BCI Advisor, Service Manual

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Chapter 3: Block Level Description
3-8 Advisor
®
Service Manual
Main Daughter Board - Impedance Respiration option (Isolated Side)
Respiration is calculated by measuring the rate of change in AC resistance at 48 kHz across two ECG electrodes.
A 48 kHz sine wave is generated by U10 and resistors R23 – R29. It is high pass filtered by C30 and R30 and
amplified by U9 and the surrounding circuitry. The resulting signal is sent out the RA connection. The sine wave
signal is inverted and also sent out either LA or LL. The DSP selects which lead to drive. The MMBD1503A diode
packs and 10k carbon composition resistors protect the output stages from defibrillators. The pair of 2.2nF
capacitors AC couple the sine wave signal and the 10 K resistors transform the sine wave excitation signal into a
current.
The respiration receiver circuitry measures the voltage amplitude of the 48kHz sine wave that is created when
the current from the transmitter is passed through the body. As the patient breathes, the amplitude of the
received 48kHz sine wave voltage fluctuates. Leads LA, RA, and LL are processed. The 20K carbon composition
resistors and MMBD1503A diodes protect the front end circuitry from defibrillators. The 20K resistors and
56pF capacitors form a single pole low pass filter. The signal coming from RA is buffered and drives one leg of
an instrumentation amp U3. The other input to the instrumentation amp is driven by LA or LL. The signal LDII
controls the CMOS switch and selects which lead to pass to the instrumentation amp. The instrumentation
amplifier is running with a gain of 2. The output of this stage is AC coupled and buffered by C25, R8 and U4. The
next stage in the receiver circuitry is a synchronous demodulator. The output of the demodulator is proportional
to the amplitude of the 48kHz sine wave feeding it. The demodulator is simply a stage that alternates its gain
from +1 to –1. The signal RCLK48KHZ (48kHz) controls the switching of signal gain from +1 to –1. Capacitor C3
filters the rectified sine wave output and creates a slow moving baseband signal that fluctuates at the rate of
respiration. The output of the demodulator is converted to a digital value by the sigma delta converter U6 at a
rate of 30 Hz.
Main Daughter Board - IBP & Temperature option (Isolated Side)
The IBP/Temperature board has two channels of invasive blood pressure acquisition circuitry. The two circuits
are independent and identical. The blood pressure transducer is a bridge style strain gauge with an output
sensitivity of 5mV/mmHg. The transducer is excited with a DC voltage of +4.096V. U6 (ADR292) generates the
excitation voltage. The excitation voltage is passed through an op-amp (U8 and U9) that acts as a short circuit
protection device. If the board senses a short circuit on the excitation voltage then the op-amp will shut down
and remove power from the excitation pin. The excitation voltage is also divided by 2, through circuitry around
(U8 and U9), which provides the input instrumentation amplifier and ADC with a reference voltage based on
each channels excitation voltage.
The excitation voltage is applied from one side of the bridge to ground. The other two legs of the bridge provide
the input signal to the circuit board. These signals come in on J1 pins 2 and 3 for channel one and J1 pins 7 and 8
for channel two. They are protected by MMBD1503A diode packs. The input signals fed into an instrumentation
amplifier, U2 and U3. The instrumentation amplifiers gain the signal by 100. The output of the instrumentation
amplifier is referenced to half of the excitation voltage. The signal then goes into a 72Hz low pass Bessel filter. The
output of the Bessel filter feeds the signal to the main board ADC.
The IBP/Temperature board provides two channels of temperature monitoring. The circuitry is designed to
interface with YSI series 400 thermistor based temperature probes manufactured by Yellow Springs Incorporated.
Connector J1 pins 11 and 13 are used to bring the temperature probe signals onto the board. The signals are
protected by MMBD1503A diode packs. A multiplexer is used to select which temperature probe or calibration
resistor is read by the main board ADC. The DSP has control of the multiplexer and it will select which thermistor
or calibration resistor will be read by the ADC. An additional DSP line will select between two calibration resistors
that are sent to the multiplexer. The board has three calibration resistors, R38-R40, that have a tolerance of 0.1%.
After the multiplexer an amplifier is used to gain the signals by 3.5. A constant voltage source of 3.8V is used to
drive the thermistors and calibration resistors. The voltage source is generated by U12 plus surrounding circuitry.