What is Shield Grounding
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On: 28 Jul, 2019
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Interference Types and Reduction points out that the shield of a conductor is effective only when it is connected to a constant voltage. When shielding ampliflers, the shield must be connected to the reference voltage for the enclosed circuit, whe...
Interference Types and Reduction points out that the shield of a conductor is effective only when it is connected to a constant voltage. When shielding ampliflers, the shield must be connected to the reference voltage for the enclosed circuit, whether it is grounded or not. Figure 3.48a shows the correct connection. If the shield were not connected or connected to a different voltage, there would be a parasitic feedback from the amplifier output to its input that could even lead to oscillations. Figure 3.48b shows the case where the shield is left unconnected. Figure 3.48c shows the equivalent circuit for its analysis. Figure 3.48d shows that connecting the reference point for an amplifier to ground when its shield is not connected does not solve the problem of external interference. Figure 3.48e, the equivalent circuit for that case, shows that in order to have minimal coupling from % to the shield, Xg,, must be very small; that is, it must be short-circuited. When grounding amplifier shields, the internal circuit must be connected to the shield at a single point, for example, as shown in Figure 3.49a for the
figure 3.48 amplifier shielding
case ola grounded shield intended to reduce power line interference. Other- wise, if there is more than one point connecting the circuit to the shield, resistive interference such as shown in Figure 3.49b may appear across Yas. We must choose the single connection point carefully in order to avoid currents coupled to the shield from circulating along the same path as signal currents. For example, if the signal were grounded and the shield-amplifier connection were as shown in Figure 3.50a, then the interfering voltage Vi would couple through Cis a current to ground via S-2-b, that is, it would share the segment 2-b with the signal.
Thus we should choose a grounding scheme such as the one in Figure 3.50b where the reference point for the amplifier ("2") is connected to the shield not directly in the amplifier but at the signal source. Then external interference does not share any path with the signal. The situation in Figure 3.50b can be described by saying that the amplifier has a "floating" input, that is, point 2 is not grounded within the amplifier.
When grounding a cable shield with a single ground connection, we must
shield and circuit must be connected at a single point (a); otherwise
decide which end to connect: the one at the signal end or the one at the amplifier end.
If the signal is not grounded and the amplifier is, then the best solution is to connect the shield to the input reference terminal for the amplifier, as indicated in Figure 3.51. If the shield were connected to the reference termi- nal at the signal side (connection A, dashed), all interference currents cou- pled to the shield would flow to ground along one of the signal lead wires (the one at terminal2; the amplifier is assumed to have a high input impedance)' If connection B were used, the interfering voltage at the input of the ampli- fier would be
shield grounding 5
figure 3.50 selection of the grounding point for a shield
figure 3.51 ground connection for a shield cable when the signal is not
If the shield were grounded at the amplifier side, connection D, the inter- fering voltage would then be
shield grounding 1
Therefore, if the signal source is not grounded but the amplifler is, then the shield must be connected to the reference terminal for the amplifier, even if it is not grounded.
If the signal is grounded but the amplifler input is not, then the situation is different urd .o is the solution. Now it is better to ground the shield on the signal source end, as shown in Figure 3.52. If instead of that it were con- nected to ground at the signal end (connection B), we would have
shield grounding 2
We should not connect the shield to the reference terminal at the amplifler input because then all shield coupled currents would flow to ground along one of the signal lead wires. If connection D is used, the input interfering voltage would be
shield grounding 3
shield grounding 4
figure 3.52 ground connection for a shield cable when the signal is grounded
Note that the situation in Figure 3.52 connection A is the same as the one in Figure 3.50, but now we have included the possibility of a nonperfect grounding connection and the presence of an interfering voltage between the signal reference point and ground, which are connected by a low-value impedance. If both the signal source and the amplifier are grounded, perhaps the compromise solution is to connect the shield to ground at both ends. But depending on the difference in voltage between grounding points and on the magnetic coupling to the newly created ground loop, the resulting interference may be increased. If this is the case, the loop must be opened by using differential input amplifiers or isolalion amplifiers.
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