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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