VTI Instruments Ametek EX1200A Series, User Manual

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EX1200A Triggering 81
DMM F
UNCTIONS
This section of the manual discusses each measurement function of DMM and aids in
understanding some of the DMM specifications. Understanding the operation of the DMM and
each of its functions helps optimize the configuration of the DMM for measurement
accuracy/speed. Note that DMM performance is the same whether the front panel or DMM Bus
interfaces is used. The examples provided in the following sections provide signal routing using
only the front panel. Some functions are common to the majority of measurements taken by the
DMM. These functions are explained below.
R
ANGE
The selected measurement range of a function affects both the ultimate digits and accuracy of the
measurements as well as the maximum signal level that can be measured. The multimeter can
automatically select the range, using auto-ranging, or a fixed range can be set using the manual
ranging options. Auto-ranging is convenient because the multimeter automatically selects the
appropriate range for each measurement. However, if the range is known to the user, manual
ranging can provide for faster measurements by reducing the processing overhead associated with
determining the appropriate range for each measurement.
The range setting for each measurement function is saved in volatile memory when changing
functions. Selecting a function returns the instrument to the last range selected for that function.
Note, however, that when the instrument is reset or power cycled, the functions return to their
default values Table 4-2 lists the valid measurement ranges for each measurement function:
Function Ranges
DC Voltage 100 mV, 1 V, 10 V, 100 V, 300 V
AC Voltage 100 mV, 1 V, 10 V, 100 V, 300 V
DC Current
1 mA, 10 mA, 100 mA, 1 A, 3 A
AC Current 10 mA, 100 mA, 1 A, 3 A
2-/4-Wire Resistance 10
Ω,
100
Ω,
1
kΩ
, 10
kΩ
, 100
kΩ
, 1
MΩ
, 10
MΩ
, 100
MΩ
Frequency
1.5 MHz
Time Period
0.3333 s
Temperature Fixed based on the type of transducer selected
T
ABLE
4-2: DMM F
UNCTION
R
ANGES
Over-Ranging
Each range can measure a signal that is within 120% of its range. This is, however, not applicable
for ranges whose limits are maximum specified levels (such as the 300 V dc range). Input values
that exceed the maximum readings cause an
“
OVERFLOW
”
error, indicating that the DMM
cannot measure the input signals at the current range.
Auto-Range
When auto-range mode is enabled, the EX1200A DMM automatically selects the input range. This
is done by validating the input signal, then the range with best resolution to cover the entire
dynamic range of the signal is selected. The thresholds for auto-ranging are 6% and 120% (lower
and upper) for voltage-based measurements (ACV, DCV, frequency, and period) and 10% and
120% for current measurements (ACI and DCI) . Thus, if the unknown signal is more than 120%
of the initially assumed range, then the DMM automatically switches to the next highest range.
Similarly, if the unknown signal is less than 6% of the assumed range, the DMM automatically
drops to the next lowest available range.