Service manual

Chapter 2 Principles
1) Input circuit: The input circuit protects the ECG input level, and filters the ECG signals
and external interference. The ECG electrode is connected to the input circuit through the
cable.
2) Buffer amplifying circuit: This circuit ensures extremely high input impedance and low
output resistance for ECG.
3) Right-foot drive circuit: The output midpoint of the buffer amplifying circuit is fed to the
RL end of the 5-lead after the inverse amplification, so as to ensure that the human body
is in the equipotential state, decrease the interference, and increase the common-mode
rejection ratio of the circuit.
4) Lead-off detection: The lead-off causes changes in the output level of the buffer
amplifying circuit. Therefore, the lead-off can be detected with a comparator, and the state
of lead-off can be converted TTL level for the Micro Controller Unit (MCU) to detect it.
5) Lead circuit: Under the control of MCU, the lead electrodes should be connected to the
main amplification circuit.
6) Main amplification circuit: The measurement amplifier is composed of 3 standard
operation amplifiers.
7) Subsequent processing circuit: This circuit couples the ECG signals, remotely controls
the gains, filters the waves, shifts the level, amplifies the signal to the specified amplitude,
and sends the signal to the A/D converter.
■ RESP
The PM-9000 Express patient monitor measures the RESP based on the impedance
principle. While a man is breathing, the action of the breast leads to impedance changes
between RL and LL. Change the high-frequency signal passing the RL and LL to
amplitude-modulation high-frequency signal (AM high-frequency signal), which is
converted to the electric signal after being detected and amplified and then sent to the A/D
converter. The RESP module consists of the RESP circuit board and coupling transformer.
The circuit has several functions: vibration, coupling, wave-detection, primary
amplification and high-gain amplification.
2.4.3 NIBP
The NIBP is measured based on the pulse vibration principle. Inflate the cuff which is
on the forearm till the cuff pressure blocks the arterial blood, and then deflate the cuff
according to a specified algorithm. While the cuff pressure is decreasing, the arterial
blood has pulses, which are sensed by the pressure transducer in the cuff.
Consequently, the pressure transducer, connected with the windpipe of the cuff,
generates a pulsation signal. Then, the pulsation signal is filtered by a high-pass filter
(about 1Hz), amplified, converted to the digital signal by the A/D converter, and finally
processed by the MCU. After that, the systolic pressure, diastolic pressure and mean
pressure can be obtained. For neonates, pediatric and adults, it is necessary to select
the cuffs of a proper size to avoid possible measurement errors. In the NIBP
measurement, there is a protection circuit used to protect patient from over-high
pressure.
The NIBP measurement modes include:
1) Adult/pediatric/neonate mode: To be selected according to the build, weight and age
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