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Grasp shielding, guarding in high impedance apps

Posted: 12 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:shielding? grounding? measurements? high impedance? electrostatic field?

Insufficient shielding and bad grounding are typically blamed when measurements are inaccurate, especially in high impedance applications. In fact, shielding and grounding problems are frequently responsible for measurement errors, but many test system developers aren't quite sure why. Many measurement errors can be traced back to currents from external fields that have become coupled into the measurement test leads.

Review of electrostatics
Charges or charged particles are point sources of an electrostatic field (E-field). Field lines always emanate from the positive charge(s) and terminate on the negative charge(s). The force between charged particles is attractive when the charges on each particle are complementary and repulsive when the charges are identical. The E-field stores energy; the amount of energy stored is proportional to the total number of lines of flux (or to the total charge). The error current coupled into measurement conductors is directly proportional to the strength of the field. At any given voltage, the capacitance describes the relationship between charge and voltage on two conducting bodies. The energy stored in the field is equal to one-half of the capacitance multiplied by the square of the voltage: E (Joules) = ?CV2. Wherever a voltage is present, there's also a distribution of positive and negative charges, even if one of the conductors is grounded.

Voltages generate a high impedance field. Currents (magnetic fields) generate a low impedance field. The field impedance is always the ratio of the electric field to the magnetic field for any electromagnetic wave. Shields work by both reflection and absorption of field energy. If the terminating impedance of an electromagnetic wave is orthogonal to the wave impedance, reflection will predominate. If they are of similar impedance, absorption is the only possibility.

Electrostatic coupling
Charges unassociated with the measurement circuit are responsible for numerous measurement problems. If a charge is fixed in space around an unshielded measurement, the E-field from the charge will radiate to the measurement lead(s) and terminate on an image charge (a complementary charge of opposite polarity). Due to the E-field, a DC leakage current could potentially flow into the measurement leads. If a charge or a conductor with a charge distribution is moving in space with respect to the measurement circuit, an AC current (I = V.dC/dt, where C= the capacitance between the charge or the conductor and the measurement circuit) will flow into the measurement leads. External conductors at a different voltage than the measurement circuit behave in exactly the same way as point charges do. When the voltage on the external conductor changes, a current equal to I = C.dv/dt will flow into the measurement. Both of these cases, i.e., point charges and differing voltages, will couple noise and error currents into the measurement. Any E-field line terminating on the measurement leads has the capacity to couple current into the circuit. E-fields dominate the interference landscape except when high currents are involved or whenever the instrument or measurement is operated near a transformer or a magnetic source. Ideally, all E-field lines from external sources should fall on a shield or guard instead of the measurement leads. Also, all E-field lines from the measurement and the instrument should fall on either the shield or the guard, never on outside conductors or charges. When the external E-field lines fall on either the shield or the guard, rather than the measurement leads, the measurement will be unaffected by these external electrostatic error sources.

RF coupling
Radio frequency (RF) energy is ubiquitous. Any conductor of reasonable length, including the cables that connect instruments to devices, can act as an antenna for this energy. Although this radiation is outside the bandwidth of the source/measure instrumentation, the electromagnetic radiation will generate currents that travel up and down the antenna (in this case, the measurement leads). When these currents come in contact with the amplifiers inside the instrument, these currents may be rectified, causing a DC offset in the measurement. For this reason, both the HI and LO terminals require a shield to ensure that this current flows in the shield rather than in a measurement lead. The safety shield is usually used (outside the instrument common shield) as the shield for this source of noise. However, in order to provide complete shielding at these frequencies, the shield must not have any apertures (holes or slots) greater than ?/2, where ? is the wavelength of the interfering radiation.

Magnetic coupling
Magnetic coupling is unrelated to currents flowing in measurement leads but rather to the generation of voltages as predicted by Faraday's law of induction. The magnetic field (M-field), unlike the E-field, is a low impedance field. Conductors suitable for use as shields provide a matching impedance (unlike the high impedance E-field) to the M-field; as a result, they will not reflect the energy away from the measurement conductors inside. To shield an M-field, either the magnetic lines of flux must be diverted through a magnetic material (this works well at DC and low frequencies with?-metal) or the shield must be thick enough to attenuate the field by absorbing the energy. To absorb the energy, the shield must be thick enough to attenuate the magnetic energy. For guidance on determining how thick the shield should be, I recommend Ron Schmitt's "Electromagnetics Explained: A Handbook for Wireless/ RF, EMC, and High-Speed Electronics" as a good reference.

The purpose of shielding is to reduce or eliminate noise currents from coupling into electrical measurements. These currents can originate from point sources of charge, generating E-fields and voltage distributions. For example, people carry static charges with them wherever they go. AC line potentials in and around the laboratory or production environment can elicit AC E-fields, which, in turn, generate error currents. When devices under test are grounded outside the confines of the instrument, a different ground potential (different from the instrument) is responsible for yet another E-field generating current in the measurement ground lead. The isolation capacitance in the instrument's power transformer completes the circuit that supports the error current. Thunderstorms and environmental changes can cause electrostatic field changes. Radiation from RF sources can also generate currents in test leads, causing EMI rectified offsets in the amplifiers internal to the measurement instrumentation. Even in fair weather, the earth itself has a field with respect to the upper atmosphere of ~100V/m.

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