(6) The notch filter attenuates approximately 20dB at 60Hz (50Hz for some
export models) with a signal amplitude at 20Hz. A signal follower U46 isolates its output
from a 4 pole low-pass filter with a slope of approximately 20dB per octave above 30Hz.
This attenuation combined with the notch filter gives approximately 40dB loss at the line
(7) The CMMR circuits plus the filters provide a ratio of close to a million to
one before the interference becomes great enough to saturate the last stage of U45.
Practically all the signal gain of the preamplifier is accomplished in the last stage. Power
supply voltages are stabilized 6.2v, 5 percent derived from the common supply, which
drives other sections of the defibrillator board.
s. Functions of the Circuits in the Delivered Energy Display.
By definition: watts-sec = joules = I^2R/sec.
(1) The MRL defibrillator compensates for wide variations of load resistance
by changing the pulse duration. By measuring the current delivered to the load and the
time during which the current is flowing, you can obtain a reading of delivered energy in
(2) The current is sensed by the voltage drop across the 4 ohm resistor which
is in series with the load.
(3) This signal, after filtering high frequency components, is divided down to
an amplitude suitable to be applied to the input of an analog integrated circuit (first section
of U37). It is also applied to the input of a voltage comparator (section of U37).
(4) If the pulse is large enough (well above the noise level), the output of the
voltage comparator will go negative, opening the analog gate U42. From this moment on,
C73 is charged at a current directly proportional to the signal amplitude present at pin 5 of
U37 at any instant. Consequently, its final charge will be the integral of I, where I is the
current flowing through the load.
(5) At the end of the trapezoidal pulse, pin 8 of U37 returns to its initial
(positive) state triggering the timer U36. In this instant, C73 is disconnected from the
charging circuit and the other plate is connected to ground. Its charge is then returned and
applied to a high input impedance voltage follower.
(6) This dc signal is amplified to a suitable level to enter the square law
generator whose output will be essentially proportional to I^2.
(7) This voltage is applied to a digital volt meter and displayed on three digits
as delivered energy in joules.