Which Mode Is Best for Measuring Floating Signal Sources? Hardware: Multifunction DAQ (MIO)
Problem: I have a floating signal source. Should I set the data acquisition (DAQ) board to differential, referenced single-ended (RSE), or non-referenced single-ended (NRSE) mode to get the best measurement results? Solution: The recommended measurement mode for floating signals is differential mode. It offers the advantage of common mode rejection which eliminates noise that is present on both leads thereby improving the signal to noise ratio. Therefore, if you are measuring low-level voltages (mV level, as with thermocouples) that may be some distance from the computer, it is advisable to use differential mode. The disadvantage of using differential mode is that it reduces the number of available DAQ device channels by half compared to referenced single-ended (RSE) or non-referenced single-ended (NRSE) mode. If you have high-level voltage signals (i.e., above 2 V or so) and need more inputs than what a differential setting can offer, you should use either the RSE (for floating signal sources only) or NRSE. If you are using a source with high output impedance in differential or NRSE mode, you may see an offset voltage (usually detected with an "out of range" error). This is caused by charge build-up from the instrumentation amplifier on your DAQ board. To reduce this problem, it is recommended that you either use a unity-gain op-amp to reduce your source impedance or connect bias resistors across your measurement lines. More information is in the Developer Zone documents below. Consult the user manual which came with your DAQ board or is available from the Product Manuals link below for more details including signal connection diagrams. Related Links: Product Manuals Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals Developer Zone Tutorial: Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device KnowledgeBase 0GI9S503: Which Mode Is Best for Measuring Grounded Signal Sources? KnowledgeBase 1VREA1PH: What Is a Differential Measurement System? KnowledgeBase 0GIA6T03: Voltage Reading of Floating Signal Saturates to the Full-scale Voltage Value Attachments:
Report Date: 12/19/1995 Last Updated: 07/23/2007 Document ID: 0GI98T03 |
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