Problem reading right voltage values (PCI-6024E)

[geokaf2]

Hallo all,
I am new to this amazing world but ia have a problem reading right voltage values using the PCI-6024E card! I want to read an analog input from a temperature sensor (0-10V -> -10 -40 Celcius). My sensor has 2 inputs ( Vcc 24V and GND) and one output (voltage). So it is a grounded signal source.
I have choose differential input mode (see attached picture) for my card. I have connected sensor's output (desired voltage value) to ACH1, sensor's ground to ACH9 (differential mode) and i have grounded AIGND to my PC case. I use AI_VRead with gain -1 (-10 -10V input range) but the measured values are not the same with the sensor's output! I have used AI_Configure but nothing changed. Is anyone knows what is wrong!

[AnalogKid2DigitalMan]

You could be dealing with a floating signal source- Is the GND connection of your sensor connected to a power supply whose GND (or negative) connection is tied to earth ground? If not, then it is floating, akin to operating off of a battery.

You may want to try  DIFF mode for floating and use the bias resistors.

[geokaf2]

Hi,
My sensor's ground is connected to the GND ouput of a power supply (24V) which is connected to a wall plug (ground to earth i hope )

Power supply                   Sensor                                          DAQ Card
----------------                     ---------------------                            ---------------------     
|  VCC24V  |-----------------|   Vcc         Out | ---------------------|ACH1                |
|                  |                     |                          |                     |----|ACH9                |
|      GND    |---------------- |   GND               |                     |     |             AIGND  |
----------------            |        ----------------------                     |     ----------------|-----
                                |____________________________|                          |
                                                                                                                   |      
                                                                                                          GND PC Case  
                                                                                                                  

[geokaf2]

Sorry for the schematic above. It didn't went as i wanted. I give the schematic of my connection in the picture (attachment)!

Thnx

[Brian F.]

Hello. 

Have you tried measuring the sensor's output using Measurement and Automation Explorer?  If so, how did that turn out?  Were you able to see the correct output voltage of the sensor?

Also, what values are you reading under the current setup? 

Finally, have you tried connecting the inputs to a different pair of AI channels (such as AI0 and AI8)?

These troubleshooting tips and the results will allow us to get a better idea of exactly what is happening. 

Brian F
Applications Engineer
National Instruments

[geokaf2]

Hello Brian,

I have tried to measure the sensor's output with Measurement and Automation Explorer. While the sensor's output varies between 6.4-7V the explorer gives 6.9-7.5 V (not respectively).

I also have tried to use different pair of channels but the results are the same! With the C++ code the results are near 5.4 , 6.8 but after a random number of measurements the measured value remains to 9.9Volt.

I believe that if the problem is not in the PCI card then is a connection problem. I wonder if i have made a connection fault. I have considered that my input is a ground signal source and i measure in DIFF mode. I have connected the output of my sensor to AICH1 (for example),  the ground of 24V power supply in AICH9 and the AIGND to my PC case. I have tried also with NRSE mode with the appropriate connection (instead of AICH9 i use AISENSE) but the results are the same. I give the circuit schematic for the DIFF mode.

Thnx for you support!

[Brian F.]

Hello. 

Thanks for the additional information.  I have a few questions regarding your last post.  First, you state that the output of the sensor ranges from 6.4 to 7 Volts, but MAX gives you 6.9 to 7.5 Volts.  This offset of .5 Volts, and the fact that the measurement is relatively close to the expected value, is interesting to me.  How have you verified that the sensor is outputting between 6.4 and 7 Volts?  Do you have another device measuring this output?  Or is that range simply listed in the sensor documentation? 

Also, can you please clarify the statement "With the C++ code the results are near 5.4 , 6.8 but after a random number of measurements the measured value remains to 9.9Volt."  How does your C++ code change the readings of the DAQ card?  Are configuring it differently?  Additionally, I am confused by the appearance of 9.9 Volts in that sentence.  Can you clear this up for me? 

Brian F
Applications Engineer

National Instruments   

[geokaf2]

Hallo,

I finally found the problem. As I was expected there was a connection fault. My sensor is a floating signal source (non a grounded signal source). I found that after some connection tests.  So in Diff mode i also connected AIGND with the ACH(-) using a resistor of 100KΩ.  The circuit is given in the attachment file.

Now I am trying to connect Java with C code (using JNI) to take measuremets from a java application!

Thanks for the help and your time!!!

[AnalogKid2DigitalMan]

Thanks for heeding my advice