Designing a EMC Compatible Electronic Meter using AD7755

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Designing a EMC Compatible Electronic Meter using AD7755

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Design For Immunity to Design For Immunity to Electromagnetic Disturbances 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


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.

Movement of Electrons

RECEPTOR

Energy Meter

SOURCE PATH

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


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.

The AD7755 has passed 60 years life test


ADE775x Family Architecture

High Resolution Data High Resolution Data ConversionConversion Fixed Function DSPFixed Function DSP

101011...

111001...

D.C. to 5KHz

D.C. to 5KHz

16 bitADC

16 bitADC

16 bitADC

16 bitADC

Multiply

&

LPF

Digital-To-Frequency

Serial Port

Interface


EMC Certified Electronic Energy Meterbased on AD7755

The fixed function DSP in the The fixed function DSP in the AD7755 is highly effective AD7755 is highly effective against radiated EMC against radiated EMC disturbances, however, disturbances, however, external protection is external protection is essential to minimize the essential to minimize the noise coupling through noise coupling through conduction.conduction.

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AD7755


Implementation: PCB LayoutImplementation: PCB Layout


PCB Layout PCB Layout

Layout considerations - Analog Performance 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.



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



Layout considerations - EMC Layout considerations - EMC

Separate grounds were used to isolate the AD7755 from electromagnetic disturbances.

Power and ground connections are made through ferrite bead.


PCB Layout: Some key areas

Large digital/power supply ground plane (noisy)

Ground shielding helps separate strong voltage signal from affecting weak current signal

Weak differential signal lines are shielded by ground fill and are parallel and closed to each other

Large analog ground plane (quiet)


Implementation: Circuit DesignImplementation: Circuit Design


Inductance of the ShuntInductance 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!



ESD (IEC1000-4-2) ESD (IEC1000-4-2)

Ferrite

RC Filter

R > 1k

Attenuation network

R > 1M

EMI Filter on

PSU

Passed 25kV Contact discharge Specification requires for 8kV



EFT (IEC1000-4-4) EFT (IEC1000-4-4)

Ferrite & RC slows down edge. EFT appears as CM signal

Ferrite & C slows down edge. MOV suppresses large differential EFT

PCB layout isolatesAD7755 from EFT in the signal ground

Passed 5kV @ 15kHzSpecification requires for 4kV @ 5kHz



HF Field (IEC1000-4-3) HF Field (IEC1000-4-3)

EMI Filter on PSUshunting cap acts

as bypass for RF

Ferrite & RC make up effective HF

filter PCB layout/grounding

Passed 20V/m near fieldSpecification calls for 10V/m


Conclusion 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.


EMC Can Be Achieved with Care in EMC Can Be Achieved with Care in Circuit Design and LayoutCircuit Design and Layout

Emissions Emissions

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Designing a EMC Compatible Electronic Meter using AD7755