Omega OMB-DAQ-2416_4AO Input isolation, Safety, Ground loops, Common mode rejection

OMB-DAQ-2416-4AO User's Guide				Functional Details
			Resolution = 19.6 bits
	Resolution = 20.6 bits
		640 µs delay before
		next sample taken

Figure 13. OMB-DAQ-2416-4AO data rate vs. resolution example

Input isolation

The OMB-DAQ-2416-4AO is an isolated data acquisition device. The analog I/O, 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 PC 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 PC, while the "isolated" ground is not.

All analog measurements are made relative to the isolated ground. See Figure 14 for details.

	Isolation
I/O connector	barrier
AIN/TIN
Aout		Non-
Isolated	Digital
		isolated	USB
µC	isolator
		µC
DIO

Counters

Isolated ground

DSUB-37 connector (OMB-AI-EXP32)

Non--isolated ground

Figure 14. OMB-DAQ-2416-4AO 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.

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