Input circuit and lead selection.
(a) All inputs are protected against damage by defibrillation pulses, by
resistors (R4, R6, R8, R11), and 90v spark gaps (K1 through K4). This combination limits
any applied pulse to 90v maximum against reference and floating ground. This pulse is
further voltage and current limited to a safe value which can be tolerated by the input
stages of the buffer integrated circuit U1.
(b) The buffer outputs are connected to a resistive network (Eindhoven
triangle) composed of resistors contained in resistor Pak R16 and R17. This, together with
switch (sw) SW2, permits you to obtain the traditional lead configurations I, II, III, AVR,
AVL, AVF and V, with the use of a five-lead patient cable.
(c) The analog gates contained in U2 switch the unused leads for every
lead configuration to the right leg of the patient. This avoids noise pick-up which, through
the respective buffers, would appear at the resistor triangle.
amplifier (first three sections of U3) with a gain for differential signals of approximately five
times and a rejection for common mode signals of at least 10,000 times (80dB) with R30
and R31 correctly adjusted.
(b) The amplified signal on pin 14 of U3 is applied through R32 to a
blocking capacitor C2. This capacitor together with R34 and R35 defines the low
frequency response. It also blocks any direct current applied to the input, such as
(c) The fourth section of U3 and first section of U4 provide the real gain
drives a voltage to frequency converter U7 which converts it to a frequency modulated
signal, which, through Q6, drives the infrared emitter diode of opto-coupler K7. The photo
transistor in K7 (on the grounded side) detects this FM signal and applies it to a frequency
to the voltage converter (see paragraph c, Grounded Analog Section).
Quick recovery circuit.
(a) Pin 7 of U4 delivers the signal to the fast recovery circuit which
switches the amplifier characteristics, on the low frequency end only, from diagnostic to
monitor response during overload conditions and back to diagnostic after that condition