Series 3700A System Switch/Multimeter Reference Manual
Section 4: Basic DMM operation
3700AS-901-01 Rev. D/June 2018
4-63
To minimize pick-up, keep the voltage source and the Series 3700A 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.
Thermoelectric EMFs
Thermoelectric EMFs (thermoelectric potentials) are generated by temperature differences between
the junctions of dissimilar metals. These can be large compared to the signals that the Series 3700A
can measure. Thermoelectric EMFs can cause the following conditions:
•
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).
To minimize the drift caused by thermoelectric EMFs, use copper leads to connect the circuit to the
Series 3700A.
A clean, oxidized-free, copper conductor such as #10 bus wire is ideal. For switching modules, use
#20 AWG copper wire to make connections. The leads to the Series 3700A may be shielded or
unshielded, as necessary.
Widely varying temperatures within the circuit can also create thermoelectric EMFs. Therefore,
maintain constant temperatures to minimize these thermoelectric EMFs. A shielded enclosure around
the circuit under test also helps by minimizing air currents.
The relative offset control can be used to null out constant offset voltages.
AC voltage offset
The Series 3700A, at 5½ digits resolution, will typically display 100 counts of offset on AC volts with
the input shorted. This offset is caused by the offset of the T
RMS
converter. This offset will not affect
reading accuracy and should not be zeroed out using the relative offset feature. The following
equation expresses how this offset (V
OFFSET
) is added to the signal input (V
IN
):
Example:
Range= 1 VAC, Offset = 100 counts (1.0 mV), Input = 100 mV
RMS
Therefore, the displayed reading is 0.100005 V.
The offset is seen as the last digit, which is not displayed. Therefore, the offset is negligible. If relative
offset were used to zero the display, the 100 counts of offset would be subtracted from V
IN
, resulting
in an error of 100 counts in the displayed reading.
