Schiller CARDIOVIT AT-1, Service Handbook

Page: 43 / 84

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SCHILLER AT-1 / AT-1smartprint ECG Unit
SERVICE HANDBOOK Issue b June 2001
Art.No. 2.540022
Main Board MK 11-10 (cont.)
Paper Mark
The pulsed paper mark signal from the printer is fed to a comparator. A detected paper
mark supresses any (logic 0) pulses of PMARK at the output of comparator U38.
Power On Reset
The Power on reset circuit controls the master reset of the CPU. This circuit has two
functions as follows:
• To provide a delay on initial switch-on to ensure that the power supply is fully
stabilized and give the 200ms reset time required by the 68332 processor.
• To disable the unit if the +5V rail drops below +4.75V.
Stepper Motor Controller
The printer stepper motor controller sets the speed of the printer motor with a clock
frequency dictated by the master CPU.
The purpose of the stepper motor controller circuit is to ensure that the motor speed
requested by the microprocessor is achieved and maintained.
ECG Isolated Power Supplies
DC/DC converter circuits produce all the isolated power voltages required by the ECG
Amplifier circuit.
The -2.0Vi and the 2Vi isolated reference voltages are generated from the -5Vi supply.
Note: When taking measurements always ensure that the isolated ground is used
for reference.
ECG Signal
The incoming ECG signals RA, LA, and C1 to C6 are low-pass filtered (approximately
10kHz) and applied to non-inverting operational amplifiers giving a gain of 11. The
signals are further low pass filtered (approximately 400Hz) and amplified by 23 before
being applied to the multiplexer.
The multiplexer sampling rate is 1000Hz.
Noise Damping
The right leg electrode to the patient is the signal ground reference signal. To assist in
cancelling some patient noise and thus reducing incoming signal distortion, the incoming
signal from the patient left leg electrode is phase shifted 180
o
. This phase shifted signal
is then used by the signal ground reference to cancel (or reduce) patient induced
noise.