scb-68 connection to the building ground

[aramis78]

Dear NI experts,
I need to measure the light emission of a laser by means of a photodiode. I have a standard desktop PC connected to a DAQ acquisition board by a USB cable. Then the DAQ is connected to a SCB-68. First question: I must perform a differential acquisition or a single referenced aquisition with a photodiode? Second question: if I chose a single referenced acqisition I need to connect the SCB-68 case directly to the building ground? Or the System PC+DAQ+SCB-68 is already connected to the ground? Sorry for my elementary questions but I not very familiar with electricity...
Thank you so much
Best regards

[AlessioD]

Hi,

If you have less than 8 signals, you should use Differential mode. This mode does not care about grounding. The only thing that you may want to do is to connect a 100 kOhm resistor between (-)terminal of your AI channel and GND, so that the signal is biased.

If you are going to need more than 8 channels, then Single Ended is the way to go for your connections.

- Referenced> use it if your sensor is connected to ground and the ground is the same as the ground for your PC
- Non Referenced> use it if your sensor is connected to a ground different from the ground of your PC or if it is not grounded.

Cheers,

AlessioD

[aramis78]

Thank you Alessio.
There is a little final question. My pc plug-in has two pole: I think the first connected to the (+) and the second connected to the (-) of voltage city network. The same is for the DAQ transformer. So my question is: where is the building ground connection?
Thanks
Bye
Moreno

[AlessioD]

How many signals do you need to measure?
Less or equal to 8 --> use differential mode and you do not have to worry.
More than 8 --> Use Non referenced Single Ended (negative pole of your signal goes to AISENSE pin) and you are fine anyways.

AlessioD