THD error measurement

[Diren]

hi everybody,
i'm doing a project on harmonics control and analysis in low voltage single phase applications. im using the USB DAQ 6008 as my data acquisition device. im stepping down the 230V rms to 12 V to feed m DAQ card. under no load, the THD is varying from 5.2 to 6.4%. When i plug my PC or any other load, the percentage THD is almost same as if difference. Why THD is not zero under no load and no change when under loaded conditions.
Thanks for replying. I've uploaded my VI.

[Diren]

i dont think its the DC component from the input signal.
The harmonics levels were as follows during no-load.
0 1 2 3 4 5 6 7 8 9
80 240 3 5 3 8 5 3 4 5

can anyone tell me why the THD is not almost zero under no load. and theres not change under loaded conditions. i tested the hardware using matlab...the THD is almost 1% under no load and 4-5% under loaded conditions. Why with labview, the values are wrong?

[Diren]

i dont think its the DC component from the input signal.
The harmonics levels were as follows during no-load.
0 1 2 3 4 5 6 7 8 9
80 240 3 5 3 8 5 3 4 5

can anyone tell me why the THD is not almost zero under no load. and theres not change under loaded conditions. i tested the hardware using matlab...the THD is almost 1% under no load and 4-5% under loaded conditions. Why with labview, the values are wrong?