Filter Pins, Part 1 - John Dunn, Consultant, Ambertec, P.E., P.C.

The physical structure of a single pin of a filter pin connector is represented as follows as a ferrite bead over the center pin with shunt capacitors to the shell on each end:    

 

At the time I was looking at these things, the shunt capacitances to the outer shell were each either 200 pF (the high frequency filters) or 4000 pF (the low frequency filters).

Using recursive differential equations, we studied equivalent circuits of these filter pins for responses to ESD discharges per the "human body model":  

 
We got these results:     

 
The peak voltage excursions for the low frequency filter pins looked good with no alarmingly high voltage excursions, but it was a little bit surprising to note a low amplitude ringing waveform for e1-e2 at approximately 36.6 MHz.

Our particular system was in no jeopardy from this ringing, but other equipment using these filter pins might have EMI susceptibility issues at or around that frequency.

Perhaps this observation should simply be taken as a cautionary tale in the use of these things.

Comments

[Joel Koltner]

John -- Out of curiosity, do you find an advantages to "rolling your own" simulations (as described here and in other posts of yours) over just using SPICE?

Thanks...

[John Dunn]

Hi, Joel.

Actually, yes I often do. I have total flexibility in preparing my results for presentation and sometimes I find SPICE to be somewhat cumbersone. For instance, I can generate arbitrary functions more quickly using just an equation in a single line of code, I can derive odd-ball results with just one or two lines of code as with e1-e2 above, I can make arbitrary parametric value changes happen during a simulation and so on. These things can be done in SPICE too, but I often prefer to go home-brew.

[Tom Terlizzi 10-19-2011]

John,
Sometimes you think you are fixing one problem but cause another one.
As always great information. I am performing a failure analysis on an ESD overstress on some thin film resistor chips in an input circuit.

Had a few questions.
How did you come up with the value of L1 & C1/C2?
Do the result change if you use a distributed LC model for the filter pin connector?
Does the response change for different load resistances or are you assuming the load takes time to respond?
thanks
T2

[John Dunn]

Hi, Tom.

The shunt capacitance were catalog specifications for the filter pins. The L and R1 values came from discussions with the manufacturer's engineer. I moved these two values around a little just to see what would happen, but nothing fundamentally changed.

The value of R2 was actually entered as 1E12 ohms, I think, close enough to qualify as an open circuit but numerically tractable. Making it smaller would have diminished the voltage excursions whose maximum values I was looking to establish as part of a worst-case analysis.