1. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 1 Grounding Grounds: –Common reference for the circuit –Safety What is the potential problem of grounding? –Improper grounding may induce noise on the transmission lines Black White Green Add independent safety ground

2. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 2 Grounding Problem: Faraday’s Law of Induction  Avoid ground loops Ground loop current

3. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 3 Spatial Dependence Within a cabinet vs. Between two buildings D

4. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 4 Shielding Techniques Metal Box Noise Source Relative voltages inside the metal box are always equal Connect the shield to the signal ground to reduce feedback effects induced by the shield. C1C1 C2C2 C3C3 C1C1 C3C3 C2C2 C3C3 C1C1 Short this impedance

5. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 5 Where to Ground the Shield? C Noise Voltage AC loop through signal wire C Noise Voltage AC loop through shield Shield should be connected to the earth ground at input reference. What if we interconnect two devices in different buildings?

6. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 6 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

7. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 7 Balanced 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 Noise Voltage + -

8. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 8 Crosstalk due to Bad Grounds Ideal Model of Actual Resistance of bad ground

9. 07 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 9 UTP Revisited Faraday’s Law of Induction: Direction of the induced current. A. -A/4 A/4-A/4A/4