USB-6259 Analog output ground has offset voltage enclosure sheath.

[shuwei]

Both the USB-6259 and an amplifier (FEMTO DLPCA-200) are installed in one larger box, which is grounded to the power ground.

The enclosure sheath of USB-6259 and amplifier sheath are contacted with the larger box. Which means that the amplifer and USB-6259 sheath are gounded to the power ground.

What we found is that USB-6259 Analog output ground has an offset voltage to the enclosure, which is about 0.06 volts. However, the FEMTO amplifier input ground has the same potential with sheath. In our application, the USB-6259 analog output is used to drive an circuit, which provide output to the FEMTO amplifier. Because this 0.06 volts of offset voltage, the circuit does not work properly with the amplifier.

Question, is this 0.06 volts offset normally existing in most of the USB-6259 box?

Another question, how to solve this problem?

Please advise,

Thanks,

[Speedy_Badger]

Hello Shuwei,

To summarize the situation, you have a USB-6259 and an amplifier tied externally to a box that is in turn tied to the power ground. I have a few questions regarding the power setup of the system.

Are you using an external power source for the USB-6259?

Is the power ground that the outer container is tied to the same as the power ground of the USB?

The USB-6259 has internal circuitry which ties the analog output ground to the internal USB power ground. If you are using USB bus for power then the analog output ground is referenced to the system which is powering it. If you are using an external power source then the analog output ground is going to be referenced to the external power source ground.
 
It sounds like the power ground that the sheath is tied to is not the same as the USB power ground. I would try making sure that the amplifier is tied to the same ground as the actual USB power source to properly match the two components.


Regards,

[changlin]

Hi

I also found that my USB 6259 has the offset problem(0.6V). I check the power ground of my desktop and my 6259. It looks the ground is ok. I measured the output by a fluke multimeter with rectangular wave output from 6259. This offset(0.6V) is always there. Is there any good way to eliminate this problem?

Best regards  

[Jordan F]

How exactly do you have everything connected (what pins are you measuring, are you using external power)?  Also, do you see the same behavior if you loop the analog output back into the analog input on the USB device and view the signal in Measurement and Automation Explorer?

[hegb_85]

I have the exact same problem with a PXI 6723 AO's card. I have an offset of around -0.5V to -0.7V, so when a change the AO value to 2V the voltage that my proportional valve will see is 1.5V.

I found this KB (it's in spanish) but it says that when you create the task you must explicit set the "input terminal configuration" to RSE if you are using this kind of measurement. Is this configuration making a difference?

Thanks!

[hegb_85]

sorry about the last post, it doesn't make sense to have an Analog Output in differential mode it must be RSE by default... any other ideas?

[JustinP]

Hi,

I need to know a little more about your setup in order to answer this question more thoroughly.  You mentioned that you set the output to be 2 VDC and were only outputting 1.5 VDC.  Do you know the resistance of the the load that is connected to this channel?  The drive current per channel on this module is 5 mA MAX and if the load resistance is too low then the circuit will hit the maximum drive current and not get to the 2 VDC. 

Regards,

Justin Petry

[hegb_85]

Yes... the Input Impedance is 4.75kOhms.

Today i re-checked the wiring and I think that is a problem with the grounds and/or voltages.

[JustinP]

Hi,

I am attaching a link to a resource that may help you with any wiring issues that you are running into.  Let me know if you have any questions about this document.

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Regards,

Justin Petry

[JÞB]

See this view 

Use the grounding screw to properlt ground the device.  the USB cable is not intended to be the only path to ground