AC Coupling

[Baji]

Hello,

    I acquire pressure transducer signal from Picoscope, Picoscope has a option AC/DC Coupling and they say if I use AC Coupling option, any DC component of the signal below about 1 hertz will be filtered out. I acquired the same signal used NI DAQ (my DAC doesn't have AC Coupling option). I tried to use High pass filter by setting lower cutoff frequency as 1 hertz. Still I couldn't match the signal of picoscope and the signal acquired from DAQ. I saw a VI in Electrical Power Suite which does what I need (http://zone.ni.com/reference/en-XX/help/373375B-01/lvept/ep_acdc_coupling). I don't have the toolkit to try that.

 I attached the graph and Raw data of acquired signal from picoscope in AC coupled mode and DC coupled mode (both executed different time). Any suggestions or Ideas are welcome. 

[Buechsenschuetz]

Not all filters are created equal. When filtering a signal, the results do not only depend on the cut-off frequency but also on the characteristics of the filter. In the era of analogue filter circuits, many different filter configurations were designed, they all had their advantages and drawbacks - and they all will yield different results even if the cut-off frequency is the same. 

The same applies more or less to today's switched filter configurations and even to software filters. You always have to take into account the filter parameters. The most important is steepness (damping level indicated in db/octave) and far-off damping (suppression of signals far off the cutoff frequency).