Page 2.5
Chapter 2
Functional Overview
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
MK 14-10 Main Board (cont.)
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.
