Such an acquisition involves careful design of all elements in metrology chain: transducers, hardware connection, acquisition sampling rate and algorithms.
Just as general rules, I suggest you to set up input channels in differential mode to maximize noise rejection, to exclude any low pass filter you may have enabled on the signal and to acquire at the highest rate you can, exploiting the maximum out of the 250kS/s capability of your card: if your signal have a high harmonic content, you may be filtering it out with a low sampling rate. Did you know which is the sampling rate of your power meter?
Additionally, are you using the same transducer on the power meter and the DAQ cadr? If not, are you sure that voltage and current transducers have sufficient bandwidth?
