Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes 2771 Noise Rdg Hi R1

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2771 Noise Rdg Hi R1


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                                    A   G R E AT E R      M E A S U R E     O F   C O N F I D E N C E




Problem: Noisy
Readings in
High Resistance
Measurements
Noisy readings prevent accurate measurements
and reduce the effective resolution

Probable Cause:                                           the circuit being measured. With this                any shock hazard. It is also important to
Electrostatic Coupling                                    shield in place, the noise current generat-          shield the cabling.
   Electrostatic coupling is the most com-                ed by the electrostatic voltage source and        B. Guarding is the use of an added low im-
mon reason readings may be noisy when                     the coupling capacitance flows through               pedance conductor, maintained at the
making high resistance measurements                       the shield to ground, rather than through            same potential as the high impedance cir-
(>10M). Electrostatic interference occurs                 the signal conductors. The easiest shield            cuit, which will intercept any interfering
when an electrically charged object is                    to make is a simple metal box or mesh                voltage or current. The guard can be con-
brought near an uncharged object. At low                  screen that encloses the test circuit and is         nected to the instrument's guard termi-
impedance levels, the effects of the interfer-            connected to the low impedance point on              nal, if available, or fed from an external
ence are not noticeable, because the charge               the measurement instrument. If this point            buffer amplifier. A guard does not nec-
dissipates rapidly. However, high resistance              is floating above ground, then observe               essarily provide electrostatic shielding.
materials do not allow the charge to decay                special safety precautions to prevent
quickly, which may result in unstable meas-
urements. Delays could be in the range of
                                                                                             Test Fixture
tens of seconds to several minutes, depend-
ing on the application. Shielding and guard-                                                                           Metal Shield
                                                                                                 DUT
ing are both important with high resistance
measurements.
                                                                                              Insulators                               HI
                                                                     + HI
                                                                                                                      Measured
Remedies:                                                 V-Source                                                     Current     A        Picoammeter
A. Shielding usually is the use of a metallic



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