Floating analog Output/input

[alfredph_wmx]

I'm using BNC-2090 breakout box for controlling equipment from computer. Both the output and input signals need to be floating. I have two questions here.

1. the manual says in order to use DIFF input to make signal floating, a resistor should be connected to AIGND. Is AIGND "Analog Input Ground"? Is it connected to the building ground through the power supply of the breakout box? Looking at the shematica drawing of internal circuit, there's "COMMON" that is connected to the outer shield of the female BNC connector. Is the "Common" connected to the building ground?

2. For the floating output, I'm not sure if this device is able to take command from the labview program and output floating voltage.

Hope someone here will kindly answer these questions.

Thanks!

Alfred

[Sean_C.]

Hello Alfred,

I am a little confused about your question. You say that you need your signals to be floating. But you cannot make a signal float by configuring your DAQ device. A floating signal simply means that there is no reference to ground at the signal source. Examples of floating signal sources are batteries and thermocouples. When you measure a floating signal source in differential mode, you need to connect a resistor to AI GND (Analog Input Ground) so that the ADC has a path to ground through which to bleed off any built up charge.

"Is AI GND "Analog Input Ground"? Is it connected to the building ground through the power supply of the breakout box?"
Yes, AI GND is Analog Input Ground. It is connected to the building ground, but through the PC's power supply, not the breakout box power supply.

"Looking at the shematica drawing of internal circuit, there's "COMMON" that is connected to the outer shield of the female BNC connector. Is the "Common" connected to the building ground?"
This depends on your measurement mode. If you are measuring in differential, then the "common" line of AI CH0 is routed to AI CH8. In NRSE mode, the "Common" lines of all channels are routed to AI SENSE (which is not connected to building ground). In RSE, all "Common" lines are routed to AI GND (which is connected to building ground).

Please see Chapter 2 and Figure 3-2 in the BNC-2090 User Manual for more information about this:
http://www.ni.com/pdf/manuals/321183a.pdf


"2. For the floating output, I'm not sure if this device is able to take command from the labview program and output floating voltage."
I am not sure it makes sense to output a floating voltage. You certainly cannot configure LabVIEW to do this. When you generate an analog output voltage, a voltage is generated on the output channel, referenced to AO GND. If you really wanted to, you could generate an output voltage on 2 AO channels then reference one to the other for a true "differential" output. For example, output 1V on AO CH0, and 6V on AO CH1, then measure the voltage difference between AO CH1 and AO CH0 to be 5V. Is there any reason you want to do this? Will a single analog output referenced to AO GND not suffice?

Regards,
Sean C.

[alfredph_wmx]

Thank you Sean!

The reason I need floating output is that i'm using external voltage to control a power supply which will in turn supply large current to a superconducting magnet. It is from power supply's manual that the external control voltage should be floating.

Also for the input signal. Basically it's to monitor the induced emf induced during ramping up or down the current through the magnet. In this case, I don't want any touch to building ground.

Alfred

[Sean_C.]

Hello Alfred,

The only way to achieve a true floating Analog Output signal would be with an Isolated Analog Output signal conditioning module. The SCXI-1124 or the SCC-AO10 can both provide isolated voltage generation. Please see the following links for more information on these two products:

SCXI-1124:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/1870

SCC-AO10:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/12044

Please let me know if you have any further questions.

Regards,
Sean C.

[AnalogKid2DigitalMan]

A much cheaper option than SCXI may be a 5B49 module NI P/N 777234-05. Will need the SC-205x breakout and 5B01 backplane.