Grounding and shielding used in electronic instruments, Electrical Engineering

Q. How and why are the grounding and shielding used in electronic instruments?

Sol. In electronic instruments grounding and Shielding techniques are available in order to avoid any kind of interference.

Grounding can be defined as a collection of measures and precautions taken to present stray currents entering electronic sensitive parts by connecting the appropriate parts of a circuit to earth or ground.

Shielding is the technique of placing electronic systems in metal carrying to prevent electrostatic or magnetic fields entering sensitive components.

Every part of the system in which small signals are present is basically sensitive to interference. Directly or indirectly the mains is one of the most common sources of interference but other sources may exist as well. The following list gives nine other potential sources of interference. Interference can be caused either by sources which can produce an electromagnetic field or by sources which have a thermal or mechanical influence on the system.

1.         A temperature coefficient can be assigned to every part of a component is always a function of temperature and temperature change. Temperature can influence the biasing of active components considerably. In very sensitive parts we have to check on with temperature gradients caused by dissipation differences in components. This may cause thermopile voltages due to the See beck effect. Temperature differences can be present due to dissipation differences of components mounted on printed circuit boards.

2.              Mechanical shocks can influence poorly installed and soldered connections.

3.              Ignition equipment in cars produces a brad bandwidth of frequencies. The ground frequency lies between 50 and 200 Hz but higher frequencies are also present, ranging from 30 to 300 MHz and for these frequencies, the spark-plug cables can act as a transmitting antenna.

4.              In digital equipment, pulse shaped signals are applied and the present frequency bandwidth is a function of the slope of the applied pulses and ultimately determines the magnitude of the influence.

5.            Switches in power distribution systems can produce large pulses and then act as an interfering source. This is also the case in large production equipment systems in which a large number of switches must be controlled and activated. A very popular type of power controller producing a large amount of interference is the thyrisor. The steeper the slope, the greater the interference. Switching at the precise moment the voltage crosses the zero can educe this type of interference.

6.          The collector of an A.C. series electrometer can be interpreted as a fast switch, operating with a frequency between I and 10 KHz determined by the amount of lamellas and the number of revolutions per minute. This mechanical switch produces pulses with steep and slope thus a broad frequency spectrum will result.

7.           Very high voltages can produce corona effects which cause a continuous interfering spectrum with a large bandwidth.

8.          In a gas discharge lamp, a periodic discharge is generated producing an interference noise signal level above 1 MHz. A filtering element such as an induction coil can reduce this influence.

9.          Finally, interfering sources with a very small frequency bandwidth should be mentioned, such as HF generators, welding equipment and all types of transmitters.

 Interference of these kinds can be eliminated by two measures:

(a)    the application of an isolating transformer,

(b)    the use of shielding leads and cabinets.

  Applying a type of guard and/or shield is a good method to prevent interference.

 In many instruments, a metal link or lead, the so-called 'Low' can be connected to the safety earth terminal, linking the common of the system to ground. If this is the only connection to earth the measurement can be performed straight forwardly, but great care must be taken to ensure that the system under test has no other connections to earth because this can introduce errors in sensitive measurements. In this case the 'Low' and earth have to be disconnected, resulting in a 'floating' common preventing ground loops.

Ground and Earth: Another aspect to be considered in earth connections is the very common pollution of earth terminals in the mains plug. In many real circumstances the earth terminal is not clean at all, but polluted with large interference and can even have several volts with respect to ground level.

Ground can be defined as any reference conductor that can be used for a common return. Any ground should be an equipotential for the whole system involved. Earthing is just a particular case of grounding. There is a metal link that provides the possibility to make a connection between low and ground to ensure safety earthing.

A Grounded System: With this in mind, consider the case when two points in a measuring system are connected to ground as depicted in Fig. It will be clear that a type of ground loop is created if the transducer and the measuring instrument are both connected by relatively long cables. We may think of thermocouples or strain gauges for the transducer and a chart recorder for the measuring instrument. In this circuitry R1 and R2 represent the respective cable resistances. In general, there are always stray or erratic current in earth and these currents combined with the coil resistance form an imaginary voltage source Vcm causing a current to flow in R1 and R2.

 It is obvious that the current in R2 will be larger than in R1 due to the relatively high input impedance Z of the measuring instrument; hence due to Vcm' unequal series voltages are generated in R2 and R1 and will be added or subtracted to the differential voltage to be measured. A common-mode voltage Vcm is converted into a differential-mode voltage due to asymmetries in the input stage of the measuring instrument. The introduced error is called a series mode voltage error.

The same situation arises when a collection of instruments is mounted in a metal rack and each instrument is electrically through the rack via the signal ground terminals, no matter whether the instruments are connected to ground or not. This current can flow because in large racks several hundreds of millivolts may exist between different drawers. Another aspect is the feedback mechanism of interfering signals to the input likely to be coupled back into the circuit by capacitive or electromagnetic induction and again a series mode error is introduced in the measuring system.

Suppose now the cable resistances R1 and R2 are neglected and a thermocouple transducer with given internal resistances Rtrl' and Rtr2 are connected to the input of an amplifier with input impedance Z, then the configuration in. will result. A thermocouple is a welded connection of two different metals with different specific resistances and if long thermocouple wires are involved, large differences in resistance can occur. Hence in this configuration the electrical, equivalent of the thermocouple is considered as a voltage source with two equally divided but unequal resistors.

When this system is earthed at two points as shown, due to Vcm and Zi two different ground loops are created, resulting in two currents in magnitude and developing two different series mode voltage across the Rtr s.

In many measuring applications, a type of bridge circuitry is applied which can be fed by a D.C. or A.C. voltage source Vcc .very often, one of the bridge nodes is earthed and it will be obvious that the voltage levels of points A and B, with respect to earth are 1/2 Vcc and both are determined by the accuracies of the bridge branches. Hence, a bridge configuration will always deliver a so-called transducer common-mode voltage to the input of the measuring system. This is illustrated.

Aspects of Shielding: Shielding can be defined as the placing of sensitive electronic parts and components in a metal casing to prevent electric and magnetic fields entering that casing.

A shielding cabinet: Suppose we have placed a non-inverting amplifier in shielding cabinets as illustrated. Inside the cabinet three parasitic capacitors C1 and C2 and C3 can be distinguished, of which C1 and C2 form a parasitic feedback path from output to input. This can result in undesired oscillation or jamming of the amplifier. The feedback path can be eliminated by connecting the shield to the common and thus short-circuiting C3 as shown.

From this example, it will be clear that a shield is only effective when the shield is connected to the common or zero-signal reference potential of the shielded circuitry, eliminating the undesired feedback path.


A shielding cabinets as part of an electronic circuit: illustration the use of a shielding cabinet as a part of an electronic system. The shielding cabinet is coupled to the mains via parasitic capacitor which will result in two currents i1 and i2 flowing through the cabinet to the earth connection. If within the cabinet different common connections are made, suppose at A and NB, a voltage difference VAB between A and B can be developed by Vcm. These results in a series mode voltage with Vss forming an input signal for transistor Q2 via R1 and clearly distortion will occur.

The use of more than one shielding cabinet: Suppose two instruments are required of which need shielding, more than one shielding cabinet is involved and it is recommended that only one earth connection is used for the two cabinets. But sometimes, a problem can arise, as illustrated. If the right-hand cabinet is earthed and both cabinets are connected to the mains via parasitic capacitors, a current   i1 can flow through the parasitic capacitor Cv the cabinet 1 and the shield of the coaxial cable to the ground connection of cabinet 2.

Posted Date: 7/12/2012 6:09:26 AM | Location : United States

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