Here, the stimulus signal (ECG-signal) is fed through 10 attenuators
that set up the correct level for the outputs. The output impedance to
earth is 500 ohm for all outputs. Filters on the outputs ensure damp-
ing of high-frequency components in the signal.
7.5 Pacer Unit
(Refer to QA-45 ECG Pacer Unit Component Location Diagram
and Schematic Diagram 8)
The pacer module consists of 3 blocks. These are the resistor group,
stimulus output and input amplifier.
1. Resistor Group. The value of the load resistance for the pacer
input is set using 6 relays, which are connected over a range of
resistors. The relays are controlled by U4, which in turn is con-
trolled from the microprocessor via a serial interface. The relay
configuration is determined in the software, and the relays are
operated in such a way that the load resistance can be varied
from 50 to 2300 ohm, with 50/100 ohm resolution. The relays
are of the side stable (latching) type, and will therefore use no
power from the batteries except when they are operated. Before
the relays are set, they are all reset via pin 18 on U4.
2. Stimulus Output. The stimulus signal that is generated on the
processor board is fed to U1, which, together with U2, makes
up a programmable gain block. The signal is amplified with U3,
and fed differentially over the load resistance via R17 and R18.
As the load resistance may be varied, and R17 and R18 are con-
stant, the amplitude out from U3 must be adapted to the load
setting. This is done using the gain block (U1 and u2), which is
controlled from the microprocessor via the same serial interface
as U4.
3. Input Amplifier. The input signal is measured over the load re-
sistor with the differential amplifier U5. The measurement sig-
nal is fed to the A/D converter's MUX from the output on U5.
U6 contains 2 amplifiers that each have a gain of 4 times (12
dB). The outputs from these amplifiers are also added to the A/
D converter MUX, so that one can choose between 3 different
dynamic measurement ranges. The measurement ranges are se-
lected according to the load setting.
5
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