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Grounding, shielding in high impedance apps

Posted: 05 Nov 2013     Print Version  Bookmark and Share

Keywords:shielding  grounding  measurements  high impedance  electrostatic field 

The safety ground shield should never be used as the electrostatic shield. Even well designed instrumentation generates currents that travel down the safety ground in the line cord. Current from the power supply Y capacitors and higher frequency currents from a switching power supply can generate noise voltages on the instrument chassis with respect to external safety ground as the current flows through the inductance of the power line cord. The resulting noise voltage appears as a common mode voltage from the safety ground to the instrument chassis. This voltage is troublesome because instrumentation measurement common is not completely isolated from the instrument chassis (earth grounded). Every instrument generates some DC and AC leakage current across the instrument mains isolation barrier, and a finite capacitance from instrument common to the instrument safety ground. This capacitance is what facilitates the flow of AC current. We do not want these currents to flow through any part of the measurement pathway (figure 4).

Figure 4: An illustration of the power system components that generate common mode current, as well as the isolation capacitances that support AC currents generated externally.

These currents create voltage drops in the measurement leads, as well as voltages across other impedances in the measurement circuit. Because instruments may be designed to float hundreds of volts above earth ground, and shield ground should be connected to the instrument measurement common, the measurement terminals and electrostatic shield should always be considered unsafe.

Grounding the shield
Should the instrument shield (which is instrument LO) be connected to safety ground? Only if the application does not drive LO, and it should be done in a way that does not allow currents to flow in the measurement leads. From an instrumentation perspective, the only reason to connect LO to safety ground is to keep the measurement terminals within the common mode specification of the instrument. Given that the measurement LO terminal is floating in many instrument designs, a higher value (~100 kΩ) resistor can be added from safety ground to the measurement LO terminal.

Common mode current
In the section titled "Safety Grounding," I mentioned that the instrument(s) themselves are responsible for some of the current that generates the common mode voltage, Vx (figure 4). These common mode currents are a direct result of the magnitude of the voltages on the primary and secondary windings of the power transformer acting on the unshielded capacitance across the transformer.

Figure 4 illustrates a typical instrument power transformer designed with primary and secondary shields. The shields within a power transformer perform the same function as the instrument shields already discussed. In the case of the instrument shield, when a portion of the measurement remains unshielded, external field lines can inject current into the measurement.

The same is true with the power transformer, except due to the proximity of the primary to the secondary windings, as well as the magnitude of the voltages, the currents could be much higher if the transformer shields were absent. The capacitor C1 represents the unshielded capacitance from the secondary winding to the primary shield. This is the portion of the primary that remained unshielded. Likewise, the capacitor C2 represents the unshielded capacitance from the primary winding to the secondary shield. The total common mode current is the sum of the currents through each of these capacitors. The common mode current will increase as the primary and secondary transformer voltages increase or when the frequency of the power supply operation increases. The unshielded capacitance offers increasingly lower impedance to higher frequency edges, increasing the magnitude of the common mode current. Common mode current originating on the primary flows through the capacitor C2 into the secondary circuits, into the chassis through the measurement leads, and eventually returns to the primary ground that generated it. Common mode current originating on the secondary flows through the capacitor C1 into the primary circuit, into the chassis at the power entry module, then through the measurement leads, and eventually returns to the secondary ground that generated it. The net common mode current causes a voltage drop in the instrument power cord inductance, as well as a voltage drop in the ground connection between the DUT and the instrument. For this reason, it is best to use the chassis connection provided by the instrumentation whenever possible to avoid introducing a new safety ground to the system. The unshielded capacitance and, to a lesser degree, the DC resistance across the transformer can couple noise currents from other sources that generate differences in safety grounds throughout the building.

Figure 5a: With a single SMU, grounding the DUT, either directly or through a capacitance, can channel ground current into the measurement LO lead.

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