oopps...
I´ve just measured the output of the isolated power supply module that feeds the ampmeter (the one in the link at first post) with the oscilloscope and I see the oscillation comes from the module. I know inside this thing there are some typical commutation power supply / oscillating circuits but peaks of 150mV in its output over the output voltage is... VERY bad.
How can I minimize thar? Will a small TO92 LM317 linear regulator after the ouput reduce the peaks?.