Anti Aliasing Filters for (USB-) DAQ Devices

[Klaus_M]

Many users (such as our customers) expect from devices in the mid-price segment like the USB-62xx family a proper input signal handling adapted to the selected input sampling rate.

Proposal:

Include anti-aliasing filters in mid-class hardware,such as the USB-62xx family. As an immediate action, please include at least a warning remark in the user manual of the devices.

Every application with variable sampling rate needs appropriate and adaptable input signal filtering.

Many NI DAQ devices do not contain anti-aliasing filters corresponding to the sampling frequencies (e.g USB-62xx family).

Signals containing higher frequency components than nyquist frequency will be folded to lower frequencies causing wrong spectrum information.

Many applications need different sampling frequency settings but use the same external hardware. In these cases, hardware including filters have to be designed for the highest possible frequency. This situation leads to unrecoverable errors in the frequency spectrum, if input signal components do not meet the nyquist criteria.

Thanks

Klaus

www.rfbeam.ch

[johnsold]

While this might be convenient, I can see ways where it might create more problems than it solves:

1. Having a fixed filter chosen for the maximum sampling rate of the device only works when the device is operated in single channel mode and at the highest rate.

2. How should NI decide what filter characteristics to use? Filter type, number of poles, ... The worst case filter would probably add a considerable amount of expense but would be overkill for a large majority of applications of the device.  And it would not be appropriate for use at lower sampling rates.

3. Users who do not understand aliasing (and there seem to be quite a few of them), might be fooled into thinking that they did not need to worry about this if a built-in filter existed.

4. Adaptable, non-sampled data filters are not easy to design and build, especially if they need to be tuned over a wide range as would be required to implement this idea.  Sampled data filter, which can be quite adaptable, have the same aliasing issues as the A/D converters.  They cannot be used as anti-aliasing filters.

5. Filter settling time would be added to the multiplexer switching and settling time.  This might significantly reduce the maximum achievable sampling rates for some multiplexed systems.

6. For some applications intentional undersampling is used.  Any device with built-in filters would need to have an option for disabling the filters for such applications.

Clear documentation of the analog bandwidth and settling time and an indication that no anti-alias filter is included is desirable.

Lynn

[Chon.Mech]

I like this idea a lot.  I spend a lot of my time designing Anti Aliasing filters for my NI-6221s.  A built-in filter would not only save me a tone of time and money but also open the possibility of controlling the filter programmability as part of the task.    Just think how awesome it would be to set the Sample rate, and the Filter frequency from the same place.    

[EngYasir]

HI

I have  NI USB-6251 and I have to use maximum sampling rate 1250 Khz how i could put anti-aliasing filters? help please

 also I have NI 9234 to receive a signal from the 4-Channel with sampling rate 50 khz for each channel should I put anti-aliasing filters in this case or not?
thanks