User manual

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example, diodes connected in anti-parallel, Zener diodes connected in anti-series, or

eventually voltage limiters consisting of two silicon transistors. Low-voltage discharge tubes

are sometimes used. To prevent the overcurrent (current overload) of the voltage limiter, there

is usually a serial resistor, and a spark gap for large discharges.

Amplifiers of the biopotentials

The amplifiers increase the level of the signal in such a way as to make it suitable for

further processing, and at the same time they ensure the impedance conformity of the

apparatus’ input with the signal sources (the patient), and they can also suppress undesirable

disturbances - when the most common source of external disturbances is the penetration of the

mains frequency via the electrostatic coupling, the magnetic induction or directly galvanically

into a part of the described system. These disturbing elements must be effectively suppressed

by the amplifier.

Essential qualities of the biopotentials’ amplifier with typical values:

• differential input impedance (> 2x200 MΩ)

• common mode input impedance (> 400 MΩ)

• common mode rejection ratio CMRR (> 100 dB at 50 Hz)

• input bias current (< 1 nA)

• frequency characteristics (0.05-120 Hz, 0.5-40 Hz - e.g. Holter)

• output impedance (< 10 Ω)

• gain (200 - 1200)

• noise voltage at input (15 µV)

• recovery time at overdrive (overload, saturation) (10 s)

• input protection from damage by surge – overload protection (5000 V)

The Driven Right Leg circuit

This circuit is often used by the designers to suppress the common mode signal. The

common mode signal is taken away from the internal structure of the operating amplifier

(instrumentation amplifier - IA), to be impedantly modified via a voltage follower and

released back to the examined object via inverting amplifier (the interference (common mode)

signal has a reverse phase). The resistor with a value of 390 kΩ is used as a current limiter of

the excitation signal.