Marquette Hellige GmbH MicroSmart V 2.xx Page 22
Servicing Instructions 227 470 35 B - 97.12
Electrode labeling
The 18 bit result of the analog-digital conversion shows if one or more differential signals are
overloaded, i.e. if the differential voltage with reference to the patient input is greater than 0.6V. A
hysteresis of 15 mV (0.6 - 0.615 V) is provided for the query. The query takes place simultaneously
for all 8 channels. The information (1 bit/channel) is transferred to the CPU via the serial port in
the word Electrode label.
The overload of a channel can be caused by excessive polarization voltage (>600 mV) or by a
detached electrode. In the latter case a voltage of 1 V is transmitted to the amplifier inputs via the
100 M resistors on the protective input circuit.
One more circuit section is provided which handles the electrode error signal for the R and N
electrodes, because these cannot be detected individually with the converter overload. The
information is transmitted via INP1 and INP2 of the input port of ASIC DIGI_HEC2 in the status
word.
Lead labeling
Different leads can be connected to the patient input. The MicroSmart is designed for use with a 5
and 10 wire patient lead. Lead labeling is identified by means of different voltage values. For this
purpose the 10 wire lead holds a 402 resistor which, together with the series resistor, generates a
voltage in the range of 8.66 mV - 9.19 mV. A voltage in the range of 4.76 mV - 5.05 mV with a
resistance of 221 is generated through the 5-wire lead. The voltage is measured with the ninth
modulator of the ASIC SDM_HEC2. This means that the chip set must be configured for the
transmission of 9 channels.
Current supply
A DC-DC converter is used which generates two alternating secondary voltages from the primary
5V with 125 kHz cycle. Two stabilized direct voltages of +5V and -5V are then generated from both
of these alternating voltages. The 125 kHz cycle is delivered by the CPU. To suppress radiated
noise, a reactor is provided in the current supply.
Reference voltage editing
The reference voltage has values between + 2.5 V and - 2.5 V. Special emphasis is placed on low
intrinsic noise because it directly affects the results of the modulator. The low pass immediately
following the reference element with a cut-off frequency of 8 Hz serves the same purpose. A
compromise had to be found between low noise and rapid stability of the reference voltage
immediately after enabling.
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