An N4L application note (3) succinctly deals with accuracy: "While the digital design of any digital instrument is clearly important, the accuracy of all these instruments is dictated by the analog design and calibration." In particular, the capacitance of voltage inputs and the inductance of current inputs limit achievable performance.
According to the PPA3500 datasheet, N4L selected the DFT for signal decomposition because it is not restricted to the [2.sup.n] sample window size as is the FFT.
Current shunt design is the subject of another N4L application note.
To put the importance of a shunt's frequency response into perspective, the N4L PPA3500 has a built-in 30-A shunt, and the current accuracy is specified as 0.04% of reading + 0.1% of range + 0.005% x F(kHz) of reading + 900 [micro]A.