Easy EMC
Don't Make Circuitry Noisier Than Needed
In the vast majority of cases, the following are recommended to minimize the inherent noise from circuitry or the susceptibility of circuitry to noise.
Of course, there are always exceptions. But here, we’re focusing on the actions to be used in the majority of situations.
Where it’s reasonable to do so, note will be made of exceptions. However, there will be no in-depth discussion of the unlikely cases.
- Use one solid return net. In a stackup using multiple return planes, the planes should all have the same net name with numerous vias connecting all the planes together.
- If you follow the first recommendation then you won’t encounter the noise problem created by running traces over split return planes.
- As previously mentioned; if you follow the first recommendation then you won’t encounter the noise problem created by placing an IC over a split return plane.
- Include sufficient bulk decoupling capacitors for the circuitry. Typically, design engineers have no problem determining the amount of bulk decoupling capacitance. But if there is a shortage of bulk capacitance, it will likely be discovered quickly as it will probably impact the functional performance of the circuitry.
- Decoupling (aka bypass) capacitors should be placed, 1 per IC power pin. A value of 0.1 uF is sufficient. Of course this assumes there is sufficient bulk capacitance as previously mentioned.
- Isolated Circuitry
- When using an isolated power supply there will inherently be an isolated return plane and a non-isolated return plane. There will be a device (often a transformer) straddling the gap in the return plane. Some of the device’s pins connect to the isolated plane and some connect to the non-isolated plane. Add a capacitor across the return plane split and right next to the device straddling the gap (often, a transformer). Even better is the addition of another capacitor on the opposite side of the device. The capacitor must be rated for the expected voltage across the gap and to meet any other requirements including safety related requirements.
- Continuing from the previous mention of return plane gaps caused by the requirement for isolated power supplies; signals that need to cross the gap should NOT be transported by traces across the gap. Instead, devices designed for isolated communications should be used. For example, optical isolators, transformer coupled or capacitor coupled devices are typical for such applications.
Filter
- Referenced here are both Differential Mode (DM) filters and Common Mode (CM) filters.
- Use common mode filters at power entry locations in most cases. However, at AC inputs use both DM and CM filters. A pre-packaged “purchased” filter is a good choice.
- At board-to-board connectors (and cables) use a differential mode filter. Specifically a shunt capacitor from V+ to RTN. A capacitance value of 0.1 uF is a good choice.
- Switch Mode Power Converters are a good place to use DM filters in a Pi configuration (shunt capacitor, series ferrite, shunt capacitor) on the input and output nets.
- Use common mode chokes on differential signals (NOT single-ended signals) that leave the circuit board (i.e. signals leaving the PCB on a cable).
- USB 2.0 is a good application for a Pi filter on the V+ line. The data pair should have a common mode choke of 90 ohms @ 100 MHz. The ferrite should be rated for a current 2x the maximum current through the ferrite.
- Circuit board mounted ferrites (1 line ferrite, not a common mode choke) have a current rating. Select a part rated at 2x the maximum current through the ferrite. Select a part with sufficient impedance (typically >300 ohms @ 100 MHz.
- Circuit board mounted ferrites (1 line ferrite, not a common mode choke) have a current rating. Select a part rated at 2x the maximum current through the ferrite. Select a part with sufficient impedance (typically >300 ohms @ 100 MHz.