5105 anti-alias filter

[hankypanky]

Hello, I was wondering if the 5105 has a built-in configurable anti-alias filter...  From the spec, I can see that it has a 24Mhz anti-alias filter, but does this mean its bandwidth (24Mhz) is hard-coded, or is it configurable?? Ideally, I want to capture data, passing all frequencies below an arbitray max frequency (and stop all frequencies above it).  I've tried setting maxInputFrequency to 10k and using the niScope_ConfigureChanCharacteristics() function, but simply changing maxInputFrequency this way appears to have no effect (the higher frequencies go unfiltered).. do I need to do something else??

Thanks...

[Matt_E.]

Hello,

The 5105's anti-alias filter is hard wired to 24MHz.  This frequency was selected based on the sample rate of the ADC with the internal oscillator.  The maxInputFrequncy variable is used to select which filter on an NI digitizer is used.  Some digitizers provide an anti-alias filter and a 20 MHz noise filter so to select the noise filter you would set maxInputFrequency to 20M and for the anti-alias filter you would set it to whatever the cut off frequency for your particular board is (24M in the case of the 5105).  To select no filters you can either enter the full bandwidth -3dB frequency or -1 which will look up the full bandwidth for your board.

The 5105 has no noise filter so your only options are 24M and -1.  If you enter anything else the value is coerced to the next highest value.

If you are using the digitizer in the 50 ohm input mode you can buy external BNC 50 ohm filters to get the cut off frequencies you need.  It is possible to buy variable 50 ohm filters but they are difficult to build and can be expensive.

You could also consider applying a digital filter to the fetched data from your digitizer.

Hope this helps

Regards,

-Matt