How to use electrostatic shielding in the circuit to reduce electrostatic interference
“When a charged object approaches the input end of the circuit under test, electrostatic coupling and interference will occur. Under low impedance, because the charge dissipates quickly, the influence of interference is not obvious. However, high-resistance materials do not allow the charge to decay quickly, which may produce unstable measurement results. Since erroneous readings can be caused by DC or AC electrostatic fields, electrostatic shielding helps to minimize the effects of this electric field.
When a charged object approaches the input end of the circuit under test, electrostatic coupling and interference will occur. Under low impedance, because the charge dissipates quickly, the influence of interference is not obvious. However, high-resistance materials do not allow the charge to decay quickly, which may produce unstable measurement results. Since erroneous readings can be caused by DC or AC electrostatic fields, electrostatic shielding helps to minimize the effects of this electric field.
DC electric fields may produce noisy readings or undetectable errors. The movement near the experimental circuit (for example, the movement of the operator of the instrument or other movement in the adjacent area, etc.) causes the electrometer to Display the reading fluctuations, which reflects the existence of this field. In order to quickly check for the presence of interference, place a charged plastic object, such as a comb, near the circuit. A big change in the meter’s reading indicates that the shielding is not perfect.
AC electric fields can also cause trouble. AC electric fields are often caused by power supplies and RF fields. If the AC voltage at the input terminal is large, part of the signal is rectified, so errors are generated in the measured DC signal. This can be checked by observing the analog output of the electrometer or picoammeter with an oscilloscope. The limited waveform indicates the need for improved electrostatic shielding. Figure 2-42 shows the limiting waveform observed at the 2V analog output terminal of the electrometer. In this example, the limiting effect reduces the DC reading by approximately 50%.
For SMU, connect an oscilloscope between its protection end and common end to observe the AC interference situation.
Figure 1 shows an example of AC electrostatic coupling. An electrostatic voltage source near a conductor (such as a cable or a wire on a printed circuit board) generates a current proportional to the rate of change of charge and the rate of change of coupling capacitance. The current can be calculated as follows:
i = C dv / dt + V dC / dt
For example, two conductors with an area of 1 cm 2 and a distance of 1 cm in the air have a capacitance of about 0.1 pF. If the voltage difference between the two conductors is 100V, the capacitance change due to vibration is 0.01pF/sec (10% capacitance change), then 1pA AC current can be generated.
In order to reduce the influence of the electric field, a shield can be made to surround the circuit under test. The easiest form of shielding is a simple metal box or metal mesh that surrounds the circuit under test. The shielding box is also available on the market.
Figure 2 is an example of shielding. The shield made of conductive material is always connected to the low impedance input of the electrometer or picoammeter, or to the LO output (or common) of the SMU. If the LO end of the circuit is floating to the ground, special safety measures must be taken to prevent personnel from touching the shield.
The cable between the HI end of the instrument and the device under test also needs to be shielded. Enclosing the signal conductor with a metal shield connected to the LO end can greatly reduce the capacitive coupling between the source of electrostatic noise and the signal conductor or cable, as shown in Figure 3. With this shield, the noise current generated by the electrostatic voltage source and the coupling capacitor flows to the ground through the shield, instead of flowing through the signal line.
In general, observing the following guidelines can minimize the current generated by electrostatic coupling:
* Keep all charged objects (including persons) and conductors away from the sensitive area of the test circuit.
* Avoid movement and vibration near the test area.
* When the measured current is less than 1nA, surround and shield the device under test with a metal closure, and connect the closure to the common end of the test circuit.
Shield and protect
Shielding usually means the use of metal closures to prevent electrostatic interference from affecting high-impedance circuits. And protection means the use of additional low-impedance conductors maintained at the same potential as the high-impedance circuit to prevent possible interference voltages or currents. Protection measures do not necessarily provide shielding.