BCI Capnocheck Plus - LED Drive and Signal Processing

Chapter 4: Oximeter Board Circuit Description
LED Drive
LED Drive circuitry is used to turn on and off the Oximetry probe LED’s,
precisely controlling the current through them. The second half of the variable
potentiometer U11 (sheet 3) is used to generate voltage between 0 and
2.5VDC. U11 is controlled, serially, by the microcontroller. Operational
amplifier U10B together with transistor Q3 create a constant current sink. The
current is proportional to the voltage generated by variable potentiometer. The
H-bridge Q4-Q5 (sheet 4) is used to activate either LED, red or infrared, inside
the Oximeter probe. The following table describes the states of the H-bridge:
RED-DRV IR-DRV /RED-ENBL AR-ENBL Function
1 0 0 1 Red LED is on
0 1 1 0 Infrared LED is on
0 0 1 1 “Open circuit” state
In this table 1 indicates logic 1, 0 indicates logic 0.
The “Open circuit” state is used to check for a possible probe cable fault. In
case of probe cable fault, one of the LED wires can short to the ground shield
causing high current through the probe LED. To prevent this, before turning
any LED on, the microcontroller checks the cable by “floating” the H-bridge. If
the cable is shorted, CON3 and CON2 connections are pulled low, and
comparator U13B generates /PRB_FAULT signal, which causes the
microcontroller to shut down the LED excitation cycle and generate
appropriate message.
Analog Signal Processing
The differential transconductance amplifier (sheet 4), formed by U12 and U9B,
converts the photodetector’s current output to a voltage at TP9 (V-AMB).
Amplifier U8B offsets the signal at TP9 so the signal baseline is at 3.4 VDC,
allowing a wider signal range for the negative-going pulses at TP9.
Comparator U13A is used to inform the microprocessor if U9B is saturated by
an excess of ambient light.
V-AMB is passed through blocking capacitor C19 (sheet 3) to remove the
signal’s DC component. The signal is then buffered and amplified by U9A.
Gain is defined by the first channel of digital potentiometer U11. The output
of U9A is routed to the integrator-filter-offset circuitry U8A, controlled by
4-2
Capnocheck Plus Service Manual