Single ended thermistors on a PXI-6225 DAQ board

[Munk100]

Hello,
We have a new PXI system running LabVIEW 8.5/DAQmx with a M-series 6225 DAQ board,  connected to a SCB-68 terminal board.  I would like to connect a number of RSE thermistors to the system to maximize the sensor count on the board.  I'm able to easily connect the thermistors in differential mode, but we need more than 32 sensors.

I've tried the AI custom Voltage with Excitation, and then attempted to use various math functions to properly scale the voltage input to accurately display and record temperature.  I'm not entirely successful with this right now; I was wondering if anyone knows how to modify the existing DAQmx create virtual channel to *easily* create RSE channels in AI Thermistor Vex mode.

Thanks in advance,

Munk

[dansch]

Hello Munk,

I understand that you are trying to connect a large number of thermistors to your PXI-6225 M Series device. The PXI-6225 has 80 Single-Ended channels and can be used to measure temperature from a thermistor by utilizing the “AI Thermistor Vex” DAQmx task. Because you are using an external voltage excitation source with the thermistors and you require a channel count above 40, you will want to connect them to the 6225 in a Single-Ended - Nonreferenced (NRSE) configuration. This configuration requires that you connect one lead of the thermistor to the AI channel and the other lead (the ground of the excitation source) to AI SENSE. If you were to use a regular RSE configuration, you would be connecting each thermistor to the input channel along with the AI GND channel. This would cause a ground loop to occur between the ground of your voltage source and the ground of the 6225, which could damage the card. Please refer to the “Field Wiring and Noise Considerations for Analog Signals” tutorial for an overview of the different terminal configurations and the necessary connections for each.

In software, you will want to use a DAQmx Channel Property Node to specify the input terminal configuration you are using. Please see the attached screenshot which shows how to find this property and what the configuration looks like. Also, I would suggest starting from the LabVIEW example program “Acq Thermistor Samples-SW Timed.vi” which can be found under Hardware Input and Output » DAQmx » Analog Measurements » Temperature in the Example Finder. To open the LabVIEW Example Finder, navigate to Help » Find Examples. When using this example for your application, you will have to change the DAQmx Create Channel VI instance to “AI Thermistor Vex” and add the input terminal configuration property node as shown in the screenshot below. Hope this helps,

Message Edited by dansch on 06-04-2008 07:51 PM

[Munk100]

Hello,

Sorry I haven't responded earlier to your helpful response.  During the time between I posted the question, and your response, we have created a solution to the task.  I used the standard AI Voltage DAQmx tool, then created a new sub vi to handle the 2D array data and scale the thermistor voltages to indicate temperature.  To scale the thermistor AI, I used the Convert Thermistor Reading.vi. 

This solution works, but I will also try the adroit solution you have suggested.   On another subject, I just tried to load NI-DAQmx V 8.6 on this machine running XP Pro x64, but I could not install because Signal Express does not suport XP Pro x64. 

Thanks for your help,

Munk100

[dansch]

Hi Munk,

I am glad to hear that you obtained a solution for this. Regarding the machine running XP Pro x64, you are correct that the operating system is not supported by DAQmx. Cheers,