Connect 3 wire sensor to NI 9209 correct way

[Blokk]

Hello,

I have several absolute pressure sensors with three wires (a common wire, an excitation+ and a signal out+). I have attached its data sheet in pdf. I need to connect about 6 such sensors to my NI9209 card inside a cDAQ chasis. I also have a 24 VDC power supply. I would like to ask about the correct connection if I want to use differential voltage measurement. 

Should I just wire up the elements in the following way (left picture), or should I follow the recommended wiring as described in the manual at page 14, the "floating differential connections" part ( http://www.ni.com/pdf/manuals/376909c.pdf ), right picture? Thanks!

Edit:

If I should use the 1 MOhm resistor, is it enough to use a single resistor connected between the COM pin of the NI card and all the COM ports of the pressure sensors?

[Blokk]

Based on this post, I do need a resistor between the COM pin of the NI card and the negative lead of the power supply. If I only use the simple wiring without grounding the com pin, I get somewhat lower voltage than I should (4.5 Volts using a handheld voltage meter, 4.0 Volts displayed by the NI card).

The only remaining question is that whether I need to use separate resistors for each sensor, or I could go with a single one...anyway I am going to test it on Monday, when I get some resistors...

Edit: I think I need to use a single resistor, since if I use multiple, it is just like a parallel network of resistors, so even I get a net resistivity lower. So I think I just connect the COM pin of the NI card to the negative lead of the power supply via a single 1MOhm resistor (the several pressure sensors are connected to the psu parallel obviously).

[Matthew_Kelton · Accepted solution]

Youneed to have the COM input on the AI module referenced to your common.  However, if all of your sensors are running off the same common, you do not need the 1M resistor.  The only reason Ni mentions this hookup is for incoming signals not referenced to the common.  Since this sensor is only giving you a single ended output, it would be much easier to just run the signal outputs to the AI module and connect the COM to the power supply.  Otherwise, you are basically jumping the COM signal to the low side of each measurement and measureing reference to COM anyway.

[Blokk]

You mean I should just use single ended wiring to the AI module, like this way?

[Matthew_Kelton]

Yes.

[Blokk]

Thanks very much for your help! May I have another small question regarding to this config? At the same time, I will need to measure one channel (analogue voltage output) of a MKS vacuum gauge controller ( https://www.mksinst.com/product/product.aspx?ProductID=457 ) too, using the same NI9209 device.

I imagine I should use a dedicated differential input channel of the NI9209, and not single ended for this signal (to separate this signal from the pressure sensors)? As much as I know, I can setup single ended (RSE) and at the same time also differential channels for a DAQmx device? Of course using two "DAQmx Create Channel" VIs... 

[Matthew_Kelton]

I didn't see any specific details of the analog output connection.  If they provide a common, you can choose to tie it to the 24VDC COM, or use the 1M resistor to connect the differential pair to the COM as shown in your earlier drawing.

You can create a task with both RSE and Diff.  You can also RSE all of it and do your own math, or jump the COM to all the low sides of the pressure measurements, and do everything in Diff.

[Blokk]

The manual says the analog output has a 100 Ohm impadence.

Edit: page 59. shows the pins of the analog outputs, there is a common ground...

[Matthew_Kelton]

The 100 ohms can be ignored, I believe.  You need to see if that ground is shorted to chassis ground.  If so, I would go the 1M resistor route, as you may get some line noise into your measurements.  Depending on your 24V supply, COM may be connected to GND anyway.  If not, it may not like the COM being connected to GND.

If the supply is connected to GND, then it doesn't matter and you can just connect the GNDs.

[Blokk]

Thanks again for the excellent help and info, and have a nice weekend!

(i am going to test the config on Monday)