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Selecting analog cutoff frequency for NI-USB 6251
pinguim
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07-16-2013 11:39 AM
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Hello everybody!
-I´m trying to acquire voltage analog signals by using a BNC 2110 and a bord NI USB-6251. Does anybody know if a programmable analog lowpass filter is available for NI USB-6251? If it is available, how can I set a value for the analog lowpass filter on labview?
thank you in advance,
best regards!
John_P1
Trusted Enthusiast
07-16-2013 12:51 PM
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07-16-2013 01:46 PM
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Thank you for this information John Passiak!
-If there is no programmable analog lowpass filter for NI-USB 6251, so I will need to use the standart cutoff frequency "fc" of the analog lowpass filter (fc= 750 kHz, as mentioned in the user´s manual) for acquiring my signals. By doing so, it will be necessary to use a sampling rate of at least 1,5 MHz to digitize the signal, in order to avoid aliasing! The maximal sampling rate "fs" available is fs= 1,25MHz (according to the user´s manual). Besides, the duration of the signals which I need to digitize take about 25s. That would lead to a considerably higher number os data, and problems to open and save the signals in Labview! How can I solve this problem?
thank you again for your help!
best regards!
LarsR
NI Employee (retired)
07-18-2013 02:26 AM
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Hi pinguim,
You could use a Producer/Consumer design in your application.
http://www.ni.com/white-paper/3023/en/
Regards,
Lars
07-18-2013 06:41 AM
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Hi Lars,
-Thank you for the tip! A guess this tip could be a possible solution for handling large amount of acquired data, but would not solve the problem regarding aliasing. What would you say about it?
Best regards!
LarsR
NI Employee (retired)
07-18-2013 07:15 AM
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Hi pinguim,
to avoid aliasing you should build a hardware filter at the input of your device. You can't avoid aliasing due a digital filter after A/D conversion.
Why Should I Use Anti-Aliasing Filters?
http://digital.ni.com/public.nsf/allkb/68F14E8E26B3D101862569350069E0B9
Anti-Aliasing
http://letfdaq2.drupalgardens.com/content/anti-aliasing-ni-article
Regards,
Lars
07-18-2013 12:56 PM
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Hi Lars,
-Thank you a lot for the links!
-I know about the problem regarding Aliasing! That´s exaclty the reason why I am looking for a programmable analog lowpass filter for NI USB-6251, as described on my first post!
best regards!
LarsR
NI Employee (retired)
07-22-2013 05:58 AM
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Hi pinguim,
such filters are not available for the NI USB-6251.
Either you build your own low pass filter or you buy other hardware with implemented programmable analog filters.
Regards,
Lars
