USB 6210 noise from bus power and external power supply

[mmiller]

I am measuring an analog DC voltage using a USB 6210. Using a digital multimeter on the screw terminals, I measure the signal I expect from the source. However, from the DAQ device to the computer I am picking up a large amount of periodic noise that seems to be coming from the combination of multiple different power supplies. From visual inspection the noise seems to have at least 2 periods. 

I have removed my computer's A/D power adapter, since it does not have a grounded shield (2-prong instead of 3-prong cord). This removes a large quantity of noise.

Since there is no ground through the power supply, I have attached the computer chassis to the building ground through a power strip, which does reduce the noise amplitude further.

Shielding the DAQ device and the USB cable with aluminum foil and connecting the foil to the building ground through a power strip also reduces the noise considerably, but not satisfactorily for my application.

Moving my mouse causes the noise to increase in magnitude. This makes me think that the power being supplied to the DAQ device from the computer through the USB is causing the problem. Is there some way that drawing power to the mouse is changing the signal to the USB? Is the USB that was sent with the USB-6210 shielded?

Is the DAQ device supposed to be connected to the earth ground through the computer? Or is there another way to connect it to the earth ground through one of its ports? I am measuring a floating signal in differential mode, so my negative terminal and the AI ground are connected.

[Brian-G]

Hi mmiller,

For differential measurements, the negative terminal should not be connected to AI GND.  You will need to utilize two AI terminals for the differential configuration, one for the positive lead and one for the negative.  If you have the positive lead connected at AIX, you will need the negative lead connected at AI(X+8).  For example, if you are using AI1 for a differential measurement, you will need to use AI1 and AI9 for the positive and negative lead, respectively.  Please refer to the 621X User Manual for more information about the measurement types.  Let us know how it works!

Brian

[mmiller]

Hi Brian,

I am following the connection diagram shown in that manual on page 4-24 for a floating signal source. I do not understand how I would connect the negative lead at AI (X+8) since I only have 7 terminals (AI1-7). Can you explain further?

Also, can you clarify whether the AI GND should be attached to earth ground? Or is it trying to use the same ground as the computer as a reference? It is not clear from the wiring diagrams.

[mmiller]

I just looked at the pinout chart again. I do have my positive lead in AI1 and my negative lead in AI9. They just happen to be next to each other and have some confusing stickers on them, provided by NI, that label them AI1+ and AI1-.

Can you still help me understand what AI GND on the DAQ device is referenced to?

[Brian-G]

Hi mmiller,

The AI GND terminal on the USB-6210/11/12 devices is non-isolated, meaning it shares the USB Ground.  Depending on your computer then, AI GND is probably referencing it's chassis ground.  You want to then make sure that you are using the guidelines under "Using Differential Connections for Floating Signal Sources" on page 4-27 of the 621x User Manual.  I also suggest grounding the USB chassis to the same point as you are grounding your computer chassis as demonstrated on page 3-4.  Let us know how it goes with these suggestions.

Brian

[mmiller]

Hi Brian,

I do not have a BNC device, which is what is shown on page 3-4. On my screw-terminal NI USB-6210, there is no nut that is clearly marked as the chassis. Can I accomplish this by attaching AI GND to earth ground?

[Brian-G]

Hi mmiller,

Right now your AI-GND is referencing the USB ground on your computer.  If the USB ground is shared with the computer's chassis ground, then they should have the same reference point at earth ground. If not though, I think it may be helpful to connect the AI GND terminal on the USB-6210 to earth ground also.

Brian