Trio™ Service Manual
0070-10-0591-01
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Theory of Operation
Parameter Circuit Descriptions
1.4
Parameter Circuit Descriptions
1.4.1
ECG
The main functions concerning ECG are:
• Lead: 3-lead, 5-lead
• Lead Method; I, II, III, avR, avL, avF, V, CAL
• Floating Input
• Right-Leg Drive
• Lead-off Detection
The ECG circuit is responsible for processing the ECG signals of human body. The circuit
consists of following parts:
Input Circuit:
The ECG electrodes are connected into the circuit through the cable.
This circuit is mainly used to protect ECG input stage and filter the signals so as to
remove the outside interference.
Buffer Amplifying Circuit:
Used to convert the impedance of ECG signals, so as to
ensure that the ECG has a very high input impedance but only low output impedance.
Right-Leg Drive Circuit:
The middle output point of the buffer amplifying circuit is
reversely amplified and then fed to the RL of the 5-lead ECG to maintain the human body
in a equipotential state. This method can reduce the interference and raise the common-
mode rejection ratio of the circuit.
Lead-off Detection:
Based on the theory that the lead-off may cause the output of the
buffer amplifying circuit to change, we can use the comparator to accurately determine
if the lead has fallen off. In this way, the level can also be converted into TTL level for the
MPU to test.
Main Amplifying Circuit:
A measurement amplifier consisting of three standard
operation amplifiers.
Last Stage Processing Circuit:
Used mainly to couple ECG signals, program control
of the gain amplifier, filter the waveform and move the level, amplify the signal and send
it to the analog-to-digital converter.
1.4.2
Respiration
Respiration is measured by the thoracic impedance method. When a person is breathing, his
chest moves up and down. This movement equals the impedance change between electrodes
RA and LL. The monitor converts the high-frequency signals passing through RA and LL into
amplitude-modulated high-frequency signals, which are then demodulated and amplified into
electronic signals varying with the respiration changes and then transmitted to analog-digital
converter. The RESP module is made up of a respiration circuit board and a coupling
transformer. The circuit includes stages such as: oscillation, coupling, demodulation,
preliminary amplification and high-gain amplification.
