Low Level Considerations - Keithley 2701 User Manual

Model 2701 User's Manual

Low level considerations

For sensitive measurements, external considerations beyond the Model 2701 affect the
accuracy. Effects not noticeable when working with higher voltages are significant in
microvolt signals. The Model 2701 reads only the signal received at its input; therefore, it
is important that this signal be properly transmitted from the source. The following
paragraphs indicate factors that affect accuracy, including stray signal pick-up and thermal
offsets.
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 2701 input low (particularly for low
level sources). Improper shielding can cause the Model 2701 to behave in one or more of
the following ways:
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To minimize pick-up, keep the voltage source and the Model 2701 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 temperature differences
between the junctions of dissimilar metals. These can be large compared to the signal that
the Model 2701 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 2701.
For front panel inputs, a banana plug generates a few microvolts. A clean copper
conductor such as #10 bus wire is ideal for this application. For switching modules, use
#20 AWG copper wire to make connections. The leads to the Model 2701 may be shielded
or unshielded, as necessary. Refer to "Shielding," page E-9.
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).
Basic DMM Operation 3-15