I am trying to use a current source for applying current pulses (0-20mA and 10-20ms width) to a resistive load inside an MRI scanner room. Pulses are applied when an optic trigger is detected by the MCU. The MCU also controls the selection of the source/sink channels of the multiplexer. A diagram is given below.
The RF signal of the MR system appears at the output of the optic to voltage converter, and thus MCU receives false triggers. This issue was solved by disabling the interrupt of the MCU for a while after the first trigger pulse is detected.
The RF noise appears also at the address selection lines and at the current channels. I tried using Faraday cage by putting the entire circuit inside an aluminum box, using coaxial cables for current channels and grounding their shielding at the near endings. However, RF signal still observed.
Any suggestions to eliminate this RF signal? Do LP filters will help here? if yes, what are your recommendations for the filter design?
Notes: 1- Current channels are passing to the MRI room through a hole in the wall.
2- The output of the optic2voltage circuit is (0-3V).
3- All the circuit components are fed by a single power supply (+15/-15v).
A view of the detected RF signal between the terminals of a current channel is shown below