Having finished a couple of oscilloscope measurements, I wonder if anyone has run into a similar issue before. Specifically, I have a current transformer (CR Magnetics 8348-2500-N) and a small transformer (2000:1) mounted fairly close on a PCB. The intended use is power measurements. Both devices feed a differential signal into a MCP3911, a analog front end meant for power measurements.
Today's oscilloscope readings on the analog pins in question were a bit worrying, as the pair of pins dedicated to current showed a sinusoidal differential voltage even with no load attached (i.e. probe of oscilloscope to CH1+, and GND of probe to CH1-). When comparing the output of one leg of the CT relative to AGND instead of the other leg, only noise was apparent.
I am wondering if my CT is picking up a signal from the voltage transformer, which is less than 1/2" away. The CT is at a right angle to the transformer, i.e. the axis that the current carrying wire is aligned with is parallel to the axis from low-to-high on the voltage transformer.
I presume that greater distance and rotating the CT by 90* would help? What minimum distance would you use between a 0.08VA transformer and a sensitive magnetic sensor like the CR Magnetics CT? I'd like to stick with the voltage transformer, if I can, to help with isolation from high voltages.