Robert Stanton, Omnetics Connector Corp.
Today’s medical treatment suites are crowded with increasingly complex electronic devices designed to monitor, display, assist and alarm the staff involved in patient services.
The newest generation of medical device electronics are smaller and more compact to allow more instruments into the area, causing electromagnetic noise to become an even more important concern.
The good news is that designers of military and satellite systems used in space have years of experience tackling problems around electromagnetic interference (EMI) and cable noise control in tight spaces. Cable and connector experience from that industry — including know-how at suppliers including Omnetics — could help pave the way for safer cable EMI control within the medical industry.
Lessons learned from designing cable for military and satellite systems matter because much of the newer ICU instrumentation runs on digital signals running in gigahertz speeds while being crammed immediately close to other electronic instruments.
Often times, it helps to define if a cable being routed within a rack of instruments is going to be a victim (receiver of unwanted EMI), or if the cable may be the transmitter of EMI noise to other equipment. In either case, the EMI noise can cause poor performance or reduce accuracy of the instruments supporting practitioners.
There are a number of solutions for the medical cable and connector designer to control EMI noise problems in today’s medical instruments:
One can begin solving the EMI issue by finding the frequency of the interfering noise and adding selective filtering circuits on the instrument circuit board or by adding filters to the connectors on the cable harness. Thin film capacitor and resistor networks can drain off unwanted signal noise by grounding them near the noise source. Unfortunately, cable and connectors are required to be small, and physical size limits some of the effectiveness of in-line filter circuits within the connector shells or the cable itself.
2. Overall shielding
Braided over-shielding has been a mainstay when it comes to isolating complex cable from the outside world. Braiding is often the best shielding solution, and it easily offers up to 85 dB of isolation from outside noise.
To reduce the amount of wiring to and from systems, medical equipment devices are utilizing hybrid cables and connectors that combine power, signal and reference triggers into one connector and cable system, much like the cable pictured above. Note that within the connector, there are power pins on one side and signal pins on the other. More importantly, however, is the hidden braided shield over the cable from each metal circular connector to the main interface connector.
Shielding must maintain a 360-degree seal from the metal connector to each of the smaller circular connectors. If there is a gap or disconnected shield, the cable becomes an antenna at its open loop, and EMI will escape to all sections of the instrument rack in the ICU unit.
Factors including braiding thickness, metal selection, type of weave and even plating on the outer portion of the shielding can help determine EMI shield effectiveness. The system designer may want to work closely with the cable designer to select how much shielding is necessary.
3. Drain lines inside cable
A number of cable designs support the highest speed digital processing systems such as live imaging or active subcutaneous inspections done by the practitioner. Cable built to IEEE 1394, similar to “fire wire” used by phone companies, has helped significantly. Each set of high-speed digital wires are wrapped along with a “drain” wire. The twisted pair of wires handle signals in the high gigabit-per-second range, while the drain wire assists with fast return signals to the source as well as pulls off potential EMI noise and jitter that happens as signals run extremely fast. This method also has a lightweight overall shield underneath the overall jacketing of the medical cable. (See image above.)
In many cases, an aluminum foil shield is used to keep the cable small and limp but still adds to the jacket protection from EMI.
4. Conductive over-molding and shrink jackets
A clever solution for some EMI cable applications is to use special jacketing made of polyolefin heat shrinkable tubing that is coated with unique silver-plated copper filled coatings from Parker Hannifin’s Chomerics division. The manufacturer’s tests show they insulate the active electronics up to 50-60 dB attenuation from 30 MHz to 18 GHz signals. Unique connector over-boots are also included.
As medical technology speeds up and is condensed, protecting cable and the systems they operate from EMI is a complex challenge, but it’s also solvable.
Robert Stanton is director of technology and new business development at Omnetics Connector Corp., based outside Minneapolis. Stanton is an electrical engineer who spent the majority of his career at Tektronix in Oregon designing and developing technology for portable electronic test equipment. His graduate work at Stanford University included materials technologies applied to high-speed miniature electronics.