I was checking out the TI E2E Analog forums this week and saw a number of EMI design based inquiries – it seems that a lot of us find ourselves struggling with the influence of EMI/RFI related issues. While we all know that it exists and can be a problem, we don’t always know how to recognize it or the best way to deal with it when it's there. One question in particular seemed to be – “how do I filter this?”
Not being an expert I decided to dig into the TI tech-day presentation “Tackling EMI/RFI at the Board and System Level” to find out (and to learn a little more about EMI/RFI design). The presentation was put together by Senior Applications Engineer Thomas Kuehl, and after spending a little time reviewing it I found out that there are three key elements for EMI response in your circuit system: 1) a source of EMI or RFI 2) a medium to couple electromagnetic interference to the circuit and 3) a sensitive receptor. The nice part of recognizing that it takes all three of these conditions to experience the trouble of EMI interference is that if you are able to remove any of them your EMI issues are removed as well.
The presentation and speaker notes explain some of the key sources of EMI (switching power supplies, clock, power mosfets to name a few…) and the ways that it can appear in the circuit for troubleshooting. To answer the question of ‘how do I design an EMI filter?’ there is a great set of example circuits from slides 19-23 that address AC line filtering, RC filtering and adding a common mode transformer for low frequencies as well as an explanation of some of TI’s newer operational and instrumentation amplifiers that have EMI filtering, like the OPA378 or INA333. The final few slides of the presentation cover techniques for minimizing EMI/RFI including the selection of passive component, layout techniques and balancing analog and digital design.
Overall, I found this presentation to be a great starting point for anyone is beginning their design and more importantly a great stopping point before committing to a layout.
After spending some time on the topic I happened to notice this little beauty on a colleagues desk - I'll be sure to send him to this presentation just in case there might be a few unseen RFI issues...