Input channels displaying 10.5 V DC on SCB-68A connected with USB-7855R

[nde88]

Hi All,

I want to acquire a voltage signal. I have a NI USB-7855R connected to a SCB-68A board. I am using the subVIs and FPGA personalities in the Balanced I/O zip file, found here: http://www.ni.com/example/5961/en/

I am using differential connections, an the single bias resistor config. as instructed by the SCB-68A manual:

I have compiled the code for DIFF input on all input channels. Channel AI 0+/0- works fine, and I can acquire a 100 mV pp voltage signal on it, which seems correct.

However, Channels AI 1+/1-, AI 3+/3- and AI 4+/4- do not seem to work, as a DC voltage of 10.5 V is displayed on the waveform graph as soon as I connect a source.

Could it be an incompatibility between the terminal board and the FPGA? Any ideas, fellow NI'ers?

Thanks in advance for your time.

[Sam_Sharp]

I suspect that either:

1) There is a connection issue and the input signal is floating high

2) The signal is outside of the common-mode voltage range (e.g. due to lack of bias resistor) or the AIGND is not connected.

Of course, 10.5V is the maximum voltage of the input so the overvoltage protection is kicking in.

[nde88]

Hi Sam_Sharp,

Thanks for your reply.

1) It is unlikely that the bad connection is human error, since I connected to AI 4+/4- the same source that was connected to AI 0+/0- (which worked fine) and the problem persists. I am suspecting there may be an internal connection problem (inside the board), but I hope not.

2) The signal is in the mV range (sensing a weak magnetic field). I connected a bias resistor and it didn't make a difference (input still at 10.5 V DC) - Also tried connecting my Agilent function generator output; still no success. (Note:  the working channels do not have bias resistors and for some reason the signal looks as expected)

Regarding biasing using resistors, I found two semi-conflicting instructions. In the USB-7855R manual, it advises to connect two resistors:

While in the SCB-68A manual, it advises to connect using only one. Which one is best?

I appreciate your help in this. I would really like to get this working as I have been struggling with this issue for quite some time.

[Sam_Sharp]

As a test, have you tried shorting the channels (+ve and -ve) to AI GND? If they still show 10.5V even when connected to AI GND then you definitely have a board problem - you should see 0V with very little noise.

[johnsold]

Are your signal sources truly floating? I wonder if you have a ground loop or some excess common mode voltage.

Measure the voltage between your sources and AI ground with and without connecting them to the DAQ device. Use a battery operated multimeter, not a DAQ device for the measurements.

Lynn

[nde88]

Hi Sam_Sharp,

I did what you suggested and I still see 10.5 V; I guess the board has a problem. What do you think I should do? (I suppose I should contact NI now).

Also, on the SCB-68A Quick reference label it says "M Series and X Series Devices" - shouldn't they have also put "R Series" since they ship this with the USB-7855R as well?

Thanks again.