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EMC Basics #4: Analog devices as critical circuits

[Editor's note : we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. There is a link to previous entries at the end of this item.]

If a circuit board includes analog circuits, we also like to focus on those for an EMC board review. Similar to voltage regulators (discussed in the previous posting), analog circuits can also cause radiated emissions and susceptibility problems. Once again, the emissions are the result of parasitic oscillations, and the susceptibility the result of RFI (radio frequency interference.)

Analog circuits differ, however, from voltage regulators. In analog circuits, the RF susceptibility problems predominate. Parasitic oscillations (resulting in unwanted emissions) can occur, but are less likely. Since voltage regulators are already feedback-based devices, unwanted oscillations are more likely than with non-feedback based devices.

The RF susceptibility problems are typically due to rectification. A CW (continuous wave) RF threat can result in a DC offset, which can often be blocked with a capacitor. With a modulated RF threat, however, a demodulated signal is the result.  If the demodulation is within the expected signal passband, there is no way to filter it.

That is why modulation is used during most RFI tests. A 1000-Hz modulation is typical, but that may be further constrained depending on the equipment under test. For example, most medical devices require modulation within the passband of the physiological function to be measured (a few Hertz or less is typical.) The goal is to uncover unwanted effects in the presence of RFI.

Fortunately, the problems are easy to prevent at the circuit level. Start with high-frequency filtering on the analog circuit inputs. As a minimum, place small capacitors (100 to 1000 pF typical) placed directly across the component inputs to “short out” the RF energy. For additional protection, these capacitors can be augmented with series resistors or ferrites. The goal is to prevent the RF from reaching critical input circuits in the first place.

Due to circuit bandwidth, the allowable input filtering may be limited. Fortunately, most analog circuits operate at audio frequencies (or below), so the small amount of capacitance needed for RF protection usually does not affect normal operation. The key is to use enough filtering, but not too much. In some cases, even 10 pF across inputs is enough to protect against RF threats.

Added protection may be needed. Power decoupling should include high-frequency ceramic capacitors (1000 pF typical) in addition to larger low-frequency capacitors. These should be installed adjacent to the analog devices. In extreme cases, output filtering and even local shielding may be necessary for very sensitive devices (or very large threats.)

To recap , consider high-frequency protection for your analog circuits. Don't assume that just because they operate at low frequencies they won't be affected by high frequency (RF) threats.  With today's modern devices, sooner or later RFI problems will occur unless protection is included. 

Previous entries in the series

EMC Basics #1: Welcome!; and Clocks: critical circuits for EMC
EMC Basics #2: Resets as Critical Circuits
EMC Basics #3: Voltage regulators as critical circuits

About the author

Daryl Gerke , an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/.

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