Onboard Filtering on PCI 6284

[K_seeker]

We are acquiring a group of signals using PCI 6284. Some signals are connected as RSE and some as Differential . Things work fine when Filtering is not enabled. When the filtering is enabled the graph shows  the same signals with different (Lower than actual) amplitude values. The Blue signal (Which is RSE) also shows a negative offset. I connected the scope later to check and I do not see any change in the signal values. What could be causing this in my setup? Is the onboard filtering not liking the RSE connections. 

I was also wondering if the onboard filtering can be enabled for only selective channels?

Thanks.

[JoeFriedchicken]

Hi K_seeker,

I am going to try to recreate the behavior you're seeing, but it will take me a little bit of time. Would you mind trying a few more experiments while I do mine as well?

Can you isolate the RSE vs differential behavior? In other words, if a task only has RSE channels, how do the filtered and unfiltered waveforms appear in comparison to each other and to your mixed RSE/diff measurements? Likewise, what happens to only differential measurements? This will further characterize the board's behavior.

Do you have any other hardware in your system?

From what I can tell, your input signal has a frequency of about 400 Hz. This is well-below the input filter (40 kHz) and you shouldn't be seeing this kind of attenuation. I don't know why you're using a control to specify the hardware filter's cut-off frequency: this value is fixed in hardware and not programmable. You can only enable or disable it.

And in response to your last question, analog input filtering is all or nothing: you cannot set some channels to use the filter and others to not.

[JoeFriedchicken]

Hi again,

I was able to recreate the behavior you observed with my hardware as well. I made a VI that used a counter to provide the sample clock and noticed that the faster I sampled, the more pronounced the errors were. This means that the signals were being ghosted when the filter is enabled [1]. You'll notice that if you handle errors for this case, you'll get a warning:

---

Warning 200038 occurred at DAQmx Start Task.vi:2

Possible reason(s):

Measurements: Data may be invalid because the settling time of the enabled filter exceeds the period between two conversions on the analog-to-digital converter (ADC) for a task with more than one channel.

Disable the filter by setting AI Lowpass Enable to false, increase the time between two ADC conversions by reducing the AI Convert Rate, or acquire data from only one channel in the task.

---

I was able to work-around this problem by both lowering my sampling rate and adjusting the interchannel delay [2], just like the warning suggested. Setting my sample clock to 5 kHz and the convert clock to 10 kHz, I didn't notice any ghosting between my differential and RSE channels.

[1] How Do I Eliminate Ghosting From My Measurements?
http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument

[2] How do I Increase Interchannel Delay Using NI-DAQmx or Traditional NI-DAQ (Legacy)?
http://digital.ni.com/public.nsf/allkb/65E7445DB1AA5DC586256A410058697B?OpenDocument

[K_seeker]

Joe,

Thanks for teh response. That's very useful information. I'll try the settings you suggested and let you know how it goes on my side. Thanks again.