Keithley 2750, User Manual, Page: 104 / 452

Model 2750 Multimeter/Switch System User’s Manual Basic DMM Operation 3-13
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 2750 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 Model 2750.
For front panel inputs, a banana plug generates a few microvolts. A clean copper conduc-
tor 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 2750 may be shielded or
unshielded, as necessary. Refer to
“Shielding,” page E-9 .
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.
AC voltage offset
The Model 2750, at 5
H
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 TRMS converter.
This offset will not affect reading accuracy and should not be zeroed out using the REL
feature. The following equation expresses how this offset (V
OFFSET
) is added to the signal
input (V
IN
):
Example: Range = 1VAC
Offset = 100 counts (1.0mV)
Input = 100mV RMS
= 0.100005 V
The offset is seen as the last digit, which is not displayed. Therefore, the offset is negligi-
ble. If REL 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.
Displayed reading V
IN
( )
2
V
OFFSET
( )
2
+ =
Displayed reading 100mV
( )
2
1.0mV
( )
2
+ =
Displayed reading 0.01V 1 10
6 –
V
× ( )
+ =
2750-900-01.book Page 13 Wednesday, August 3, 2011 7:56 AM