BCI Advisor - Main Board - Digital Signal Processor (Isolated Side); Main Board - Patient Signal Acquisition (Isolated Side); Main Board - ECG (Isolated Side)

Chapter 3: Block Level Description
Advisor
®
Service Manual 3-7
Main Board - Digital Signal Processor (Isolated Side)
The main board uses a DSP to acquire and lter data from the following parameters: ECG, IBP, Temperature, and
Impedance Respiration. The DSP has a supervisory chip (U27) that can reset the DSP program. The DSP operates
from 1.8V and 3.3V. The 1.8V is used internally on the DSP. The 3.3 is used for the DSP I/O ports. U30 through U33
convert the 3.3V signals from the DSP to 5V signals used elsewhere on the isolated side of the board. U29 is used
to convert 5V signals to 3.3V signals that the DSP reads. The DSP is clocked by X4, which runs at 18.432MHz.
Main Board - Patient Signal Acquisition (Isolated Side)
The patient signal acquisition is performed by U35. U35 is a 16bit Successive Approximation Register (SAR)
analog-to-digital converter (ADC). This on board ADC will acquire the following parameters: ECG (leads I, II,
III, and V), both Temperature channels, and both IBP channels. An 8x1 multiplexer will be used select which
parameter will be read by the ADC. Three controls lines from the DSP control the multiplexer. The signal from the
multiplexer is passed through a unity gain amplier before it reaches the ADC. Two control lines from the DSP
will acquire the data from the ADC. The ADC has one control line that informs the DSP when data is ready to be
read. The input range of the ADC is ±2.048V. Data is acquired from the ADC at 240Hz.
Main Board - ECG (Isolated Side)
J19 is the input connector that brings the signals in from the side panel mounted ECG cable. There are 5 signal
lines that come from the patient. They are left arm (LA), right arm (RA), left leg (LL), right leg (RL) and the chest
(V) leads. RL is the signal reference and the other 4 leads provide the ECG signals. Each of the leads that come in
J19 and are protected with ½ watt 20KW carbon composition resistors and the MMDB1503A high conductance
diode packs. The 56pF capacitors and 20kW resistors provide a single pole low pass lter at 142kHZ. A second
low pass lter follows the rst low pass lter and it consists of a 110kW resistor and a 330pF capacitor. The second
low pass lter provides a single pole at 4.4kHz. The two series 22MW resistors provide a soft pull up that will raise
the input signal to +2V if an ECG electrode comes o the patient. The TLC2274 op amps (U44) provide a unity
gain buer stage for each signal lead. The outputs of the op amps feed a series of instrumentation ampliers,
it also provides the lead fail signal. These instrumentation amps generate the ECG leads lead I (LA-RA), lead II
(LL-RA), lead III (LL-LA), and V (C-(LA+RA+LL)/3). The instrumentation ampliers also provide a gain of 5.76. The
outputs of the instrumentation ampliers are passed through a series of low pass lters and gained again by
17.15, (U42 plus surrounding circuitry). The resulting signals are then sent to the ADC.
The ADG612 switch (U38) is used to provide 3 lead ECG support by selectively shorting one of the leads RA, LA,
or LL.
The CD4052 4x1 multiplexer (U41) is used to select which signal will be tested for lead fail. Two control lines from
the DSP are used select which lead is tested. The output of the multiplexer is sent to a voltage comparator (U46).
If a lead voltage is above 1.2V then the output of one of the voltage comparators will be high, indicating a lead
fail condition. If a lead voltage is below -1.2V then the output of the other voltage comparators will be high,
indicating a lead fail condition.
The ECG circuitry also provides pacemaker detection capability. The output of the dierential ampliers (U75 to
U78) feeds a multiplexer. One of the leads is selected for pacemaker detection. The chosen lead signal appears
at the output of the multiplexer and is low pass ltered at 8.6kHz by the circuitry surrounding U43. The signal is
dierentiated and clamped at approximately 0.5V by U43 and the supporting circuitry. The output of this stage
is gained by 7. The output of this stage drives two comparators. The comparators have thresholds of +0.45V and
-0.45V. A pace pulse passing through the detection circuitry will cause one of the comparators to go low. The
comparators interrupt the DSP to mark the time of a pacemaker pulse.
4.096 V
2
16
counts
=
62.5 uV
count