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Designing a EMC Compatible Electronic Meter using AD7755

Designing a EMC Compatible Electronic Meter using AD7755. a. Design For Immunity to Electromagnetic Disturbances.

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Designing a EMC Compatible Electronic Meter using AD7755

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  1. Designing a EMC Compatible Electronic Meter using AD7755 a

  2. Design For Immunity to Electromagnetic Disturbances "the meter shall be designed in such a way that conducted or radiated electromagnetic disturbances as well as electrostatic discharge do not damage nor substantially influence the meter” -Section 4.5 IEC1036

  3. Movement of Electrons Energy Meter SOURCE PATH RECEPTOR What is EMC?Source-Path-Receptor Model • EMC Stands for Electro-Magnetic Compatibility. • The interference generated by the movement of electrons will travel through a path and result in the energy meter. Common coupling effects: • Frequency less than 30MHz (e.g. EFT) – Conduction • Frequency between 30MHz to 300 MHz (e.g. RFI) – Cable Radiation • Frequency larger than 300MHz (e.g. ESD, RFI) - Radiation

  4. The AD7755 has passed 60 years life test EMC Requirement on IEC1036 IEC1036 specifies that the energy meter must be able to withstand: • Electrostatic Discharge (ESD) up to 8kV • Radio Frequency Interference (RFI) up to 10V/m near field • Electrical Fast Transient and Surge (EFT) up to 4kV The AD7755, core of the energy meter, offers some protection against the above interferences. However, the energy meter circuit and PCB layout should be designed to divert the interference away from the AD7755 to ensure the expected lifespan of the energy meter is not affected.

  5. 101011... 16 bit ADC Digital-To-Frequency Multiply & LPF D.C. to 5KHz 16 bit ADC Serial Port Interface D.C. to 5KHz 111001... ADE775x Family Architecture High Resolution Data Conversion Fixed Function DSP

  6. a AD7755 EMC Certified Electronic Energy Meterbased on AD7755 The fixed function DSP in the AD7755 is highly effective against radiated EMC disturbances, however, external protection is essential to minimize the noise coupling through conduction.

  7. Implementation: PCB Layout

  8. PCB Layout Layout considerations - Analog Performance • Separate grounds were used to isolate the AD7755 and low level analog signals from the rest of the design • Ground plane should be made as large as possible to keep the common ground impedance to a minimum • Care was taken with the routing of the ground return current from the capacitor based PSU.

  9. PCB Layout Ground Plane helps reduce the impedance of the current path in an ESD/EFT event and reduce interference picks up through radiation.

  10. PCB Layout Layout considerations - EMC • Separate grounds were used to isolate the AD7755 from electromagnetic disturbances. • Power and ground connections are made through ferrite bead.

  11. PCB Layout: Some key areas Large analog ground plane (quiet) • Large digital/power supply ground plane (noisy) Weak differential signal lines are shielded by ground fill and are parallel and closed to each other Ground shielding helps separate strong voltage signal from affecting weak current signal

  12. Implementation: Circuit Design

  13. Inductance of the Shunt Shunt inductance can create a high impedance path at high frequency: ESD/EFT and RFI picked up through the cable will travel to the AD7755’s signal inputs V1P and V1N. Protection is needed!

  14. Passed 25kV Contact discharge Specification requires for 8kV Design For Immunity to Electromagnetic Disturbances ESD (IEC1000-4-2) Ferrite RC FilterR > 1kW Attenuation networkR > 1MW EMI Filter on PSU

  15. Passed 5kV @ 15kHz Specification requires for 4kV @ 5kHz Design For Immunity to Electromagnetic Disturbances EFT (IEC1000-4-4) Ferrite & RC slows down edge. EFT appears as CM signal PCB layout isolatesAD7755 from EFT in the signal ground Ferrite & C slows down edge. MOV suppresses large differential EFT

  16. Passed 20V/m near field Specification calls for 10V/m Design For Immunity to Electromagnetic Disturbances HF Field (IEC1000-4-3) Ferrite & RC make up effective HF filter PCB layout/grounding EMI Filter on PSUshunting cap acts as bypass for RF

  17. Conclusion • Good EMC design is essential to help ensure the lifespan and accuracy of a solid-state energy meter. • Large ground plane, short analog signal lines help greatly reduce EMC problems. • Components’ parasitic characteristics will usually dominant in a EMC situation. • EMC Protection does not have to come with additional cost to a meter if the right component is used. For example, RC used in anti-aliasing filter can be effective EMC filter. • Mechanical meter is also susceptible to EMC issues.

  18. EMC Can Be Achieved with Care in Circuit Design and Layout Emissions Immunity

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