Transcript High Resolution Data Conversion Fixed Function DSP 101011

a
Designing a EMC Compatible
Electronic Meter using
AD7755
http://www.analog.com/energymeter
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
http://www.analog.com/energymeter
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
SOURCE
Energy Meter
PATH
RECEPTOR
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
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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
http://www.analog.com/energymeter
ADE775x Family Architecture
High Resolution Data
Conversion
D.C. to
5KHz
D.C. to
5KHz
16 bit
ADC
Fixed Function DSP
101011...
Multiply
Digital-ToFrequency
&
16 bit
ADC
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LPF
111001...
Serial
Port
Interface
EMC Certified Electronic Energy Meter
based on AD7755
a
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.
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Implementation: PCB Layout
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PCB Layout
Layout considerations - Analog Performance
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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.
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PCB Layout
Ground Plane helps reduce the impedance of the current path in an
ESD/EFT event and reduce interference picks up through radiation.
http://www.analog.com/energymeter
PCB Layout
Layout considerations - EMC
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Separate grounds were used to
isolate the AD7755 from
electromagnetic disturbances.
Power and ground connections
are made through ferrite bead.
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PCB Layout: Some key areas
Large analog
ground plane (quiet)
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
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Large digital/power
supply ground plane
(noisy)
Implementation: Circuit Design
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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!
http://www.analog.com/energymeter
Design For Immunity to
Electromagnetic Disturbances
ESD (IEC1000-42)
Ferrite
RC Filter
R > 1kW
Attenuation network
R > 1MW
EMI Filter on
PSU
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Passed 25kV Contact discharge
Specification requires for 8kV
Design For Immunity to
Electromagnetic Disturbances
EFT (IEC1000-4-4)
Ferrite & RC slows
down edge. EFT
appears as CM signal
PCB layout isolates
AD7755 from EFT in
the signal ground
Ferrite & C slows
down edge. MOV
suppresses large
differential EFT
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Passed 5kV @ 15kHz
Specification requires for 4kV @ 5kHz
Design For Immunity to
Electromagnetic Disturbances
HF Field (IEC1000-4-3)
Ferrite & RC make up
effective HF filter
PCB layout/grounding
EMI Filter on PSU
shunting cap acts as
bypass for RF
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Passed 20V/m near field
Specification calls for 10V/m
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.
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EMC Can Be Achieved with Care in
Circuit Design and Layout
Emissions
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Immunity