Filter in 1762-IF4

What does it do? The help file says "This is the cut-off frequency. All input frequency components at or below the cut-off frequency are passed by the digital filter with less than 3dB of attenuation. All frequency components above the cut-off frequency are increasingly attenuated."

I have no clue what that means... please explain. What does frequency have to do with a current loop?

This allows you to set a software lowpass filter for the input. You specify what frequencies you want to keep by specifying a cut-off frequency. For example, if you set the cut-off frequency to 100Hz you will receive data with a frequency of 100Hz and below and will reduce the amplitude of data above 100Hz.

This is at best a second order filter. It is most likely a first order filter. So, given the above example, don't expect to receive 100Hz signals unchanged but not receive anything with a frequency of 110Hz. The cut-off isn't that sharp. The cut-off is a continuously increasing attenuation (gain reduction) as the frequency increases. And it won't be particularly aggressive.

Keith

I understand what you are saying, but I still don't understand what it has to do with an analog signal. It is a DC current reading... where is there a frequency component?

I am not an electrical engineer, so maybe I'm just not sure how exactly an analog signal works. If this were an RF signal it would make sense to me (any signal over the filter frequency would have its amplitude increasingly chopped). I figure it's DC, so the current should be a straight line, not a wave right?

Am I making any sense?

Noise!!. If you have a scope take a look at your analog signal you will be very surprised at what you see.

in simplest terms: think of the wires for your analog input as ANTENNAS ... now think of all of the things that MIGHT be broadcasting to those antennas ... local TV and radio stations ... walkie-talkies from the plant maintenance crew ... power lines ... variable frequency motor drives ... etc. ... etc. ... now think about how your analog input module is going to interpret all of that extraneous “hash” ... key point: the analog input module can’t tell the “real” signal from the “junk” ...

so try a filter and see if it helps eliminate some of the “junk” signals ... sort of like a shock absorber for your car ... the wheels can still flex up and down to follow the road ... but most of the jitter and shake of the little bits of gravel and crevices in the pavement are smoothed out ...

now as my distinguished colleague Keith was saying, some filters work better than others ... as do some shock absorbers ...

and according to common theory, "current" input signals generally have less need of filters than "voltage" input signals ...

final thought: if you’re not having a problem with signal noise, then just put the information from this thread on the back burner ... you may need it later ... but maybe you won’t ...

Ahh..... noise! Of course. I had thought of the fast changing signals, but I didn't think a constantly changing signal could constantly change too fast. So the real question would be... Should I set the filter to "None" if I'm not having problems, or use the default (60Hz, 450mS)?