Why is the Analog signal from the FPGA is distorted?

Hi,
I am using the PXI-7831R board with LabView 2010. I'm trying to transmit TDMS files from the computer hard drive via the analog output of the 7831R. I've used the "Basic DMA output - R series" example and replaced the generated signal with a TDMS file. When I tried it on a synthesized pure sine file, it transmitted the positive part of the sine wave (which was a little distorted) but instead of the negative part I saw 0V DC. When I tried to use a "real" TDMS file (that I sampled earlier) the signal came out completely distorted. In both cases I used a graphic output in the host VI and checked that the data coming from the file is ok. Can anyone help me with that?
I would also appreciate some explanations on the "Update Rate" block (I suspect that he's the one causing all my trouble) - how am I supposed to use it and is it mandatory?
Thanks in advance.

It sounds like when you are using the sine wave, you are clipping the data. I am assuming that this is because you are sending it negative values. R-Series cards can not handle fixed point values, so try to offset the signal so that the lowest point of the signal is at 0. See how that works for you. As far as the Update Rate goes that controls the speed of your loop. If you do not set it high enough, the loop will simply run as fast as it can. I hope you find this helpful.
Brandon Treece
Applications Engineer
National Instruments

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Why is the Analog signal from the FPGA is distorted?

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