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How Should I Ground Signals for My USB R Series Device?



Hardware: Reconfigurable I/O (RIO)>>R Series

Problem:
How should I ground the signals on my USB R Series device, and what are some grounding considerations when using the device in Autonomous Mode?

Solution:
The analog input channels on the USB R Series have AI+, AI-, and AIGND pins to which you can connect both single-ended or differential voltage signals. In addition, there is an AISENSE pin for connecting non-referenced single-ended signals. Although the wiring for the USB R Series devices is similar to that of other USB data acquisition devices, there are some special considerations for USB R Series devices.

Autonomous Mode:
USB R Series devices are grounded through the USB connection. Because of this, when the device is disconnected and running in Autonomous Mode the device is essentially floating. A floating source is a source in which the voltage signal is not referred to an absolute reference, such as earth or building ground. This is true for all grounds both analog and digital, and should be taken into consideration when developing applications that utilize Autonomous Mode.

Analog Input Connection Options:
To connect referenced single-ended voltage signals to USB R Series devices you must connect the voltage ground signal to AIGND to keep the common-mode voltage in the specified range.


Figure 1.  Connecting Referenced Single-Ended Signals

To connect non-referenced single-ended voltage signals you must connect the voltage ground signal to AISENSE to keep the common-mode voltage in the specified range.



Figure 2. Connecting Non-Referenced Single-Ended Signals

When connecting grounded or floating differential signal sources, connect the positive voltage signal to AI+ and the negative voltage signal to AI-. To connect grounded differential signals, you must also connect the signal reference to AIGND.


Figure 3. Connecting Grounded Differential Signals

To connect floating differential signals, you must connect the negative signal to AIGND through a 1 MΩ resistor to keep the voltage within the common-mode voltage range.



Figure 4. Connecting Floating Differential Signals


Related Links:
Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals

Attachments:





Report Date: 09/30/2013
Last Updated: 10/29/2014
Document ID: 6DTBCA8Q

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