filtering an analog signal

[4268]

i am taking voltage readings as analog signals and was wondering do i need to filter the signals that come from the daq assistant. i tried smoothing the signals but this did not match the values that i should be getting is there any other filter signal that would be suitable or do i need to filter my analog signals at all as they are reasonably accurate as they are but was wondering would filtering improve the accuracy that much

[johnsold]

The only filtering which is really necessary for accuracy is the anti-alias filter.  This must be done in hardware before the signal goes the the A/D converter.

What kind of signals do you have? What is your sampling rate? How much noise or interference is present?

As I said in my response to your other post, please post some code with typical data included.

Are the problems in the two posts related?

Lynn

[4268]

i have 100 samples to read and a 1k frequency. how do i put in an anti alias filter in the hardware. the values shown in the text file range from 86.001 and -0.005 up to 0.062 when no data is being recorded. so is there any need to use filtering as the values recorded are reasonably accurate

[RER]

Hey Guys!

AC-class signals, such as vibration, often require antialiasing filters. Like standard noise filters, the antialiasing filter is lowpass; however, it requires a very steep cutoff rate, so it almost completely removes all signal frequencies that are higher than the input bandwidth of the device. If the signals were not removed, they would erroneously appear as signals within the input bandwidth of the device.

But not all NI DAQ devices will have antialiasing filters built in. Devices designed specifically for AC-class signal measurement – the NI 455x, NI 445x, and NI 447x dynamic signal acquisition (DSA) devices, the NI 61xx simultaneous-sampling multifunction I/O devices, and the SCXI-1141 module have built-in antialiasing filters.

Ultimately, anti-aliasing must be carried out in hardware. But the time you have digitised your signal, it is too late to remove aliased signals. However, general noise removal filters can be implimented post-acquisition (i.e. in software).

Have you looked at the Filter Expres VI? Or, if that is not flexible enough, have a look at the low level filter vis, which can been found under the signal processing pallette. They are available in either array manipulation or point-by-point flavours.

You may not require them, but for advanced filtering, you might want to have a look at the "sound and vibration" toolkit.

https://www.ni.com/en-us/shop/product/labview-sound-and-vibration-toolkit.html

Best wishes,

[RER]

Hey Guys!

AC-class signals, such as vibration, often require antialiasing filters. Like standard noise filters, the antialiasing filter is lowpass; however, it requires a very steep cutoff rate, so it almost completely removes all signal frequencies that are higher than the input bandwidth of the device. If the signals were not removed, they would erroneously appear as signals within the input bandwidth of the device.

But not all NI DAQ devices will have antialiasing filters built in. Devices designed specifically for AC-class signal measurement – the NI 455x, NI 445x, and NI 447x dynamic signal acquisition (DSA) devices, the NI 61xx simultaneous-sampling multifunction I/O devices, and the SCXI-1141 module have built-in antialiasing filters.

Ultimately, anti-aliasing must be carried out in hardware. But the time you have digitised your signal, it is too late to remove aliased signals. However, general noise removal filters can be implimented post-acquisition (i.e. in software).

Have you looked at the Filter Expres VI? Or, if that is not flexible enough, have a look at the low level filter vis, which can been found under the signal processing pallette. They are available in either array manipulation or point-by-point flavours.

You may not require them, but for advanced filtering, you might want to have a look at the "sound and vibration" toolkit.

https://www.ni.com/en-us/shop/product/labview-sound-and-vibration-toolkit.html

Best wishes,