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how to add offsets to the channel

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Say my detector has initial offset of 10mV without any input. Now if i want to reset the signal at 0mV so that I only see the input and not the offset once my detector detects the signal for same channel. any ideas?


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What hardware are you using?

 

The hardware agnostic method is to simply subtract your known offset from your input signal.  Use the Arithmetic step to do this after you have acquired your signal.

 

If you are using SignalExpress to generate your stimulus or start your experiment, you can automate this even further (requires SignalExpress 3.0).  Take a short, untriggered acquisition and use the Statistics step to find the average.  Add a sequence break and run your normal experiement afterwards, using the output of the average in the Statistics step as an input to the Arithmetic step to subtract the offset from your signal.

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Thanks DFGray for your reply. I am using NI 9239 data acquisition board. I am still trying to figure out lot of stuff in signal express. Like my old daq software (IO tech daqview) I could add different offsets at each channel and set all to zero and then do data acquisition. so that even in real time data acquisition I used to see the signal without offset. I was wondering if I could do same in signal express. I saw something in scaling and conversion tab of processing>>analog signals, but I could not add different offsets to different channels at once.

  I was also wondering if I can do processing after data is logged. Tech support on phone suggested me that processing while recording data is my best bet, but I am just curious if I could do processing after I record the data, if yes then how? 

 



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Hi odessy27,

        You can do post processing (after logging the data to file) by using the playback mode.  This tutorial is very helpful and explains in detail how you can go about doing this. Additionally, as long as you create a separate Scaling and Conversion step for each channel, you should be able to add different offsets to different channels.  Have a good one!

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Accepted by odessy27

My apologies for not answering sooner.  I had it open to reply to a week ago, got a higher level interrupt, and never got back.  In any case...

 

Most National Instruments hardware does not offer offset capabilities, so it must be done in software.  SignalExpress usually requires you to do everything explicitly.  This has usability issues, such as the one you currently have (would like to set offsets on each channel individually), but makes up for it in flexibility.  It is almost an old-style UNIX approach to data acquisition.  SignalExpress exposes the raw data acquisition and simple function blocks that you can string together however you wish.  In this case, you have a couple of options:

  1. If your signals all have the same offset, you can use the group output of the DAQ input step as the input to an Arithmetic or Scaling and Conversion step to remove the offset.
  2. If the signals have different offsets (most likely), use a separate step for each.
View the output of the scaling step as you are taking data to see the zeroed data.
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Another option to adding an arithmetic step is to use a Scale.  They can be configured in the Analog Input Acquire step, under Custom Scaling.  Here you can enter a multiplier and offset. 

 

Then if later you need to change a scale, you can note the date/time and create a new scale name, so you keep a clear record of which scales were used for your data.

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