Keithley 3730 User Manual

Section
5: Basic Digital Multimeter (DMM) Operation Series 3700 System Switch/Multimeter
User's Manual
5
-14 Document Number: 3700S-900-
01 Rev. A / August 2007
Low level considerations
For sensitive measurements, external considerations beyond the Series 3700 affect the
accuracy. Effects not noticeable when working with higher voltages are significant in microvolt
signals. The Series 3700 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 Series 3700 input low (particularly for low level sources). Improper
shielding can cause the Series 3700 to behave in one or more of the following ways:
 Unexpected offset voltages.
 Inconsistent readings between ranges.
 Sudden shifts in reading.
To minimize pick-up, keep the voltage source and the Series 3700 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 Series
3700 can measure. Thermal 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 thermal EMFs, use copper leads to connect the circuit to the
Series 3700.
A clean, oxidized-free, 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 Series
3700 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.

Keithley 3730 Specifications

General

Brand	Keithley
Model	3730
Category	Multimeter
Language	English