Philips M3000A, Service Manual

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Electrocardiogram/Respiration (ECG/Resp) Measurement
26 Introduction to the Instrument
from the CPU) controls the analog-to-digital conversion and reads out the digitized ECG
data. The CPU communicates with the ECG ASIC via a built-in serial link.
To prevent interference from the 50/60Hz power line, the common mode signal is used to
drive the right leg (RL) drive amplifier. The output from the amplifier is then returned to the
patient via the RL electrode.
Excitation Current Source
The excitation current source feeds a small current into the right arm and left leg electrodes.
This results in a voltage drop between the right arm and left leg which is proportional to the
thorax impedance.
Bridge & Amplifier
The bridge subtracts an offset from the voltage between right arm and left leg. The amplifier
amplifies the remaining signal.
The Central Processing Unit (CPU)
The CPU performs the following functions:
• Controls the ECG ASIC, and stores ASIC specific calibration and error log data. This
initializes the intended ASIC configurations such as gain, A/D sampling rate, and DAC
operation.
• Acquires the digitized ECG and Resp signals. Digitized ECG signals are used to
calculate 3 ECG leads which are then passed on to the Pace Pulse detecting software, and
the wave processing and filtering software.
• Performs ECG wave filtering. The waves are digitally filtered and scaled then passed to
the software that communicates with the system CPU.
• Performs pace pulse detection. A digital high-pass filter acts as a slope detector for the
received ECG waves. The output of the slope detector is fed into two comparators
comparing the signal to a positive and negative threshold. The output of the comparators
indicate the absence or presence of a pace pulse. The threshold of the comparators is
moving and adapts itself to the amount of noise present in the ECG wave. Detected pace
pulses are communicated to the wave processing and filtering software.
• Communicates with the system CPU. Communication is via a serial, bi-directional data
link. The ECG/Resp CPU sends the following data to the system CPU:
•
3 ECG waves,
ECG ASIC
Controller
Data
Acquisition
Pace Pulse
Detection
Wave Processing
& Filtering System CPU
Communication
System
CPU