Marquette Hellige GmbH
MicroSmart V 2.xx
Page 21
Servicing Instructions 227 470 35 B - 97.12
Pre-amplifier
The 9 connectable electrode signals are transmitted to 9 low-noise operations amplifiers behind the
protective input circuit. These operations amplifiers amplify the input signals by the factor 3.8. This
pre-amplification is necessary in order to maintain the maximum noise value of 15 µVpp over the
entire system.
The R electrode is used as reference for the other electrodes, with the effect that the difference to
the R electrode always applies after each input amplifier. This means that the signal L-R is
available at the output of the operations amplifier for the L electrode. This configuration is necessary
in order to obtain a common-mode rejection in addition to the push-pull modulation. The signal for
the push-pull modulation is taken from the R electrode. The lead-offs are computed in the software
from these differential signals, with the R electrode being ignored through the renewed differential
formation in the appropriate lead-offs. The 8 differential signals which remain from the original 9
electrode signals are transmitted to the modulators via a first order low pass with 1 kHz cut-off
frequency.
AD converter
After the pre-amplifiers the signals are transmitted to analog-digital converters. The AD converters
are the modulator type. Two ICs of type SDM_HEC2 are used, each of which include 5
converters. The components for the internal integrators, used to adapt the modulators to their task,
are connected to the pins IM2x, OUT2x, REFx, OUT1x and IM1x. Each differential signal at the
output of the AD converter is resolved to 18 bit. With reference to the patient input, one LSB
corresponds to 5µV. Conversion is parallel in all channels, i.e. without any time offset. The scanning
frequency is 1kHz. Using the appropriate control words, it can also be set for 500 Hz and 2 kHz.
A square-wave signal is visible at the outputs OUT1 thru OUT5, which occurs synchronous with
the SWITCH signal. The duty factor of this square-wave signal depends on the input signal. This
data stream reaches the IC of type Typ DIGI_HEC2.
PACE detection
After the pre-amplifiers the 8 electrode signals lead to a multiplexer 1:8. Using the 3 outputs OP1,
OP3 and OP4 of the chip Chips DIGI_HEC2 the multiplexer selects the electrode to be used for
PACE detection. The selected signal is routed via a first order high pass with 23 Hz cut-off
frequency and amplified by the factor 1,000. The signal then reache a window comparator with a
4,5 mV threshold with reference to the input.
The 2 outputs of the window comparator are put to a PIC processor for further PACE analysis. This
processor supplies a PACE bit if the appropriate signals of the window comparators apply and if the
pulse duration is 2ms. The overshoot of the PACE pulse is suppressed by the PIC processor.
