Anti aliasing filtering in DAQ

[malkoba]

Hi

Does the M 6259 multifunction DAQ , has anti aliasing (low pass filtering) for the analog input signals ?

Thanks

Mal

[Dr._Imad]

The manual never mentions anything about an antialiasing filter. So I assume it does not have.

[rpursley8]

The data sheet shows a small signal bandwidth of 1.7 MHz. This probably indicates the presence of an anti-aliasing filter.

[LV_Pro]

Actually anti-aliasing filtering is determined by your sampling rate. To prevent aliasing you need limit input signals to less than 1/2 the sampling rate, otherwise the will appear as a false signal that is the same frequency below the sampling rate as the actual signal is above, i.e. if you are sampling at 100kHZ and have a signal that is a 120kHZ that makes it past any input filtering then you will see a 20kHZ false signal in your output. So you would want a filter that limited signal to below 100kHZ, with either a sharp cutoff below 100kHZ or that rolls off to a very low level at 100kHZ, depending on how important the frequencies near 100kHz are. A sharp cutoff filter is harder (and more expensive) to design and build, but may be necessary if your signals of interest are near your sampling rate.



P.M.

[malkoba]

Hi

I am using a sakpling rate of 100 kHz and my input signal range is between 50 to 5000 Hz. That means I would not have to worry about anti aliasing?

Thanks

Mal

[rpursley8]

You should be fine. But you might want to add a filter after the acquisition to limit the signal bandwidth to 50-5000 Hz. Since the bandwidth is 1.7 MHz, you will have some unwanted out-of-band noise that would be good to get rid of. It would be better to filter the signal before you acquire it, but that would be building up your own filter.

An anti-aliasing filter is designed to prevent someone from sampling a signal higher than the Nyquist limit of the A/D since higher frequency signals will alias into this band and will appear as a lower frequency signal. For this card, the small signal bandwidth is set at 1.7 MHz. This is odd enough that I believe is it set by a anti-aliasing filter as opposed to the analog bandwidth of the A/D converter itself. I could be wrong.

In some cases you want to alias the signal (bandpass sampling), but most of the time you want to band-limit you signal with an anti-aliasing filter to assure that what you get is an accurate representation of the input into the A/D converter.

Message Edited by rpursley8 on 06-13-2005 11:00 AM

Message Edited by rpursley8 on 06-13-2005 11:03 AM

[LV_Pro]

What sampling rate do you plan to use? This boards specs indicate a maximum single channel sampling rate of 1.25 MS/s, which would limit the input frequencies with out aliasing to below 625 KHz, if you are sampling at the maximum rate. If you are sampling at a lower rate (which you would if using multiple channels) the maximum input signal without aliasing will be lower. If you don't filter the incoming signals and they are above your 1/2 the sampling rate range you will get aliasing. Once the aliasing occurs, at the sampling stage, you will not be able to filter it out, unless you know what the original frequency was that caused it, as it will appear as a legitimate signal in your band of interest. You probably won't need anti-aliasing if the possible signals (the ones your trying to see and any that happen along) are below the 1/2 sampling rate value, but if there is the possibility of signals higher than that you very well may need filtering.

P.M.

[LV_Pro]

Apparently I opened mine to type before you sent your reply. As I said, if you are reasonably confident that you won't have signal above about 50kHZ then you are probably ok without anti-aliasing. Depending on what you are connected to will determine that, if your circuit that you are monitoring has a normal 50 - 5000 Hz band range it may very well roll off before the 50KHz, providing the "anti-aliasing" itself. Try it.
Good luck

P.M

[malkoba]

Hi

Thanks for the advice.

Mal