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Voltage going to 0 on DAQs

Hello. I work in a lab at a university. We are having an issue with our two DAQs. We use a cDAQ 9172 chassis to hold our data acquisition devices. We have a 9265 and a 9205 DAQ. We power the 9265 DAQ with a 24V 1.3A power supply to get the analog current output that it provides. This current is used on four silicon diode temperature sensors. The same power supply powers two Omegadyne pressure sensors. The 9205 DAQ uses 6 of its channels to collect analog voltage inputs from sensors.  When we run our VI we get good results from the DAQs. After about 10 seconds all of the voltages on the channels go to zero on the 9205 DAQ. It stays like this for about another 10-15 seconds before going back to giving good readings.  Does anyone know what could be causing this? Are we overloading the DAQ? Is our power supply cutting out? I can't find anything about this online.

 

Thank you,

Chris Neal

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Hello Chris,

 

I am sorry you have encountered this behavior.

From your description, you are taking readings from the 9265-powered sensors with the 9205.  On first glance, I can't think of anything in that setup that would cause signal loss.  1.3 Amps should be more than enough current for the sensors.  Would you mind providing the make and model of the PSU, as well as the make and model of the sensors?

 

Have you observed any unusual current draws when the signals on the 9205 go to zero?  Any unexpected voltage spikes?  The 9205 is rated for +-30VDC.  It has an input impedance of 10G ohms (in parallel with a 100pF cap), so you may want to examine what kinds of signals are being output by the sensors.

 

To start eliminating points of failure, have you tried running the same measurements in Measurement & Automation Explorer's test panels?  If we can observe the same "cut out" in MAX, we know that the issue is not likely related to LabVIEW.  If, when running the measurements in MAX, we do not see the unusual 10-seconds-on 10-seconds-off, the issue is probably arising to something going wrong in the code.

 

Best regards.

Matthew H.
Applications Engineer
National Instruments
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Thanks for replying so quickly Matthew.

 

You are correct. We are powering our silicon diode sensors with a 10uA excitation current from the 9265. The diodes then output between 0 and 2V. The voltage output terminals of the sensors are connected to the 9205.

 

Here is a link to the information about the silicon diode temperature sensors that we're using:
http://www.lakeshore.com/products/cryogenic-temperature-sensors/silicon-diodes/dt-670/Pages/Overview...

 

The power supply information is available here:

http://www.alliedelec.com/images/products/datasheets/bm/ELPAC/70195479.pdf

 

We are using a digital multimeter to read the current and voltage from the channels of the 9265 and 9205. I put the leads from the multimeter into the DAQ channels and set the meter to collect voltage or current data. Is this a valid way to measure what the channel is seeing?

 

We have removed the ground prong from the power supply cable because it was causing an issue with our pressure sensors. We ground all of our sensors to the 9205 DAQ.

 

I will run the VI with MAX open to see if the same behavior occurs.

 

Thank you,

Chris Neal

 

 

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Hello Chris,

 

Thank you for the links.  I didn't find anything in them that would make me suspect some kind of conflict with your system's hardware.
As far as the measurements go, so long as you're reading the voltage in parallel and the current in series, the measurements you take with the multimeter should be accurate.

 

A helpful document that addresses wiring/grounding concerns is our field wiring guide:
http://www.ni.com/white-paper/3344/en

It sounds like you have a pretty good handle on things, but it never hurts to have that kind of information handy.

If it is at all possible, try to run the measurments (or similar measurements) only in Measurement & Automation Explorer.  If it is not possible to perform them without a VI present, try modifying some of our shipping examples to perform the measurements.  With any data acquisition setup, there are numerous potential points of failure in both hardware and software setup.  By using a shipping example or the MAX test panels exclusively, we can eliminate (or confirm) software as the source of the error. 

Matthew H.
Applications Engineer
National Instruments
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