Mallinckrodt NELLCOR NPB-4000, Service Manual

Page: 103 / 219

Section 11: Isolated Front End Functions -- Theory of Operation
11-7
11.5 RESPIRATORY CIRCUIT
Figure 11-6: Respiratory Circuit Block Diagram
As shown in the highlighted portion of the expanded block diagram, Figure 11-6,
the Respiratory circuitry includes the ECG leads, and some control circuitry.
Respiration is detected by measuring modulation of a high frequency AC signal
sent to a body. The AC signal is 100 kHz, 4 volts peak square-wave
complementary signals are produced by oscillator U81. These signals, via
resistors R288 and R271, generate 42 microamps p-p current which is injected in
a body through patient leads RA and LA. The AC voltage drop on these leads is
respiration modulated and fed through high-pass filters C94, R244, C95, R228,
and resistors R129 and R141 to a multiplexer U76 via buffers U75C and U75D.
The multiplexer is controlled by the same AC signal that generates current and,
in conjunction with amplifier U77C having gain of 9, comprise a synchronous
detector.
The DC component of the filtered signal is removed by 0.1 Hz high-pass filter
C134, C172, R205, and R204, and the remaining low frequency respiration
signal is fed for further amplification. The capacitors C134 and C172 can be
quickly discharged through U63D switch under software control in case of
significant baseline drift. Amplifier U72B has a gain of 667 and c2.5
volts offset via resistors R215 and R216 to convert bipolar signals to unipolar
format. Complete swing of the output is about 1.5 volts p-p, and the signal is fed
to the linear optocoupler multiplexer.
11.6 TEMPERATURE CIRCUIT
The temperature is measured in the body range of 15 to 45
ø
C using Yellow
Spring series 400 thermistor thermometers. Thermistor with the resistance range
of 3539 to 984.2 ohms is connected as a feedback of amplifier U72A. The
reference signal of 1.235 volts is supplied by voltage reference U70 through
precision resistor R217. The temperature, therefore, is measured using output
ratios. Other ratios have to be arrived at by using Yellow Spring thermistor
value chart. Resistors R203, R218 and capacitor C130 comprise a low pass
filter.