Smiths Medical BCI 3304, Service Manual

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Chapter 2: Oximeter Circuit Description
3304 Service Manual 2-5
A/D Converter
A/D converter (ADC) U8 is configured as a two channel 16-bit ADC.
The first channel is used by the signal processing circuitry. The second
channel is used to read battery voltage. The ADC’s reference voltage
input of 3.78VDC is formed by the filter-amplifier U4B. The following
sequence describes an ADC conversion cycle:
After ADC channel is chosen by the signal CH1/2, the CRS/FIN •
signal is set to log. 1 for a few microseconds to course charge the
ADC input sampling capacitor. Then it is switched back to the
fine mode to reach 16 bit accuracy level.
Signal /HOLD is generated to start conversion.
•
After a few microseconds data is read serially clocked by the CKS •
signal.
LeD Drive
The red LED drive circuit consists of a programmable current sink
formed by U3A, U6 and Q3, and MOSFET bridge Q1 and Q2. LED
current is defined by the voltage generated by the second channel of
digital potentiometer U6 referred to the output of the 2.5VDC voltage
reference U7. This voltage is compared with the voltage across current
sense resistors R26 and R27 by amplifier U3A. The MOSFET bridge is
controlled by four digital signals connected to the gates of MOSFET’s.
The combination of these signals reverses polarity of the current
through probe LED’s and “floats” probe LED’s. When LED’s are “floating,”
comparator U30B checks for a possible short of the LED control
conductor to ground which may happen during a probe cable failure. If
this happens, the microprocessor does not turn the probe LED’s on and
informs the operator of the probe problem.