USB-2416 User's Guide
Functional Details
17
In this example, channel 0 is sampled 3000 times over 100 ms, and all samples are averaged into one sample.
Then, channel 1 is sampled 600 times over 20 ms, and samples are likewise averaged into one sample.
The final samples are available to you at a maximum rate of about 8 Hz (8.245 Hz).
Figure 11. USB-2416 data rate vs. resolution example
Input isolation
The USB-2416 is an isolated data acquisition device. The analog input, digital I/O, counters, and all the digital
control/timing are referenced to an isolated ground as shown in the figure below. This ground is physically and
electrically separate from the ground use by the circuit connected to the system bus interface.
Isolation provides a barrier between the host computer and potentially hazardous voltages by physically and
electrically separating two parts of the measurement device.
The non-isolated ground is common to the chassis ground of the computer, while the isolated ground is not.
All analog measurements are made relative to the isolated ground. See Figure 12 for details.
Figure 12. USB-2416 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 PC.
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 measurement system's ability 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.
