In professional and home healthcare, it is common for medical devices to be attached to the human body to measure an increasing number of health indications. These range from surface parameters such as skin resistivity, body temperature and blood oxygen saturation levels to heartbeat monitoring using electrocardiograms (ECG) and many more (Figure 1).
Sometimes sensors are more invasively connected, internally to the body, such as during surgery. Most sensors are electronic and clearly their design needs to ensure that voltages and currents are low enough so they do not present an electrical shock hazard to the patient. There is also though a requirement that the sensor does not provide a route for damaging current to flow from other faulty equipment through the patient and back through the sensor. This means that the sensor and its power supply must not only be sufficiency isolated from hazardous high voltages but also from ground. Even battery-powered hand-held devices may have a route to ground through a connected printer, USB port or laptop for example.
In electronics, the relevant
safety standard is ‘Medical Electrical Equipment’ IEC 60601-1 and its national versions such as EN 60601-1 in Europe and ANSI/AAMI ES 60601-1 in the USA. In the standard, ‘isolation’ has a very specific meaning, effectively requiring double- safe electrical separation between hazardous voltages or high energy sources and the patient. Medical equipment must therefore have two ‘Measures of Protection’ (MOPs) so that if one measure fails, the second measure will still provide adequate protection. The MOP requirements have two frades, MOOP for operators and the higher-grade MOPP for patients.
To provide power for the sensors or to read data across the isolation barrier, miniature safety transformers or optocouplers with reinforced insulation are a practical solution. These give high withstand voltage isolation but still have a coupling capacitance across the barrier which could allow a leakage current to flow. The coupling capacitance must therefore be kept as low as possible.
If the sensor power supply is derived from an AC/DC converter, then all power supplies have safety isolated outputs as standard. However, the grade of isolation for commercial or IT AC/DC’s is not adequate to meet the medical standards. Many have only one measure of protection and the isolation coupling capacitance is normally far too high to meet the low leakage current requirements. Additionally, the outputs may be ‘floating’ but with insufficient isolation to ground. Some
‘Medical grade’ AC/DC’s are available that have ‘Measures of Operator Protection’ (MOOPs) but are not suitable for direct patient connection because input/output and output/ground isolation is insufficient and the AC/DC barrier coupling capacitance not low enough. AC/DC’s with the required Two Measures of Patient Protection’ (2MOPPs) are available but are limited in range and expensive. Multi-channel sensors will often require multiple, mutually-isolated power rails and it is certain that anything other than a fully custom 2MOPP AC/DC will not have all the necessary power rails needed.