Thanks for all the great feedback.
OK, problem solved !
I tried 1mm copper wire instead of 0.5mm and no difference
DC offset, measured with a 34470A was about 3 mV DC, so not really a problem.
The scope was AC coupled anyways and the bad readings were confirmed by a DMM
All three of these current probes that I had tested are calibrated by an official lab, therefore I did not expect a problem with the probes, especially since they are very close to each other on the readout.
BUT ... I was so wrong with this assumption.
In the pictures below, I am testing with 10 loops of copper wire (1 mm diameter) and all 4 probes are hooked up on the same loop:
- Channel 1: Tektronix A622, 1.047A (4.7% Error)
- Channel 2: Keysight N2893A, 1.006A (0.6% Error)
- Channel 3: Tektronix A622, 1.043A (4.3% Error)
- Channel 1: FLUKE 80i-110s, 1.029A (2.9% Error)
So, obviously it has nothing to do with the wires and setup, just the probes are out of calibration.
Now I can continue this project and make a few nice loop arrangements.
I am planning to make a 5, 10, 50 and 100 loop gadget and this way I would be able to test up to 1000A with a 10A input.