4-64 120 Series Maternal/Fetal Monitor Revision B
2015590-001
Theory of Operation: MECG Board
junction of D23 and D24, and 0 V at the output across C39. Under these conditions,
a conduction path exists between the input and the output of the limiter. When a
step change occurs at the input, for example a +1 V step, diode D21 will become
reverse biased turning off the current path from the input to the junction of D21 and
D22, and to the output. Diode D23 conducts, raising by +1 V the potential at the
junction of D23 and D24. This turns off diode D24, breaking the current path from
the input and from R69 to the output. The only current path left is from R68 through
D22 to the output capacitor C39. R68 begins to charge C39 at the rate determined
by the time constants of the two parts until C39 reaches +1 V and the normal
conduction paths are restored from input to output. This in effect will limit the
output of signals that have faster slew rates then the time constants of R68 or R69
and capacitor C39. The output of the limiter is buffered by a unity gain non-
inverting amplifier from an op-amp of U6. The output of this buffer connects
through FET switch U2 to a single-pole 97 Hz low-pass filter composed of resistor
R65 and capacitor C37. This filter is buffered by the common output stage from the
op-amp of U6 that provides the output of the ECG amplifier. When the pacemaker
detector gating pulse occurs, a switch in U2 opens the path between R40 and C38,
which stops the charging of C38. C38 at this point will hold its last potential for the
5 ms gate period, maintaining a constant input to the slew rate limiter during this
period. In a like manner, the output from the slew rate limiter buffer is disconnected
by another switch in U2 from the low-pass filter of R65 and C37. Due to the very
high input impedance of the final buffer stage of U6, C37 will hold its last potential,
keeping the output constant during the gating period. In effect, the pacemaker
rejection circuitry holds the ECG output at the voltage that was present at the output
before the occurrence of a pacemaker pulse, until sufficient time has elapsed for the
pacemaker pulse to terminate.
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