I am trying to record signals from four sensors (analog output, differential, 0-10VDC) connected to the AI terminals of a NI-6215USB, simultaneously with the signal from an anemometer. The anemometer generates a pulse output and I connect its + cable to the PFI1 (Gate) terminal, and the - cable to the DGND terminal for a frequency measurement. When everything is connected I get correct readings for the analog channels and erroneous readings for the measured frequency. When I disconnect the analog signals completely from the terminals, I get correct frequency readings but a residual alternating voltage is also logged in the analog channels.
I don't know if it's matter of wiring or it is impossible to simultaneously acquire several analog and counter signals. I have attached two screenshots (from Signal Express 2015 that I am using). The first one (analogs in) shows the case where the analog signals are connected, but I get unphysical high noise-like signal in the frequency measurement, in the order of thousand Hz. The second one (analogs out) shows the case where I disconnect the terminals from the analog sensors, I get the correct expected frequency of around 83Hz coming from the anemometer (with the exception of some spikes) but there is also an alternating-like voltage in the analog channels (where no sensor is connected at all).
I have trying changing the sampling period in the Counter Frequency Step Setup panel, but nothing helps. I also tried connecting the + cable of the anemometer signal to the PFI0 (Source) terminal, but the result is still the same.