Acquiring a 4 - 20mA signal

[Rachael_V]

Is it possible to acquire a 4 - 20 mA output signal using the NI 9205 analog input module on a CDAQ-9172? Or should I best use another type of module?

[Dan_K]

Hello Rachael V,
 
Welcome to the NI Forums.  I am glad you are interested in our cDAQ modules and I may be able to give you some advice on your application.
 
The NI 9205 module is a multiple input range voltage input device.  In order to read the current you are looking for you would have to connect the output to a known external resistance and measure the voltage drop on the resistor.  Any resistance less than 500 Ω should be able to provide a voltage range that would be within the limits of this device.  I would personally recommend a 220 Ω resistor, which would give you a voltage range of 880 mV to 4.4 V.  By specifying this range in your task, the NI 9205 would operate in the ±5 V range, giving you a sensitivity of 0.4 µA (assuming a very low tolerance resistor).
 
Another option would be the NI 9203 module, which is specifically designed to take current inputs.  This would have the benefits of requiring no additional hardware on the input and not requiring any post processing to convert voltage measurements to current values.
 
For more information on the NI 9203 check out it's product page and User Guide:
 
NI 9203 - Products and Services
 
NI 9203 Operating Instructions and Specifications - February 2008
 
I hope this information helps.  If any of this is unclear just let me know and I will try to clear things up.

[Rachael_V]

Thanks Dan_K.....that certainly clears up my questions.

[almo]

using the NI 9203 ther is no solution for the antialiasing (no antialiasing filter inside the module) !! 

[Dan_K]

Hello almo,

That is correct, the NI 9203 does not include an antialiasing filter for input signals. One thing to note is the the front-end bandwidth is 850 kHz, so any signals higher in frequency that that will be naturaly filtered do to the analog circuitry on the front end of the device.

[almo]

beacouse the 9203 has no antialiasing and it include also the resistors(for inverting the current signals into voltage sinals) i cant see where can i put an antialiasing filtter between my sensor and the 9203, can you advise where it possible to integrate an antialiasing filter?

anyway i prefer to use the 9205 and to put the resisitor and the antialiasing by my self.

[ManLongAtNI]

Hello almo,

I'm not sure what your asking. Applying an external anti-aliasing filter would be done independent from the module. The only suggestion would be apply any filter & sensor on a different apparatus and wire it accordingly to the module.

Regards,

Glenn