DAQ NI-USB 6001 Voltage Measurement issue

[RichKnight]

Dear NI Community:

I would like to use 3 NI-USB6001(totally have 12 differential measurement channels) to monitor the voltage of a test rig(12 21700 cells connected in series, each 4V, totally  around 50V). and another USB6001 receive the analog signal from other 3 device and control the power supply.

You can check the configuration in above image.

When I setup the test rig and connected the rig with 3 NI-6001s (3 devices connected on one laptop via USB hub), I got wrong voltage value on some channels(random value).

and if I only connect one USB6001 device on the laptop, all 4 channels measurement are good. but things will go wrong when I connected them together.

And then, disconnected Dev2, Dev3 and Dev4 from USB hub, and only Dev1 connected on the laptop, I still can not get the right data from channels of Dev1.

Could I ask the reason?

Kind Regards,

RichKnight. 

[ZYOng]

Is your test rig signal grounded? Have you tried single ended mode? See Field Wiring and Noise Considerations for Analog Signals

Are you using a Bus-Powered Hub?

[RichKnight]

Hi ZYOng:

Thanks for your reply!

My test rig is not grounded, it is a float signal, And I did not try the RSE mode, I can try in next Monday.

I am also think about if bias resistors should be connected on the AI GND port.

Yes, I am using a bus powered USB hub, 

Kind Regards,

RichKnight

[santo_13]

Warning, ensure that your signals are well within the stated maximum input voltages of the inputs. If those cells are part of battery pack, if exceeded the common mode or differential voltage of your DAQ, you will permanently damage the DAQ.

It looks like you're measuring the cell voltages of a battery pack, the battery closer to the positive end has a higher common mode voltage if measuring differentially. If measurement single ended, the voltage may exceed the 10V measurement limit and hence need a voltage divider to scale it within 10V.

With USB-6001 with a maximum input voltage limit of 10V can measure only 2 cells in series even if you have more channels available.

[RichKnight]

Hi Santhosh:

Thanks for your reply!

This device seems could measure 20V, since the input range is plus minus 10V in differential mode.

And another question, the datasheet shows each channel's input should in the range -10V~10V, so I am confused   😞

Kind Regards,

RichKnight

[santo_13]

It can measure +/-10V with respect to AIGND which is connected to earth through the computer USB port and power supply.

I am pretty sure you're using the DAQ incorrectly for your application and may learn the wrong way by damaging the DAQ.

[RichKnight]

Hi Santhosh:

Thanks for your reply!

I read the spec of the USB6001.

Because I am using the differential mode, so I do not need to share the ground between each channel, and each channel within +/- 10V.

Seems what I need to do is add a bias resistor(10K-100K) between the negative and AI GND. According to https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider... 

Kind regards,

RichKnight

[santo_13]

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@RichKnight wrote:

Hi Santhosh:

Thanks for your reply!

I read the spec of the USB6001.

Because I am using the differential mode, so I do not need to share the ground between each channel, and each channel within +/- 10V.

Seems what I need to do is add a bias resistor(10K-100K) between the negative and AI GND. According to https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider... 

Kind regards,

RichKnight

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Though you're measuring differentially it doesn't mean the common mode voltage can be in kV or MV (for an exaggerated example) though your differential voltage is just under 10V. The signals you measure must satisfy all the constraints.