FPGA only acquires positive part of signal

[nde88]

Hi All,

I have the NI USB-7855R and using the Balanced IO Example DAQ Personality (https://forums.ni.com/t5/Example-Code/Balanced-I-O-Example-DAQ-Personality/ta-p/3996455). I connected my Agilent waveform generator to AI0 (in RSE mode) and I have the following problems:

1) Waveform graph in LabVIEW is only diplaying the positive part of the sine wave I am supplying to the board (clipping at 0 V). Only by offsetting the signal on the waveform generator I am able to obtain both cycles.

2) While I am outputting 5 Vpp on the waveform generator, on the waveform graph in LabVIEW I am only seeing 1.5 mVp.

Any suggestions on what could be the problem?

Thanks in advance or your help!

[nde88]

Anyone? I just uploaded the project file, however I have removed the digital part and counters part of the example personality VI, as I don't need them.

[ToeCutter · Accepted solution]

The first graph 'analog input' in your link shows the analog inputs being converted to unsigned integers (U32) before passing to the FPGA->Host FIFO. Obviously negative values will be lost making this conversion.

[nde88]

Thanks, what you are saying makes sense, but I can't see exactly where that is implemented?

By the way I right-clicked on "AI FIFO" within the project, and changed the data type from U32 to signed FXP, but I did not see a difference.

[nde88]

OK, I just realized that the conversion occured in the bitfile VI. I removed the U32 conversion bit and now it all works as it's supposed to! Thanks!