't. Each ferrite has its own characteristics and curves, and selecting the correct ferrite involves examining them and seeing how they interact with the system. Sure, there are a wide range of things you want to filter across a large number of circuits, but a ferrite large enough to do so is too large to belong on most PCBs (which is usually why they're on the cables leading in and out instead).

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  • \$\begingroup\$ Thanks - I thought I might be missing a bit of insider knowledge, after a bit more searching I found the values buried in a forum post (600Ohm/100MHz) \$\endgroup\$ – Brendan Jan 2 '16 at 18:02
  • \$\begingroup\$ @Brendan That 600Ohm/100MHz figure is really pretty meaningless. To understand whether or no the device will help, you really need to look at the datasheet and see where impedance becomes more resistive than reactive. Ferrite selection is worth a question in itself. \$\endgroup\$ – Matt Young Jan 2 '16 at 18:27
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In the schematic, all the ferrite beads that you mentioned are being shown on power lines. Basically, these are acting as low pass filters for your power lines and there can be many values that will satisfy your needs. Probably the circuit will work even without them.

However while selecting these filter ferrite beads, you need to keep one thing in mind. The frequencies that you want to block (noise frequencies) will be spread over a wide range and there is a special class of inductors/ferrite beads for that purpose - Low Q type.

Here is an excerpt from app note: AN 583: Designing Power Isolation Filters with Ferrite Beads for Altera FPGAs

beads

You can choose a proper Low-Q bead that will eliminate the frequency range that you want to block.

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  • \$\begingroup\$ Most ferrites are fairly high Q at lower frequencies and transition to high Q at higher frequencies. You can tell by examining the typical impedance curve. \$\endgroup\$ – mkeith Jan 4 '16 at 4:36

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