Transcript Where to Ground the Shield?

Grounding
• Grounds:
– Common reference for the circuit
– Safety
Black
White
Green
Add independent safety ground
• What is the potential problem of grounding?
– Improper grounding may induce noise on the
transmission lines
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Computer Interfacing and Protocols
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Grounding
• Problem: Faraday’s Law of Induction
d
C E  d l   dt SB  d A
Avoid ground loops
Ground loop current
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ECE
ECE 766
Computer Interfacing and Protocols
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Spatial Dependence
Within a cabinet
vs.
Between two buildings
D
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Shielding Techniques
Metal Box
Relative voltages inside the
metal box are always equal
Noise Source
C3
C3
C1
C3
C1
C1
C2
C2
Short this
impedance
Connect the shield to the signal ground to reduce feedback
effects induced by the shield.
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ECE 766
Computer Interfacing and Protocols
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Where to Ground the Shield?
AC loop through
signal wire
AC loop through
shield
C
Noise Voltage
C
Noise Voltage
Shield should be connected to the earth ground at input reference.
What if we interconnect two
devices in different buildings?
Winter 2005
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Long Paths
• Problem: Two distant, connected chassis
grounded at different points may carry large
currents
 Break the chassis connection
• Solutions:
– Use larger signals to increase SNR
– Use light as medium
• Fiber optic link
• Optical isolators (phototransistor and LED in a pack)
– Use balanced (double-ended, differential) connections
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Balanced Connections
+
-
Noise Voltage
• Both chassis are isolated electrically
• Noise induced on both lines
• Signal carried on each line: s(t) and –s(t)
– Differential mode gain = ( s(t )  n(t ))  ( s(t )  n(t ))  2s(t )
( s (t )  n(t ))  ( s (t )  n(t ))
 n(t )
– Common mode gain =
2
– Differential mode gain >> common mode gain
Winter 2005
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ECE 766
Computer Interfacing and Protocols
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Crosstalk due to Bad Grounds
Resistance
of bad ground
Ideal
Model of Actual
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UTP Revisited
• Faraday’s Law of Induction:
d
C E  d l   dt SB  d A
.
A
• Direction of the induced current
.
A/4
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07 - 9