NI-9253 Differential Current Input with FlexLogger?

[amoe1006]

Hello everyone, 

I am currently using an NI-9253 in order to read a current signal from a pressure transducer.  I've read the getting started guide, and while they show the wiring diagram for a three-wire circuit, mine is essentially a four-wire circuit. 

My current wiring setup is - 

Power: Vsup (External power source connected)

Gnd: COM

Signal (+): AI0

Signal (-): COM

Currently I get readings of 12mA to 20mA, half of the full 4mA to 20mA signal.  I've also tried wiring this up like the Loop-Powered circuit with Sig+ to Vsup and Sig- to AI0, same result.  Is there anything else I can do with the wiring, or setup a differential reading between AI0 and AI1 for example within FlexLogger to get the full 4mA to 20mA signal I am after?

Thanks!

[santo_13]

Were you able to make sure that these faults did not occur?

Also, would you be able to share the transducer datasheet if there is any specific way the Sig+ and Sig- needs to be connected?

[amoe1006]

I've confirmed that none of these faults are present during testing.   The relevant part of the data sheet for wiring is below -

[santo_13]

I am not completely sure, but I have a hunch that the common ground connection to the card is causing the issue. From the connection diagram from the sensor specification, it looks like the current loop may not be ground referenced.

I would try to remove the power supply connection to 9253. So the connection would be only the ORANGE and BROWN to AI and COM. VSUP will be open and Power Supply directly connected to the sensor.

To be on the safe side, alternatively, you could connect a load resistor instead of the 9253 and measure the voltage on BROWN wire with respect to Power Supply GND. This would say if the current loop receiver needs to be ground referenced to the power supply or not.

[amoe1006]

I believe you are correct in that it is due to the common ground.  I tried powering the sensor directly from the power supply and running the leads to AI and COM, but got the same result.  

I'll try measuring the voltage next, but at this point I'm thinking it might just be a hardware limitation of the 9253.  

Thanks!

-Aaron

[santo_13]

Another experiment would be to place a multimeter in series and compare the current measurements with the instrument.

If the DMM measurements match the instrument, the next experiment could be using a load resistor and measuring the voltage/current using DMM.