GE Corometrics 126, Руководство по обслуживанию

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Revision B 120 Series Maternal/Fetal Monitor 4-61
2015590-001
Theory of Operation: MECG Board
MECG Board
Defibrillator Protection
The defibrillator protection is primarily a function of neon bulbs DS1–DS3 and 1
k
Ω
resistors located in each lead of the ECG cable assembly. When the voltage on
any patient lead exceeds 70 to 90 V with respect to isolated ground, the associated
neon bulb will light, discharging the energy on the lead to isolated ground. The 1
k
Ω
resistors in the cable serve to limit the current through the bulbs to a safe level,
and to prevent the circuit from absorbing too much of the total defibrillator current
applied to the patient. The ECG amplifier is further protected by the 20 k¾ series
resistors R4–R6 and limiting diodes D1–D6. These protection diodes are connected
in pairs to each patient lead and to ±10 V references through resistors R7 and R8.
These diode pairs will conduct when the input to any lead exceed the ±10 V
reference levels. In conjunction with the series 1 k
Ω
resistors, these diodes will
limit the input to the remaining circuitry to ±10 V. Capacitors C5–C10 are used to
dampen the voltage variation to the ±10 V diode references caused by the high
amplitude short duration waveforms normally associated with defibrillators.
Input Lead Switching
Lead switching is accomplished by a pair of dual analog multiplexes U14 and U13.
The two input leads from the ECG instrumentation amplifier, and the right leg drive
output stage each connect to an input of one of the three 1-to-4 MUX sections of
U14 and U13. The two select lines (SEL1 and SEL2) for the both U14 and U13
connect to control circuitry located on the FECG/UA Board. The four outputs of
the three MUX stages are wired for the following three combinations of lead
selections:
„ [SEL 1=0 SEL 2=0], –amp in to RA, +amp in to LA, right leg
out to RL
„
[SEL1=1 SEL 2=0], -amp in to RA, +amp in to RL, right leg out to LA
„ [SEL1=0 SEL 2=1], -amp in to LA, +amp in RL, right leg out to RA.
A fourth selection [SEL1=1 SEL 2=1] bypasses the input connector and connects
the two inputs and one output to an on board test generator.