DAQ device- floating or grounded signal?

[poli]

I am using a grounded constant current power supply to send a constant current of 50mA through an unknown resistor and i want to measure and display the voltage drop across this resistor using my NI DAQ device- 6020 E (BNC) model. The constant current power supply is grounded because it is plugged in to the building ground through wall outlet. My question is- is the signal i am tring to measure ( voltage drop across resistor) a grounded signal or floating signal? According to the signal grounding type, my DAQ device has to be configured and so i must be sure about it.

[AnalogKid2DigitalMan]

Just because the supply is plugged into a grounded outlet does not mean that the current source is referenced to ground.

What range of resistances do you intend to measure across?

You probably need to use a differential input configuration.

[PBear]

Hi Poli,

Analog Kid is right, you don't know if your current source and DAQ board on the same ground plane. But simply using a differential input configuration will remove the problem of having different ground. You could probably get away with using RSE or NRSE and not have a problem. With differential, you're measring a voltage at either end of your resistor, each referenced to the DAQ ground. The values are simply subtracted and you will have your measurement and not need to worry about ground loops. Also, the voltage across the resistor will be referenced to what ever circuit it is involved in. If it's simply jumped across the current source, it will probably have the same ground as your current source and possibly your computer/DAQ board and you could get away with RSE or NRSE. Take a look at this DevZone article.

Field Wiring and Noise Considerations for Analog Signals

[poli]

so, if i understand correctly i just need to use differential mode and do not bother about floating/ grounded mode for the input signal. At the analog input channel , on the daq6020 E(BNC)  there is a switch that must be switched to floating/ grounded - so i need not bother about this switch at all if i use differential mode- is that correct?

[PBear]

Hi poli,

This switch does change your circuit so it is something that you should pay attention to. Look at the bottom of page A-26 of your user manual for a good description of what this switch does. I'd tend to say that you'd want to select FS but I'm not completely sure what your circuit entails and GS may work for you as well. Check that section out and give each a shot to determine which gives you the best results. The FS mode gives you a bias resistor to AI GND which is recommended to for many differential connections.