Shield makes sense ONLY if connected to common ground.
So, how it works?
Suppose we have some electric field in which a detector is immersed. In case it is a stationary field - nothing happens. In case the field changes, some of that will be picked up, just like an electret microphone does. Such microphone is more efficient if impedances are high, so one way of fighting E field is lowering impedances - wherever practical.
E field when presented to some coil will produce voltage that is in average ~ half of the peak voltage achieved by E field value reduced by the coil and Rx low impedance. In case E field is screened by some shielding, no E field is seen from the coil, and half of zero is zero. In case such E field is observed from a bifilar center tapped coil, the E field reduced by low impedance and symmetry happens between center tap and both coil ends, but Rx input being differential cancels these.
Symmetry is great for fighting E field, but not too practical. You usually wish to transform it to a more practical asymmetric path. But when you do, just try hard to have a common ground point as close as possible to every high impedance and low signal device you have. In case of shielding, yes, you want it to be connected to a common ground point. Otherwise it is not a shield but an antenna.
So, how it works?
Suppose we have some electric field in which a detector is immersed. In case it is a stationary field - nothing happens. In case the field changes, some of that will be picked up, just like an electret microphone does. Such microphone is more efficient if impedances are high, so one way of fighting E field is lowering impedances - wherever practical.
E field when presented to some coil will produce voltage that is in average ~ half of the peak voltage achieved by E field value reduced by the coil and Rx low impedance. In case E field is screened by some shielding, no E field is seen from the coil, and half of zero is zero. In case such E field is observed from a bifilar center tapped coil, the E field reduced by low impedance and symmetry happens between center tap and both coil ends, but Rx input being differential cancels these.
Symmetry is great for fighting E field, but not too practical. You usually wish to transform it to a more practical asymmetric path. But when you do, just try hard to have a common ground point as close as possible to every high impedance and low signal device you have. In case of shielding, yes, you want it to be connected to a common ground point. Otherwise it is not a shield but an antenna.
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