Siemens SC 7000 Service Manual page 40

Service Manual
8.3.2 Temperature
24
Figure 2-10 Respiration Functional Block Diagram
Impedance respiration is monitored by injecting a nominally 40kHz square
wave of current into one ECG electrode and removing it at another ECG
electrode. The resulting 40kHz voltage drop across those electrodes is
proportional to the impedance. Specially balanced true current sources do
not load the ECG electrodes or distort the ECG morphology. The waveform
of the current is preemphasized to reduce bypassing effects of cable
capacitance. The returning 40kHz differential voltage is amplified,
synchronously demodulated, and low-pass filtered. The resulting dc-
coupled waveform is converted to single-ended form, further low-pass
filtered, and passed to the A/D multiplexer. An ac-coupled stage with an
"autobloc" dc-restorer feeding a separate input to the A/D multiplexer also
provides additional gain.
Figure 2-11 Temperature Functional Block Diagram
Refer to Figure 2-11.
• Designed to meet the stringent German PTB requirements including
detection of marginal accuracy due to degradation of a single
component.
• A second temperature channel is also available.
Temperature is sensed at the patient by a non-linear negative-temperature-
coefficient thermistor. This is linearized with a precision resistor network
and excited by the same reference as the A/D converter to a produce
ratiometric digital output. An input multiplexer (MUX) selects among the
external signal and internal reference dividers simulating -5 and +50°C. The
dc amplifier matches the dynamic range of the A/D by combining,
amplifying, and precisely offsetting the small signal from the multiplexer.
Power supplies whose failure would invalidate temperature measurements
are also monitored and compared against the A/D reference.
Siemens Medical Systems, EM-PCS, Danvers
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