Voltage AI signals maxing out without an input signal

[rodrigocooper]

I have been using the same LabView program to measure a voltage signal for months with no problem. Recently though, the signals did not come as expected. When trying to trouble shoot, I found out the AI signal would max out even if I had removed all inputs to the DAQ board (SCC-68). The board itself is set to "internal power" and all input connections are removed. When attempting to measure 5 voltage signals simultaneously (with no input connected), the signals would hover around 0 V (+/- 1V), but when I would remove four of the signals, the lone signal would ramp up to max within 20 seconds. This didn't happen before and is preventing any collection of data. The problem is consistent with or without input signals, including signals from an oscilloscope calibration (a 5V square signal that is read as a square signal with min voltage of 2.8 and max voltage of 3.3V) and a power supply (the collected signal typically 1-2V lower than the power supply). Is there something I can do to fix this or is this a faulty hardware problem?

[Brooks_C]

Hello Rodrigocooper,

If you disconnect all connections to an analog input it is expected behavior to see this voltage rail (float up to the maximum voltage you can measure)--see item three in this KB:  Incorrect Readings when Scanning Unconnected or Open Channels.  Also you may want to look at this KB as well for more general troubleshooting techniques:  Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels

Are you measuring signals from the same device that you've been measuring for months?  If you've been measuring a battery (or a floating signal source) and now you're measuring a grounded signal source then you'll need to change the input configuration for your measurement, and possibly connect the signals to different pins.  If you've changed the input configuration but not the pins then this might cause the behavior you're seeing because the card would be measuring open pins.

For Information on which connection configuration to use take a look at the Field Wiring and Noise Considerations for Analog Signals article.  Table one summarizes the three different measurement configurations.  Once you know what configuration you're using then check your device manual to find out what pin numbers you should be using.

If you're still having trouble after you look over these resources then it would be helpful to know the following:

1.  What DAQ card are you using?

2.  Are you using a Measurement and Automation Explorer (MAX) test panel or a LabVIEW VI or another program to test the board?

3.  What is your signal source?

4.  What pins are you connecting to the hardware with and what channel are you trying to measure with your program?

Another test you can do is to run the DAQ Diagnostic Utility (check the DAQ Diagnostic Utility 1.0 if you don't see it on the 2.0 list) if your board is supported.  Go to this site and check the Readme to see if your board is supported, if it is then download and run the utility.  Make sure you disconnect all cables from the DAQ card first to avoid a false failure.  This utility will internally route signals from the AI to AO etc. to make sure the hardware is working.

I hope this helps you debug, and please post back if you have further questions.

Cheers,

Message Edited by Brooks_C on 04-18-2008 02:29 PM