Keithley 2700, User Manual, Page: 107 / 500

3-16 Basic DMM Operation Model 2700 Multimeter/Switch System User’s Manual
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 2700, 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
negligible. 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
× ( )
+ =