MT-071| Application Note
MT-071 TUTORIAL
Analog Isolation Amplifiers
ANALOG ISOLATION TECHNIQUES There are many applications where it is desirable, or even essential, for a sensor to have no direct ("galvanic") electrical connection with the system to which it is supplying data. This might be in order to avoid the possibility of dangerous voltages or currents from one half of the system doing damage in the other, or to break an intractable ground loop. Such a system is said to be "isolated", and the arrangement that passes a signal without galvanic connections is known as an isolation barrier. The protection of an isolation barrier works in both directions, and may be needed in either, or even in both. The obvious application is where a sensor may accidentally encounter high voltages, and the system it is driving must be protected. Or a sensor may need to be isolated from accidental high voltages arising downstream, in order to protect its environment: examples include the need to prevent the ignition of explosive gases by sparks at sensors and the protection from electric sh
ock of patients whose ECG, EEG or EMG is being monitored. The ECG case is interesting, as protection may be required in both directions: the patient must be protected from accidental electric shock, but if the patient's heart should stop, the ECG machine must be protected from the very high voltages (>7.5 kV) applied to the patient by the defibrillator which will be used to attempt to restart it. A summary of applications for isolation amplifiers (both analog and digital) is shown in Figure 1 below.
Sensor is at a High Potential Relative to Other Circuitry (or may become so under Fault Conditions) Sensor May Not Carry Dangerous Voltages, Irrespective of Faults in Other Circuitry (e.g. Patient Monitoring and Intrinsically Safe Equipment for use with Explosive Gases) To Break Ground Loops
Figure 1: Applications for Isolation Amplifiers Just as interference, or unwanted information, may be coupled by electric or magnetic fields, or by electromagnetic radiation, these phenomena may be used for the transmission of wanted information in the design of isolated systems. The most common isolation amplifiers use transformers, which exploit magnetic fields, and another common type uses small high voltage capacitors, exploiting electric fields. Optoisolators, which consist of an LED and a photocell, provide isolation by using light, a form of electromagnetic radiation. Different isolators have differing performance: some are
Rev.0, 10/08, WK Page 1 of 6
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