2-16 Measurement Concepts and Connections Model 6487 Reference Manual
Measurement considerations
There are a variety of factors to consider when making low-level measurements. These
considerations are summarized in Table 2-3 and are detailed in Appendix G of this manual
and Appendix B of the Model 6487 User’s Manual. For comprehensive information on all
measurement considerations, refer to the Low Level Measurements handbook, which is
available from Keithley Instruments. Check www.keithley.com for more details on the
handbook.
Table 2-2
Example 2V analog output values
Range Applied signal Analog output value (nominal)*
20nA 10.5nA -1.05V
2mA -1.65mA 1.65V
* Output values are within ±(2.5% + 2mV) of nominal value.
Table 2-3
Summary of measurement considerations
Considerations Description
See Appendix G for details
Input bias current Offset current of Model 6487 could affect low current measurements.
Voltage burden Offset voltage of Model 6487 could cause errors if it is high in relation to the
voltage of the measured circuit.
Noise Noise generated by source resistance and source capacitance.
See Model 6487 User’s Manual Appendix B for details
Ground loops Multiple ground points can create error signals.
Triboelectric effects Charge currents generated in a cable by friction between a conductor and the
surrounding insulator (i.e. bending a triax cable).
Piezoelectric and stored
charge effects
Currents generated by mechanical stress on certain insulating materials.
Electrochemical effects Currents generated by the formation of chemical batteries on a circuit board
caused by ionic contamination.
Humidity Reduces insulation resistance on PC boards and test connection insulators.
Light Light sensitive components must be tested in a light-free environment.
Electrostatic interference Charge induced by bringing a charged object near your test circuit.
Magnetic fields The presence of magnetic fields can generate EMF (voltage).
Electromagnetic
interference (EMI)
EMI from external sources (i.e. radio and TV transmitters) can affect sensitive
measurements.
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