Noisy Analogue Signal and Lowpass Filter Questions

Question

Hi everyone,
I am getting analogue signals from a strain gauge that are a bit more noisy than I would like... Normally, I jump right in an smooth the signal using a lowpass filter of whatever order gave me the result that was acceptable, without overkill and smootig the signal to nothing... However, I am settingup a new DAQ device and would like to a little more informed about what and how am doing this...
So, here are my questions:
1. How do I determine the filter order? I usually use a 2nd order filter for smoothing.. I understand the comncepts of roll-offs and phase delay as they relate to the order numbers... On what basis should I decide which order filter to use for this specific signal?
2. I am still thinking that using a lowpass filter is the best approach... I assume the lower cutoff freq is in Hz (i.e. 10Hz) and is NOT a normalised freq [i.e. cutoff freq / (sample rate/2)]?.. The default value is 0.125, which seems too low for a Hz value? Also, on what basis should I decide what cutoff freq to use for this specific signal?
3. I need to synch the filtered strain gauge signal with other, unfiltered signals... But as the filter will add a phase delay it will affect signal synchronisation... I heard somewhere that a 4th order filter is zero-phase, but I can't see how that could be right as the filter only operates in one direction... Any suggestions for what I could do to maximise the timing alignment between the filtered and unfiltered signals?
All comments welcome!
Regards,
Jack

Answer 1

Jack,
I was the Instrumentation Engineer at a university for 24 years and worked with several execise physiologists, biologists, and neuroscientists on similar projects.  My group was the "lab/tech staff" you have little of.  There were 3.5 (engineer, 1.5 electronics technicians, and machinist) of us for the entire university.
OK.  The strain gauge signals probably have no information above a few tens of Hz (hummingbirds) or a few Hz for most larger species.  EMG is usually filtered in the amplifiers to slightly larger bandwidths (few hundred Hz) unless you are trying to see the individual action potentials.  Does the dynamometer have specifications on the bandwidth or sampling rates used?
Since your bandwidths are realtively low, I suggest that you sample all the analog inputs (and the trigger outputs, if the boards have the capability) from the same clock.  You can later decimate or average data on channels where less data is needed.  Record the trigger signals on an unused analog input if you have one available.  It can be very handy to have the information about when the triggers actually occurred in the same dataset as the response data.
Delay equalization is a complex topic. Suppose that you are sampling all your channels at 1 kS/s. Suppose you have a 10 Hz low pass filter, 2 pole Butterworth, on a strain gauge channel and a 50 Hz filter, 4 pole, elliptical, on an EMG channel and you want to synchronize the filtered data with some unfiltered channels.  The two filters have very different phase and delay responses over the 0-500 Hz frequency range. To equalize you would need to determine the maximum delay at any frequency in the range for both filters. Then you would need to design THREE all-pass filters. One would have a phase response which compensates the 10 Hz filter. Another would compensate the 50 Hz filter.  These would need to be designed so that the total delays through the low pass plus all-pass filters is the same.  Then you need a third all pass filter with that same total delay to compensate the unfiltered data.  This is may be a non-trivial job for an expert, depending on the low pass filters.
What kind of noise are you seeing and how is it affecting your measurements?  Can you post some data?
Lynn