5 Established Isolation Architectures for Medical Isolation Amplifiers

Medical isolation amplifiers provide an ultra-low conductive pathway between the input (patient) terminals and the output terminals and ground. This pathway provides what is called ohmic or galvanic isolation for a patient. In medical applications, this isolation is required for reasons of patient safety. The dc resistance between input and output terminals is typically on the order of thousands of mega ohms; at ac, capacitance between the input and output terminals is on the order of single picofarad.

There are five established isolation architectures:

1. Transformer isolation in which power is magnetically coupled using transformer to the (isolated input stage by high-frequency current and signal is coupled to the output stage also by transformer, by modulating the power supply oscillator.
2. Photo-optic coupling in which the isolated conditioned input signal is coupled to the output by means of photo-optic couplers (Using an LED and a photodiode or photoresistor). Power is still supplied through high-frequency isolation transformer.
3. Capacitive coupling of a signal-modulated, high-frequency digital carrier from the isolated input stage through a pair of 1-pF capacitors to a demodulator in the output stage.
4. Magnetic coupling using giant magneto-resistive resistors (GMRs) in a Wheatstone bridge. A  GMR’s resistance is altered by its local magnetic field. The isolated input signal is converted to a current that is passed through coils in close proximity to two GMRs in a bridge. The ΔRs unbalance the bridge, which is on the output side of the Isolation Amplifier. The unbalance is detected by a differential amplifier, which generates a current used to re-null the bridge. The re-nulling current is sensed and is proportional to the input voltage. Isolation is maintained by the ohmic isolation between the input coils and the GMR bridge resistors.
5. The flying capacitor chopper capacitor uses a small capacitor charged up by the signal voltage and then switched by a high-speed double pole double throw (DPDT) relay to an output amplifier that reads the voltage across the capacitor. Such switched-capacitor isolation amplifiers have ohmic isolation set by the relay structure and are useful only for dc or very low-frequency signals.

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