Keithley 2564 Noise in Hi Res free download

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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