wring of NI 9205

[Qingqing_Liu]

I am using NI 9205 to capture 0-10 V signal. For the Connecting Grounded Differential Signals, the diagram shows that the negative terminal (-) needs to connect to COM. Do I have to connect the negative terminal (-)  to COM? If I do not connect it to COM, what will happen? 

[nanocyte]

The real requirement is to keep the common mode voltage under 10.4V.  

I'll just quote the user manual as I think it answers your question

To connect floating differential signals to the NI 9205, you must
connect the negative signal to COM through a 1 MΩ resistor to
keep the voltage within the maximum working voltage. If the
voltage source is outside the maximum working voltage, the
NI 9205 does not read data accurately.

https://www.ni.com/docs/en-US/bundle/ni-9205-getting-started/page/overview.html

[Qingqing_Liu]

Can I connect the COM to ground but not to the negative side -?

[nanocyte]

You want to connect COM on the 9205 to GND on your sensor? Yes, that will very likely work assuming your sensor has a <= 1MOHM impedance between GND and -.

[Qingqing_Liu]

I have a 24 DC power supply. Can I connect the COM on NI 9205 to the ground terminal of the 24 DC power supply?

[nanocyte]

Are you talking about earth ground or the negative output from the supply? It's more common to connect the 9205 COM to the negative output than the earth/chassis ground. But you haven't really supplied a wiring diagram so I can't really say if what you're doing is optimal.

[Qingqing_Liu]

I put the diagram in the attachment, which is from the NI 9205 manual. You can see that the COM is connected to the negative (-) and the earth ground. Can I only connect the COM to earth ground but not the negative (-)?

[nanocyte]

I was looking for a diagram that included your power supply and sensor. I'm assuming you're not directly measuring the voltage on your power supply

The negative in that diagram is not the negative of your power supply, it's the negative of your sensor. Normally, if a sensor is outputting a differential signal, it has two outputs, a + and a - and the power supply - would be separate and go to your COM.

Check this out and see if it makes sense:

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

[Qingqing_Liu]

In the diagram, the sensor (V1 in the diagram) is connected to AI0 and AI8, also the AI8 is connected to the COM and earth ground. can I only connect the sensor (V1 in the diagram) to AI0 and AI8, but not connected to COM and earth ground?

[nanocyte]

Are we looking at the same diagram? AI8 is not connected directly to COM. There's a Vcm in between right? I can't tell you if you can connect COM to earth ground because I'd have to fully understand your system. It might be fine, or it could damage the 9205, or it could just make your measurement inaccurate. The symbol in the 9205 diagram is a chassis ground, not an earth ground.  I can tell you that it's somewhat uncommon to connect com directly to earth ground so I wouldn't do that unless you had a good reason for it.