Transcript Why “Smart Products?”

Noise Isolation
Keeping the Gremlins Out
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How noise gets into your circuits
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Key Characteristics of the noise source
Voltage
‘High’ voltage
‘High dV/dt
Current
Strong electric field
Capacitive coupling
Capacitive coupling I  C dV
dT
‘High’ current
Strong magnetic field
Frequency
‘High’ frequency
Radiation
Inductive coupling
Radiative coupling
Distance from the victim
Distance = 0
Direct contact
Conductive coupling
Distance > wavelength
Probably Radiative Not For Us
0 < Distance < Wavelength
Capacitive or Inductive
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Key Characteristics of the noise source
Voltage
Current
Frequency
Distance from the victim
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What is the most likely coupling mechanism for
Fluorescent light noise
Capacitive
High Voltage
Inductive
High Current
Capacitive
High dv/dt
Arc welding noise
Digital clock noise
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What is the most likely coupling mechanism for
Fluorescent light noise
Arc welding noise
Digital clock noise
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Conductive coupling
Motor
Sensor
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More conductive coupling
+12V
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+5V
How should I wire these up?
12V
+
G
7.5V
+ G
+ G
Motor
+ G
HC11
+
G
Sensor
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Which waveform must be conductively coupled?
Why?
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Identifying characteristics of conductive coupling
Metallic contact required
Unaffected by people or cable movement
Non-zero average value for the waveform
Techniques to reduce conductive coupling
Break the conductive contact
Use filtering
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Identifying characteristics of conductive coupling
Metallic contact required
Unaffected by people or cable movement
Techniques to reduce conductive coupling
Break the conductive contact
Use filtering
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Capacitively coupled noise
Simplified circuit
Coupling capacitance
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Physical Representation of
capacitively coupled noise
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Equivalent circuit for
capacitively coupled noise
j [C12 /(C12  C2G )]
VN 
V1
j  1 / R(C12  C2G )
1
R

If
j (C12  C2G )
C12
VN 
V1
(C12  C2G )
1
R

If
j (C12  C2G )
VN  jRC12V1
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Reducing Capacitively coupled noise
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Reducing Capacitively coupled noise
Shield
Position the shield to intercept the noise current
Connect the shield to return the noise current to the source
How many connections to the shield are required?
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Summary of capacitive noise reduction techniques
Reduce capacitive noise coupling by
1. Reducing coupling capacitance
2. Reducing circuit impedance
3. Using shielding
Capacitive shielding requires
1. Proper shield location
2. Correct shield connection
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Summary of capacitive noise reduction techniques
Reduce capacitive noise coupling by
Capacitive shielding requires
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Isolation
Why do you need it?
Conductive noise
Large voltage differential
Provide fault isolation (Safety)
Minimize leakage currents
How do you get it?
Magnetic coupling
Optical coupling
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Isolation
Why do you need it?
How do you get it?
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Isolation via magnetic coupling
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PCB mount miniature relays
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Reed Relay construction
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Optical Isolation
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Create a design to connect
68HC11
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PS2501
Opto-Isolator
L-298
H-Bridge