Trying to use the same ground for more than two channels (RSE configuration) with usb 6000

[mperaki]

Hello,

I'm currently trying to set up a LabView code in order to get readings of voltage using 5 different locations simultaneously. I want to only use the first location as ground and obtain 4 voltage measurements between the ground and each one of the other 4 channels. I'm using a usb 6000 which for now allows me to use the same ground for only two channels. Is there a way to use the same ground for 4 channels?

Thank you,

Maria

[Dennis_Knutson]

That makes no sense to me. You have as many as 8 single ended inputs and all would share the same ground reference that can be connected to 1 or all 3 pins marked as ground. At first you say you want 5 channels and then 4. Which is it?

[mperaki]

I have electrodes placed in 5 locations but one of them will be connected to the ground pin and the other 4 to the analog channels. The problem is that the technician told me that I shouldn't connect the same ground reference to more than one pin. Is that true? You have to understand that this is the first time I'm using a DAQ that's why I'm so confused.

Thank you

[Dennis_Knutson]

Having an electrode connected to ground does not make any sense. You are saying you don't want to measure that sensor. Do all sensors share a common ground. I think you should attach a schematic because what you are saying is confusing to me.

[Dennis_Knutson]

If you have 5 sensors that are supposed to output something like a 0-5V signal, then the common (gnd) of all sensors would be connected together and then to the gnd of the DAQ. The 5 outputs of the sensors would be connected to ai0 through ai4 of the DAQ.

[mperaki]

You're right it is confusing the way I describe it so I ahve attached some pictures of the set up. The first figure is the cell filled with soil and the 5 probes inserted in it. The anode is placed at the left end of the cell and the cathode at the right end but they do not appear in the figure. I'm trying to use location 1 as gournd and take measurements of voltage from the following pairs: 1-2, 1-3, 1-4, and 1-5. The second figure is the DAQ I'm using. When I was trying to set up the RSE configuration, it was ussing ground A for the channels 6 & 7 (locations 2 and 3 in figure 1) but ground B for channels 4 & 5 (locations 4 and 5 in figure 1). What I am asking is which is the best way to use the signal from location 1 as the ground for all 4 channels. Is this more clear?

Thank you for your time!

Maria

[Dennis_Knutson · Accepted solution]

The images don't explain anything. You need to explain what the sensors are. I think you are totally wrong about how they should be connected. With RSE, all sensors should be at a common ground and the outputs going to individual channels as I already explained. If you can't do that, you probably can't use rse and you have the wrong DAQ device. If you want one side of a sensor as a reference, you need differential connections.