Re: DAQCard Antialiasing

On Mon, Sep 23, 2002 at 04:53:05PM +1200, David Antliff wrote:
> 
> I am using a National Instruments DAQCard-AI-16XE-50. When using the
> comedi drivers to acquire data, for example across 16 channels at a
> combined sampling rate of 16kHz (1kHz per channel), does the DAQ card have
> a built-in anti-aliasing filter that is applied to each channel with a
> (realisable) brick wall characteristic out to 500Hz? And if I increase the
> comedi sampling rate to, say, 1200Hz, does the filter adjust (to 600Hz)?
> 
> Or does the device have no usable front-end antialiasing (or perhaps just
> a fixed antialias filter at something like 20kHz?). I realise it's
> important to consider aliasing anyway regardless, but this is something
> I've been wondering about for some time.

That particular device has an input bandwidth of 39 kHz - 69 kHz,
depending on the gain setting, as listed in the Specifications
section of the user manual.  IIRC, I've measured that it has a
roughly 6 dB/decade rolloff above that point.

That input bandwidth is actually rather low for a National Instruments
board.  The 6052E, a similar but newer and faster board, for example,
has an input bandwidth of 500 kHz, which is 50% higher than the maximum
sampling rate.  This makes it possible to (somewhat accurately) measure
signals in the 2nd and 3rd Nyquist bands, which I've used on a number
of occasions.

As an aside, this feature also makes it possible to use a data
acquisition board to listen to AM radio stations, after some
demodulation.  It's certainly very easy to see the carrier signals
in Fourier transforms of inputs.  (AM radio in the US is 540 kHz -
1600 kHz.)

So if you want analog filtering, you need to do it yourself.



dave...

Received on 2002-09-23Z18:54:37