Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes 2563 Noise in Low Curr
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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: Noise
in Low Current
Measurements
Low current measurements (current <1nA) Potential Cause: Vibration C. A useful alternative to the series resis-
often encounter noise. Here are a few likely Vibration can cause noise currents to flow tance is to put a forward-biased diode in
causes of noise in low current measurements due to the triboelectric effect. series. The diode acts like a variable re-
and how they can be avoided. sistance--low when the charging current
Remedies is high and then increasing in value as the
Potential Cause: A. Use low noise cables, which have graph- current decreases with time.
Electrostatic Coupling ite lubrication on the shield braid, to cut
This occurs when an electrically charged down the friction between insulators in
object is brought near an uncharged object. the cable to reduce this effect. Potential Cause:
B. Secure the cables to avoid any unneces- Offset Current Drift
Remedies sary vibration. Offset currents can drift over a period of
A. Shield the device/circuit--High imped- time, usually because of temperature chang-
ance materials do not allow the charge Potential Cause: es in the device and the meter.
to decay quickly. Shielding the device or Low Source Impedance
circuit to be measured greatly reduces the Remedy
electrostatic interference. Connect the Remedies Stabilize the temperature of the meter,
shield to circuit low. A. When the impedance of the source is low the device, and the general environment of
B. Movement--Avoid movement and vibra- a less sensitive measurement range can be the measurement.
tion near the test area. used.
C. Keep all charged objects (including peo- B. Adding some series resistance when meas-
ple) and conductors away from sensitive uring current from a capacitive source re-
areas of the test circuit. duces noise.
Specifications are subject to change without notice.
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