(b) If the supply voltage falls below 9.5v, pin 3 and pin 4 of U21 go from a
low to a logic high. This is presented to the gate of a P channel power field effect
transistor (FET) (Q50), interrupting the supply to the monitor.
(c) The on-off switch on the monitor or the system ON switch on the
defibrillator completes the signal pass to the gate of Q50, which turns the monitor on as
long as pin 4 of U21 is in its low state.
(d) Resistor RA20 adds a certain hysterisis so as to make the switch on
and off function have a certain snap action. U21 is connected permanently across the
battery, but its current consumption, with pin 4 disconnected, is almost seven times lower
than the normal self-discharge current of the NiCad cells.
(a) The negative-going blanking pulse emanating from the digital section
activates PNP transistor Q23 which in turn throws Q22 from normally cut-off into full
conduction. This applies the full negative bias to grid one of the CRT which cuts off its
(b) When Q22 returns to its normal cut-off condition, the present voltage
by the brightness control is applied back again on grid one of the CRT.
b. Functions of the Circuits in the Isolated Patient Cable Preamp.
(a) This preamplifier can accept a three lead or a five lead patient cable
as long as they conform to AAMI standards. An eight position selector switch permits the
signal pickup from the paddles (by enabling a separate paddle preamplifier) or lead
configurations I, II, III, AVR, AVL, AVF and V.
(b) The frequency response is diagnostic (.05-100Hz for 3 decibel (Db)
points) all the time but with a fast recovery circuit which switches the response to monitor
quality after any severe overload and back to diagnostic response 1.5 seconds after end of
overload. This feature avoids the very low frequency instability (and even loss of signal)
normally found in amplifiers with diagnostic frequency response.
(c) A signal to noise ratio greater than 50dB refers to 1mv. A minimum
common mode rejection is 60dB in direct measuring mode and above 120dB against line
ground (case and auxiliary outputs). A pacemaker pulse detector and pulse overload
protection is included and will be detailed later. Input impedance over diagnostic
frequency band is 5 meg.