Schiller AT-1 - ECG Amplifier MK II-2

Chapter 2
Funcrionai Overview
SCHILLER AT-I -1-Chunnel ECG Unit
SERVICE HANDBOOK Issue 3 September I999
ECG AmplQ7er MK 11-2
ECG Isoiated Power Supplies
DUDC converter circuits produce all the isolated power voltages required by the ECG Amplifier
Circuit.
The -2.OVi and the 2Vi isolatedreference voltages are generated fromthe-5Vi supply. The voltages
are stabilised by a &nor diode and voltage follower circuit.
Servicing Note:
When
taking measurements always ensure that the isolated ground is
used for reference.
ECG Signai
The incoming ECG signals RA. LA. and C1 to C6 are 27 kHz low-pass fdtered and applied to non-
inverting operational-ampwiers giving a gain of 11. The signals are further low pass filtered
(approximately
1 Idlz) 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 swted 180". This phase shifted signal is then used by the signal
ground
to cancel (or reduce) patient induced noise.
I
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