External bias resistor selection on PXIe-4302 with TB-4302C

[Brian_C_123]

Hi,

• I'm trying to measure 4-20mA current loops from 15 PR9113B 2-channel transmitters (so 30 channels total) using the TB-4302C terminal board connected to an PXIe-4302 DAQ card.  
• Here is a PDF schematic [Dropbox link] of my planned setup (showing only 3 of the 30 channels for simplicity)
• The transmitters are isolated and so as per Table 1 in the NI document "Field Wiring and Noise Consideration for Analog Signals" I believe I need to insert an external bias resistor between AISENS and AIGND.  
• The PR9113B is listed as having a load of <600ohms, which I will assume = 600ohms. 
• Page 2-2 of the PXIe-4302 user manual recommends an external bias resistor of Rbias = load_resistance*100 = 60kohms

QUESTIONS:

• Since I have multiple transmitters using the same external bias resistor, does that affect the resistance of the bias resistor that I need?
• How do I determine the maximum power through the resistor?  Is it negligible?  Or is the worst-case the full 23mA  going through the resistor, Power = I^2*R?  Or something else?

Any other comments on my proposed setup are welcome, I'm very new to 4-20mA current loops.

Thanks!

[rsmayer]

Hello,

I'm not sure how having 30 transmitters would affect the resistance of single bias resistor, or if it is recommended to have multiple bias resistors hooked up like this.

You are correct, you should be able to determine the power through the resistor with I^2*R, and the power through the bias resistor should be negligible but it will depend on the input impedance of the device.

-Ryan

[Brian_C_123]

Ryan,

Thanks for your input. 

I'm not sure how having 30 transmitters would affect the resistance of single bias resistor, or if it is recommended to have multiple bias resistors hooked up like this.

---

I guess my diagram is a bit unclear as to what's on the board and what I propose to add.  The 5ohm shunt resistors are already on the circuit board as purchased stock from NI.  Only the one external bias resistor needs to be added (if needed).  There is even a specific location on the terminal board where you are supposed to add the external bias resistor: "another AISENSE terminal, J21, is paired with AIGND terminal to allow insertion of an external bias resistor between AISENSE and AIGND if a floating external power supply is implemented."

So I'm only going to add one bias resistor, and it will tie all the AISENSE lines to AIGND.

Thanks!