# How to calculate EMI filter (ferrite bead) specs for digital signal?

I have a small project of mine that deals with plain old VGA output (d-sub). In my project I'm using a CM2009 IC between actual VGA controller and VGA output connector. It's mentioned in the datasheet for this IC that I can use EMI filters on HSYNC, VSYNC and DDC channels right after IC and before VGA connector. Though there are no specs for these filters in the datasheet. It is even said that EMI filter configuration in the datasheet (ferrite bead between two capacitors) is given just as an example. I inspected some old videocards and found that most of them have some type of EMI filters on SYNC and DDC channels before VGA connector. Some have just a single ferrite bead as EMI filter, others have more complex EMI filter configuration including capacitors. I decided to use more simple variant of EMI filter in my design - just a single ferrite bead, without capacitors.

So how do I calculate precise specs for a single ferrite bead playing a role of EMI filter on SYNC and DDC VGA channels? The most importnat specs for ferrite bead (of course SMD one) that I need are impendance @ 100 MHz and current rating.

Also, maybe it would be better to use more complex EMI filter type that includes capacitors? How do I calculate capacitor specs in that case?

• The impedance at 100MHz spec is about as close to meaningless as they come. What are the rise times on the signal you're wanting to filter? – Matt Young Dec 8 '14 at 21:29
• Well I understand that impedance @ 100MHz spec is derived from actual impendance-frequency characteristic, but it's still one of the base specs fro ferrite beads. – jake.libber Dec 9 '14 at 7:27
• Rise time is about 12 ns for SYNC (as measured by some sources, while VGA standard says that it should be <10 ns) and <1000 ns for DDC (it uses I2C specifications). – jake.libber Dec 9 '14 at 8:02