OMB-DAQ-2408 Series User's Guide
Functional Details
13
Figure 5. OMB-DAQ-2408 Series input isolation diagram
When making measurements in industrial environments, DAQ devices can encounter hazardous voltages,
transients, large common mode voltages and fluctuating ground potentials. Any one of these issues can
seriously degrade the measurement accuracy of the device and possibly damage the measurement instrument.
To overcome these issues, some DAQ devices provide physical and electrical isolation. Some of the benefits of
isolation include:
Safety: A DAQ device employing physical and electrical isolation helps to keep high voltages and
transients from damaging the system-side host computer.
Ground loops: Improper grounding of the signal source that the DAQ device is measuring is one of the
most common sources of noise and measurement inaccuracies. Isolation improves the measurement
accuracy by physically preventing ground loops. Ground loops – a common source of noise and error – are
the results of a measurement system having multiple grounds at different potentials.
Common mode rejection: With isolation, a DAQ device can measure small signals in the presence of large
common mode voltages. Isolation increases the ability of the measurement system to reject common mode
voltages. The common mode voltage is the signal that is common to both the positive and negative inputs
of the measurement device, but is not part of the signal to measure.
Analog/TC input
Each analog input channel has the following measurement parameters:
Voltage input range
TC type J, K, T, E, R, S, B, or N
You can select a unique input range or signal type for each channel. For example, one channel could be used for
volts and another for temperature.
Analog input mode
You can configure the analog inputs for SE or DIFF mode. The input voltage range is software selectable for
±10 V, ±5 V, ±2.5 V, ±1.25 V, ±0.625 V, ±0.312 V, ±0.156 V, or ±0.078 V.
With SE mode, connect up to 16 inputs to screw terminals
CH0
to
CH15
. SE mode requires two wires:
Connect one wire to the signal you want to measure (
CHx
).
Connect one wire to the analog ground reference (
AGND
).
Refer to Figure 3 on page 11 for the location of the SE inputs.
With DIFF mode, connect up to eight DIFF inputs to screw terminals
CH0H/CH0L
to
CH7H/CH7L
. DIFF mode
requires two wires plus a ground reference:
Connect one wire to the high/positive signal (
CHxH
).
Connect one wire to the low/negative signal (
CHxL
).
Connect one wire to the analog ground reference (
AGND
).
