EMC and EMI in products

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I would love to see this thread moved to the Drawing Board so we can have a deeper discussion...

If ever a link posed food for thought, this is it.


Justin
 
What I'm lookign for is a standard EMI filter that i could put on the front of a mic-pre (or a mic-front-end-circuit). However, Ferrite beads may not be the answer as I was told that:

Ferrite beads may also help, but will need to be able to handle the DC current (to avoid saturation, which will null the high frequency rejection characteristics of the core material).

I assume the DC current here is the PHantom power leaving the mic pre (and destined) for the micophone. That's around 4mA, no?

Also... it might be worht checking a few of these eLab presentations.
http://focus.ti.com/general/docs/gencontent.tsp?contentId=32276#03/08/2006
 
[quote author="Rochey"]
Ferrite beads may also help, but will need to be able to handle the DC current (to avoid saturation, which will null the high frequency rejection characteristics of the core material).
I assume the DC current here is the PHantom power leaving the mic pre (and destined) for the micophone. That's around 4mA, no?[/quote]
10mA max total, according to the spec; worst-case shorted for P48 is 48V/6k81 = 7mA per leg. The smallest ferrite chip bead I could find on Digi-Key's site is rated for 50mA, and that's for a 0201 part. Planning to hand-solder many of those?

Although I suppose you could saturate a ferrite toroid with P48 if you wind quite a few loops through it, I don't think that you need to worry that much if you just plan to run a wire through a ferrite bead.

JDB.
 
I second this--highly unlikely to saturate any ferrite bead with phantom power. On the other hand, you'll need to have quite a good idea about the inductivity of your ferrite bead and the resulting Q of the low-pass filter formed with the following capacitors, especially as source impedance can be dramatically lower than standard (20 ohm for a Schoeps) and contain itself an inductive component. If you've got an unhappy combination, serious peaking will result. Personally I'd either just use capacitors (for most applications that should be enough protection) or use a common mode chocke (so most--though not all--of the peaking is common mode). You might check some of my mic preamp designs for inspiration, e.g. A_r1.pdf.

Samuel
 
Don't you need a 'common mode choke' in this application? Somebody must make a standard product. I've got a balanced line project that needs one. Anybody have a part number handy?
 

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