Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes 2564 Noise in Hi Res

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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 measure-                    and guarding are both important with high               connected to the low impedance point on
ment, and reduce the effective resolution.                  resistance measurements.                                the measurement instrument. If this point
                                                                                                                    is floating above ground, then observe
Probable Cause:                                             Remedies                                                special safety precautions to prevent
Electrostatic Coupling                                      A. Shielding usually is the use of a metallic           any shock hazard. It is also important to
    Electrostatic coupling is the most com-                    enclosure to prevent electrostatic inter-            shield the cabling.
mon reason readings may be noisy when                          ference from affecting a high impedance           B. Guarding is the use of an added low im-
making high resistance measurements. Elec-                     circuit. A shield can be built to enclose            pedance conductor, maintained at the
trostatic interference occurs when an electri-                 the circuit being measured. With this                same potential as the high impedance
cally charged object is brought near an un-                    shield in place, the noise current generat-          circuit, which will intercept any inter-
charged object. At low impedance levels, the                   ed by the electrostatic voltage source and           fering voltage or current. The guard can
effects of the interference are not noticeable                 the coupling capacitance flows through               be connected to the instrument's guard
because the charge dissipates rapidly. How-                    the shield to ground rather than through             terminal, if available, or fed from an ex-
ever, high resistance materials do not allow                   the signal conductors. The easiest shield            ternal buffer amplifier. A guard does not
the charge to decay quickly, which may re-                     to make is a simple metal box or mesh                necessarily provide electrostatic shield-
sult in unstable measurements. Shielding                       screen that encloses the test circuit and is         ing.




Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.




Keithley Instruments, Inc.                    28775 Aurora Road 

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