Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

107 -

GroundingGrounding

• Grounds:– Common reference for the circuit– Safety

• What is the potential problem of grounding?– Improper grounding may induce noise on the

transmission lines

Black

WhiteGreen

Add independent safety ground

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

207 -

GroundingGrounding

• Problem: Faraday’s Law of Induction

Avoid ground loops

C S

AdBdt

dldE

Ground loop current

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

307 -

Spatial DependenceSpatial Dependence

Within a cabinetvs.

Between two buildingsD

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

407 -

Shielding TechniquesShielding Techniques

Metal Box

Noise Source

Relative voltages inside themetal box are always equal

Connect the shield to the signal ground to reduce feedback effects induced by the shield.

C1

C2

C3 C1C3

C2

C3

C1

Short this impedance

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

507 -

Where to Ground the Shield?Where to Ground the Shield?

C

Noise Voltage

AC loop throughsignal wire

C

Noise Voltage

AC loop throughshield

Shield should be connected to the earth ground at input reference.

What if we interconnect twodevices in different buildings?

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

607 -

Long PathsLong 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

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

707 -

Balanced ConnectionsBalanced Connections

• Both chassis are isolated electrically• Noise induced on both lines• Signal carried on each line: s(t) and –s(t)

– Differential mode gain = – Common mode gain = – Differential mode gain >> common mode gain

)(2))()(())()(( tstntstnts

Noise Voltage

+-

)(2

))()(())()((tn

tntstnts

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

807 -

Crosstalk due to Bad GroundsCrosstalk due to Bad Grounds

IdealModel of Actual

Resistance of bad ground

Winter 2005

ECE

ECE 766Computer Interfacing and Protocols

907 -

UTP RevisitedUTP Revisited

• Faraday’s Law of Induction:

• Direction of the induced current

C S

AdBdt

dldE

.A

.

-A/4A/4-A/4A/4