VII Leakage and Protection II
Protection can also be used to reduce leakage in cable connections. Figure 12 shows the principle of driving protection to prevent the leakage resistance of the cable from affecting the small current measurement. In an unprotected configuration, the leakage resistance of the coaxial cable is in parallel with the DUT (RDUT), creating an undesirable leakage current (IL). This leakage will affect the measurement of very small currents.
In the protection circuit, the inner core shield of the triaxial cable is connected to the protection terminal of the SMU. The shield is now driven by a unity gain low-impedance amplifier (protection). The potential difference between the Force HI terminal and the protection terminal is close to 0V, so the leakage current (IL) is negligible.
Figure 12. Using protection to reduce leakage in cable connections
To see the results of measuring the triaxial cable and the coaxial cable at very high resistance, a plot of measured current versus time for a 100 GÎ© resistor using a 10V step function is plotted in Figure 13. The triaxial cable improves the measurement in two ways by using protection: 1) it reduces the effective cable capacitance, which reduces the measured RC time constant or settling time; 2) prevents leakage in the cable from causing measurement accuracy Negative Effects.
Figure 13. Comparison of results comparing high impedance with coaxial cable and triaxial cable
As can be seen from Figure 13, the measurement is performed with a protected triaxial cable, with a small leakage current (lower pA) and a shorter settling time (approximately 10 times faster). If the SMU must be connected to a test fixture with a BNC connector, please at S
The Keithley triaxial cable is used between the MU and the test fixture, and then the BNC is connected to the three-axis adapter (with protection removed) to connect the cable to the test fixture.
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