Revision C 170 Series Monitor 5-37
2000947-004
Theory of Operation: FECG/IUP Board Theory of Operation
Pressure Channel
The input to the pressure channel is to resistors R9 and R10 with the signal names of
–PRESS and +PRESS respectively. Resistors R9 through R12 and capacitors C16
through C18 provide RF filtering, and in conjunction with diode limiters D1 and D2,
provide protection from electro-static discharge.Instrumentation amplifier U5
provides a gain of 248 which is given by (49.4E3/R13)+1. The output of U5 which
is inverting with respect to the signal –PRESS, drives the inverting amplifier U6 (pin
1). This amplifier has a gain of –4.99 and rolls off at approximately 32 Hz. The
output of U6 (pin 1) drives U6 (pin 5) with a division of two formed by resistors R18
and R19. The operation of this amplifier is best understood if analyzed from its
quiesent condition of no input signal. Under this condition U6 (pin 5) is at zero
volts, therefore U6 (pin 6) must also be at zero volts. This forces a quiesent current
of 75 µA through R20. This current is produced by U6 (pin 7) driving the LED that
illuminates the photodiode at U10 (pins 3 and 4) which is the feedback path. The
LED also illuminates the photodiode at U10 (pins 5 and 6) which are on the non-
isolated side. The current transfer ratio of this photodiode with respect to the
photodiode on U10 (pins 3 and 4) is 0.85 to 1.05. The tocotransducer and the IUP
transducer have the transfer function of 5µV/UA-Unit/Volt-excitation. Therefore
since the excitation voltage is 4 V, the input voltage is 0.002V/100UA-Units. This
corresponds to 2.475 V at U6 (pin 1) and 1.2375 Vat U6 (pin 5). This causes a delta
current of 9.3 µA through R20, which is also transferred across the barrier with the
same transfer ratio of 0.85 to 1.05. This current is input to U13 (pin 6) through R32.
Potentiometer R25 sets the gain at U13 (pin 8) to 0.5 V/100UA-Units. Potentiometer
R30 sets the initial offset voltage to +0.5 V. The output amplifier U13 (pin 8) is a
unity gain amplifier with 3 poles of filtering with its 3 dB frequency of 3 Hz. Since
the system analog-to-digital converter has an input range of 0 to 2.5 V, the dynamic
range is from –100 to +400UA-Units.
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