Digitizer V/LSB ?

[mmakster]

Hello,

I am using PXI digitizers (5122 and 5152) with LabView 8.0.  I write the data in I16 binary format to save disk space.  How can I extract the Volts per LSB information to reconstruct the voltages afterwards offline ?  I would like this information to go into a setup file that is written before any triggering takes place.  Also, what I am looking for can be reconstructed from the true range, the gain and the number of bits, so extracting these would be useful, but I am not sure that what I select as range is the actual range (i.e. selecting +-5 V for instance seems to actually give something like +-4.8 V).

Philippe

[BenjaminR]

Duplicate

Message Edité par BenjaminR le 11-07-2008 10:38 AM

[mmakster]

Sorry, but it's not really obvious what is duplicate and where I can find the answer to my question (extracting V/LSB, or gain and range, before a trigger).  Could you be more specific ?

Philippe 

[Coq_Rouge]

This is very simple. You have to find the weight of each bit. Use this formulae (daq range)/(2^n-1). n is number of bits in your converter.  As an example. My old daq-516 card has range equal to +5/-5 volt and 16 bit resolution. Then the formula will give (5-(-5))/65535 as bit weight. Multiply this number with the daq output, and you have the result in voltage

[mmakster]

Actually, it's not that simple.  With my 8-bit 5152 on the +-2 V range, your formula gives 4/255 = 15.7 mV.  Yet the I16 ADU output actually ranges from -32768 to 32512.  If I multiply these values by 15.7 mV, I get several hundred Volts.  There must be a gain someplace, but I have yet to find it.  Any ideas how to access it ?  I can get the V/ADU with Fetch Binary vi, but this implies having to wait for a trigger before getting the information.  I can't believe there isn't an easier way.

Philippe 

[Coq_Rouge]

My formula is correct:manwink: But i think you are using the wrong VI setting then are reading from the 8 bit scope. The "

"niScope Fetch Binary" is poly type. I guess the default setting  is I16. So this Vi works fine on your 14 bit data, but the output from the 8 bit scope is a mess. Select the right setting for the "niScope Fetch Binary" VI by right clicking on it and I guess your system will work fine. Take a look at this document

http://www.ni.com/pdf/manuals/371134a.pdf

[DFGray]

Most of the NI-SCOPE read VIs have a wfm info cluster which contains the scaling information you want.  Note that this is calibrated, so will not perfectly correspond to the "ideal" situation you list above.

[mmakster]

Thanks for your answer.  Yes, that is the scaling information I want, and yes I can get it from read VIs (like the wvfm info in niScope Fetch Binary), but isn't there some way to get the info before doing any reads ?  I would like to save the information before the triggering starts.  I guess I can just set up a bogus immediate trigger, do a bogus read, save the scaling info, then set up the real trigger I want, and do the acquisition, but that seems sort of complicated.  Calibrated vs ideal isn't an issue at this point.

Philippe 

[Coq_Rouge]

You can wrap your data,scaling and time stamp info in a cluster and save it as a datalog file. This way you can reconstruct your data 100%. If your cluster is pure numeric it will have very little overhead.

[mmakster]

I have to comply with an existing data format, so I don't think that would work, unfortunately.

Philippe