AddWaveformProcessing vs. FetchArrayMeasurement

[MarkL@Hon]

Can someone please explain the difference between AddWaveformProcessing vs FetchArrayMeasurement for subtracting channels for a differential measurement?  I understand that with Fetch... the subtract is done at the point when you actually fetch it.  Is it incorrect to use AddWaveformProcessing?  If either is valid, can you give me scenarios where one should be used over the other?

Thanks in advance for your response.

[Lea_D]

Hi MarkL,

I want to start with the niScope_FetchArrayMeasurement . This command will pull the waveform data from the digitizer. It will return whatever measurement array is specified. The next command you mentioned was the niScope_AddWaveformProcessing. This command will add a measurement to the list of steps to be completed before the actual measurement. 

To get to the section on subtracting channels, go to start » Programs  » National Instruments » NI-SCOPE » Documentation. In the documentation. Select "NI High-Speed Digitizers Help". From here, go to Programming » Array Measurements in NI-SCOPE » Two-Channel Measurements (Array) » Subtract Channels. From here you'll see the C/C++ equivalent of it is NISCOPE_VAL_SUBTRACT_CHANNELS. 

You can also use this help to check out some of the functions that are available in programming » Reference » NI-SCOPE Function Reference Help. Additionally check out this DevZone  for more information on NI-SCOPE and on what fetch is doing.

Hope this helps! 

[MarkL@Hon]

Lea,

Thanks for your response.  Not sure I got this from your answer but please confirm whether I have it right.

I can accomplish the subtraction either way based on my needs.

With niScope_FetchArrayMeasurement, the subtract is done on an already acquired waveform as it is retrieved from the scope.

With niScope_AddWaveformProcessing, the scope is pre-set to do the subtraction prior to the acquisition.  When the waveform is acquired the subtraction is done such that what ends up in the buffer for the active channel is the subtracted waveform?  This way I can leave the waveform on the scope and possibly perform measurements on the already subtracted waveform?

Are these statements accurate?

Thanks again

MarkL

[Lea_D · Accepted solution]

Hi MarkL,

You're welcome on the response. After reading over your description, this is accurate.

You asked for a scenario for each one. So say you're aquiring from channel A and channel B. You could use add waveform processing to make sure that each channel A and channel B goes through a filter. Then you can configure it to do the subtraction so that when you fetch the data, you're no longer doing any sort of measurements on the raw data. You would be doing measurements on the differental signal of filtered channels A and B.

Take the same scenario with the FetchArrayMeasurement. You can configure the command to do the subtraction, but that subtraction would be done on the raw data and you wouldn't get any of the filtering. 

Basically, the AddWaveformProcessing allows you to perform opperations and perform measurements on the already subtracted waveform. The FetchArrayMeasurement will perform t he subtraction on the already acquired waveform just as you have stated.

Hope this helps!