NI 9205 with cDAQ9188

[ASIM_BHATTI]

Hello all

I am using cDAQ9188 with 03 NI 9205 modules. I have little confusion about analog input connections on NI 9205 module. The manual of cDAQ9188 says that chassis must be connected to ground.It is OK. The Figure#6 of NI 9205 manual seems to show that in differential measurements mode, the negative end of all differential signals be connected to COM pin which is connected with chassis ground of cDAQ9188. Is it right? If yes, it means we are just grounding one end of differential signal?

Similarly Figure#7 shows that in RSE mode, all grounds of input signals are connected with COM pin which is connected with cDAQ9188 chassis ground. Am i right in my understanding?  

Figure#8 shows that all grounds of input signals are connected with COM pin which is short with AISENSE pin and cDAQ chassis ground.

The main confusion is that should we connect COM pin with cDAQ chassis ground or not in all three input modes? As in my case, analog input signals are very weak (few mVolts),

Thanks & Best Regards

[Mike_B_NI]

ASIM_BHATTI,

   According to page 8 in the manual: "The NI 9205 channels share a common ground that is isolated from the other modules in the system" So, this is not the same ground that is tied to the chassis. The COM on the module is used for CMR. 

   In RSE mode, you would tie the COMs of the DUT and the NI 9205 so that module can meaure the DUT with respect to its ground, similar to how you would use a DMM.

  Here's a great white paper that talks in more detail about the different settings and use cases: Field Wiring and Noise Considerations for Analog Signals

  Hope this helps.