Solutions for controlling EMI noise in medical instruments

Today’s medical devices are filled with increasingly complex electronic devices that monitor, display, assist and warn patients. The latest generation of medical device electronic devices are smaller and more compact, but can accommodate more monitoring instruments. The biggest design difficulty is how to effectively reduce the interference of external electromagnetic waves on the equipment?

The good news is that the military and satellite systems have solved (EMI) and cable noise control problems in a small space, and the cable and connector experience of the military and satellite systems has unexpectedly paved the way for the medical device industry to solve the problem of electromagnetic interference control. The mature experience of cables and connectors in this industry helps to adopt safer cables in the medical industry.

The experience gained from designing cables for military and satellite systems is important because many of the newer ICU instruments are required to operate on digital signals at gigahertz speeds in close proximity to other electronic instruments.

Often, this helps determine if the cables routed within the instrument rack are being disturbed by electromagnetic waves from the outside world (receivers of unwanted EMI), or if the cables may be transmitters of EMI noise that propagate to other equipment. In either case, EMI noise can cause serious problems with the performance of the entire medical equipment or degrade the accuracy of the instrument to varying degrees.

Medical cables and designers have a variety of solutions to control EMI noise problems in medical instruments. The following are several mainstream solutions to EMI noise:

1. Selective filter circuit

EMI problems can be solved by finding the frequency of the interfering noise and adding selective filtering circuits on the instrument circuit board or adding filters to the connectors on the cable harness. Film capacitors and resistors can eliminate unwanted signal noise by grounding near the noise source. However, the cables and connectors must be small enough. If the size is too large, it will limit the performance of the coaxial filter circuit, which will have a certain impact on the effectiveness of anti-electromagnetic interference.

2. Overall shielding

When it comes to isolating interfering electromagnetic waves from complex external environments, the use of braided shielding on cables has always been one of the mainstream methods. The industry generally recognizes that braiding is usually the best shielding solution, which can easily provide up to 85dB of isolation from external noise.

To reduce the number of wires in and out of the system, medical devices can use hybrid cables and connectors to combine power, signal, and trigger into one connector and cable system. But note that inside the connector, there are power pins on one side and signal pins on the other side. But more importantly, the cable from each metal circular connector to the main interface connector is a hidden braided shield.