Thermal Emfs - Keithley 2010 User Manual

2-18 Basic Measurements
Shielding
AC voltages that are extremely large compared with the DC signal to be measured may
produce an erroneous output. Therefore, to minimize AC interference, the circuit should be
shielded with the shield connected to the Model 2010 INPUT LO (particularly for low level
sources). Improper shielding can cause the Model 2010 to behave in one or more of the
following ways:
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•
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To minimize pick-up, keep the voltage source and the Model 2010 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.

Thermal EMFs

Thermal EMFs (thermoelectric potentials) are generated by thermal differences between the
junctions of dissimilar metals. These can be large compared to the signal that the Model 2010
can measure. Thermal EMFs can cause the following conditions:
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To minimize the drift caused by thermal EMFs, use copper leads to connect the circuit to the
Model 2010. A banana plug generates a few microvolts. A clean copper conductor such as #10
bus wire is ideal for this application. The leads to the input may be shielded or unshielded, as
necessary.
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 control can be used to null out constant offset voltages.
NOTE
Unexpected offset voltages.
Inconsistent readings between ranges.
Sudden shifts in reading.
Instability or zero offset is much higher than expected.
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).
Additional thermals may be generated by the optional scanner cards.