VTI Instruments Ametek EX1200A Series, User Manual

Page: 86 / 165

www.vtiinstruments.com
EX1200A Triggering 85
DC V
OLTAGE
M
EASUREMENTS
DC voltage (DCV) measurements are one of the simplest measurements to take with a DMM. The
DMM measures the average voltage value of the connected signal differentially. This means that
floating signals can be measured as well as signals containing large common mode voltage offsets
(refer to the
EX1200A Series DMM Specifications for maximum signal details). When in DCV
mode, there are several options that can be configured, as discussed below.
DMM Front Panel
Sense HI
Input HI
Input LO
Sense LO
DC Voltage Source
T
ABLE
4-4: DMM C
ONNECTIONS FOR
DCV M
EASUREMENTS
NOTE Although the DMM can measure up to 300 V rms (200 V from LO or Sense LO to Guard), this
may exceed the maximum switching value of the plug-in module installed in the system. Ensure
that voltage, current, and power specification for the plug-in modules are not exceeded when
measurements are taken.
Input Resistance Selection
On the 100 mV, 1 V, and 10 V ranges the user has the option of either >1
GΩ or 10 MΩ input
impedance. The >1
GΩ setting is appropriate when the device being measured
requires a very
light load, but this setting can cause measurement errors when high parasitic capacitance exists on
the input path. This capacitance can exist when long shielded cables are used or many multiplexer
cards are connected to the same bus. This capacitance must be sufficiently discharged before the
measurement is taken or the capacitance’s charge will be included in the measurement. The
10
MΩ input impedance selection provides this discharge path; unless the very high input
impedance is required it is recommended to use the 10
MΩ setting
.
The default input resistance is set to HIZ. When the mainframe is power cycled or reset, the
default value is applied.
Thermal EMF Related Errors
Thermoelectric voltages are the most common source of error in low-level dc voltage
measurements. Thermoelectric voltages are generated when circuit connections are made using
dissimilar metals at different temperatures. Each metal-to-metal junction forms a thermocouple
which generates a voltage proportional to the junction temperature (typically on order of a few
microvolts). To minimize errors of this nature, precautions should be taken that include using
copper-to-copper crimped connections.