How does Biosemi limit the current flow to the subject ?

All our systems (ActiveTwo, ActiveOne, Mark-8 and the Mark-6) have a Driven Right Leg circuit with current limiter. Besides reducing the Common Mode voltage, the DRL also protects the subject for defects in the amplifier. If one of the input stages in the Active electrodes would break down, and the electrode input would become shorted to one of the Active electrode supply rails (0V or 4V), the current limiting resistor in the DRL protects the subject. In the very unlikely case that two Active electrodes would fail simultanously and that one electrode would be connected to the 0V and the other electrode would be connected to the 4V then dangerous current would be possible in spite of the DRL current limiter. Therefor we have integrated an extra protection in the ActiveTwo AD-box which only enables the powers when no errors are detected. (see diagram below)

The subject is protected for leakage currents from the mains supply by the isolation barrier between the amplifier and the PC: the optical fiber data-link combined with battery power supply provides complete safety. Leakage currents are well below the measurement accuracy (< 1 uA). In case of the optional mains supply, the low capacitance of the used DC-DC converters limits the currents to less than 10 uA (with the BioSemi 128-channel system, the actual measurement was 2 uA with the subject touching the 220V/50Hz mains supply, the isolation capacitance of the DC-DC converter is approx. 30 pF per 128 channels).

The DRL provides an additional safeguard for mains supply currents when someone would by mistake make a ground (earth) connection to the amplifier. Please note that this would be very difficult, since the "saboteur" would have to open the cabinet to do this since all conductors on the outside of the cabinet are either electrode inputs or protected shield outputs.


On our standard DRL circuit, the output current is limited by a 100 kOhm resistor inside the DRL integrator loop. The DRL integrator runs on a 4V supply. This results in a maximum error current of 40 uA (worst case: DRL integrator swings to 4V and one input shorts to 0V, or vice versa). This 100k resistor value was chosen to comply with the 50uA max. value specified for the IEC-601 CF type isolation used in Europe.

Using a 500 kOhm resistor instead of the standard 100 k resistor, is all that is needed to meet the 10 uA spec used in the United States. We use a 100k resistor on systems for the European market because it allows the system to handle somewhat larger Common Mode Voltages. In an extreme case where the subject for example touches the isolation of a mains supply cable while the amplifier is located on a metal table connected to the safety earth, the resulting Common Mode voltage may be large enough to generate currents between 8 uA and 40 uA. In such an extreme case, the US spec system will display clipping signals, whereas the Euro spec system would just be able to keep on measuring. When you are using the 10 uA limit, you will have to take some action in order to reduce the CM voltage in such a case.

Finally, the cabinet is constructed in such a way that the subject can never touch unprotected low-impedance points, such as ground planes, power supply rails or amplifier outputs.