Keithley 2182, Руководство пользователя, Страница: 62 / 339

2-22 Voltage and Temperature Measurements
Low-level considerations
For sensitive measurements, external considerations beyond the Model 2182 affect accuracy.
Effects not noticeable when working with higher voltages are significant in nanovolt signals.
The Model 2182 reads only the signal received at its input; therefore, it is important that this
signal be properly transmitted from the source. Two principal factors that can corrupt
measurements are thermal EMFs and noise induced by AC interference.
NOTE More detailed information on thermal EMFs and other factors that affect low-level
measurements are explained in Appendix C . Also, for comprehensive information on
low-level measurements, see the “Low level measurements” handbook, which is
available from Keithley.
Thermal EMFs
Thermal EMFs (thermoelectric potentials) are generated by thermal differences between the
junctions of dissimilar metals. These voltages can be large compared to the signal that the
Model 2182 is trying to measure. Thermal EMFs can cause the following conditions:
• Instability or zero offset that is above acceptable levels.
• The reading is sensitive to (and responds to) temperature changes. This effect can be
demonstrated by touching the circuit, by placing a heat source near the circuit, or by a
regular pattern of instability (corresponding to changes in sunlight or the activation of
heating and air conditioning systems).
To minimize thermal EMFs, use clean copper-to-copper connections wherever possible in the
test circuit. See “ Connections ” for details on connection techniques and cleaning.
Widely varying temperatures within the circuit can also create thermal EMFs. Therefore,
maintain constant temperatures to minimize these thermal EMFs. A shielded enclosure around
the circuit under test also helps by minimizing air currents.
The REL (Relative) control can be used to null out constant offset voltage. The basic
procedure to use REL is found in “ Measuring voltage and temperature ,” and details on Relative
are provided in Section 4 .
Noise
AC voltages that are extremely large compared with the DC signal to be measured may be
induced into the input of the Model 2182 and corrupt the measurement. AC interference can
cause the Model 2182 to behave in one or more of the following ways:
• Unexpected offset voltages
• Inconsistent readings between ranges
• Sudden shifts in a reading
To minimize AC pick-up, keep the test circuit source and the Model 2182 away from strong
AC magnetic sources. The voltage induced due to magnetic flux is proportional to the area of the
loop formed by the input leads. Therefore, minimize the loop area of the input leads and connect
each signal at only one point.
Shielding also helps minimize AC interference. The metal shield should enclose the test
circuit and be connected to Channel 1 LO or to the chassis ground screw on the rear panel.