Novametrix Medical Systems 515 Service Manual page 20

Theory of Operation:Analog Board #2288
Service Manual; Model
(00H - 3FH). If writing then the next 16 bits sent is the value to be sto
the CS line is taken low then high and the DO line is scanned until a high 1
is read. This indicates that the data has been received and the next opera
can begin. If the read operation was selected the data would be present at
DO line once the address was sent.
1.4.5 LED Drive circuitry
The intensities of the REDLED and IRLED are controlled by the proce
through the use of IC20, an 8 bit dual digital to analog converter (DAC).
"A" DAC corresponds to the REDLED and is selected when pin 6 is
(DACA). The "B" channel corresponds to the IRLED and is selected
bringing pin 6 high (DACB). Steady state DC output voltages which repre
the digital inputs are developed at IC22 pin 1 for the REDLED and IC22 p
for the IRLED. Because both DACs use the same reference voltage (VRI
the output voltage swing from 00H to FEH is the same for each (0 - +4.
VDC).
The voltages that drive the LEDs are pulsed, this is controlled by h
pins 9, 10, and 11 for the REDLED, and pins 14, 15, and 16 for the IRL
When IC25 pin 9 goes low the switch between pins 10 and 11 is closed,
voltage at IC22 pin 1 is now present at IC25 pin 10 until pin 9 goes high. "
pulsing is controlled by the LEDl line. The same applies for the IRLED cl
nel, when pin 16 of IC25 is low the switch from pins 14 to 15 is closed and
voltage at IC22 pin 8 is present at pin 15 of IC25. The IRLED pulsing is (
trolled by the LED2 line. Resistors R53, R54, R55, and R56 form a voltage
vider for each channel to bring the maximum pulse voltage down from 4.1 \
to 1 volt.
The pulses at the voltage divider of R53 and R55 drives pin 5 of IC22 wl
with pins 6, 7, and Q7 is configured as a unity gain amplifier. The voltage
appears at pin 5 of IC22 will appear at the junction of R60 and the emitte
Q7, the maximum of which will be 1 volt. Since the collector of Q8 is tied to
cathode of the REDLED, when Q7 is pulsed by IC22 pin 7 the REDLED
conduct. Resistor R60 limits the current through the LED to 303 mA
volt/3.3^2). Diode D7 is used to limit any negative offset that may be ir
duced by IC22 to -0.7 volts. Pins 12, 13, 14 of IC22 and Q8 form the unit j
amplifier for the IRLED channel. The operation of the circuit is the same as
the RED channel. Resistor R61 limits the current through the IRLED to
mA (1 volt/5.6Q) and D8 protects against any negative offset.
1.4.6 Photodiode Return Path
The photodiode in the sensor is connected to pins 9 and 10 of IC21 whic
configured as a current to voltage convertor. When light strikes the photodi
it induces a small current to flow from anode to cathode, this creates a cur
flow into pin 10 of IC21 (non-inverting input). This current flow will produc(