Nellcor N-180 Service Manual page 27

N-180 Pulse Oximeter Service Manual
These pulse pairs are coupled to the first of two identical filter and
amplifier stages, each having a gain of approximately two. The signal is
then coupled to the last stage having a gain of two. The DC offset of the
resulting low frequency patient pulse waveform is proportional to the
ambient light intensity at the measurement site. The peak-to-peak
amplitude of the patient's pulse waveform is a result of the factors
expressed in the Beers-Lambert Law is used to calculate oxygen
saturation.
The pulse waveform at the red filter and amplifier output labeled RED,
==: must be a positive voltage, because the next step is to digitize the
waveform in the measurement system. To ensure that the waveform
does not move into a negative voltage region, the final amplifier stage
input has a +2.5 millivolt input, which guarantees a minimum positive
offset.
é Control signals:
Refer to figure FO10-1 (sheet 4 of 7 and sheet 7 of 7). Measuring SpO2 is
controlled by logic signals from micro controller U5. The control signals
are listed and defined below:
IRGATE* is a result of micro controller output PWM1* ANDed with output
CMT1. IRGATE* controls the transmission of the IR on and off levels
into the IR Filter and amplifier.
IRLED* from micro controller output CMSR4. IRLED* controls the selection of
the LED drive level at U4B and the direction of current applied to the
sensor LEDs.
REDGATE* is a result of micro controller output PWM1* ANDed with output
CMTO. REDGATE* controls the transmission of the red on and off
levels into the red filter and amplifier.
REDLED* from micro controller output CMSR5. REDLED* CONTROLS the
selection of the LED drive level at U4A and the current direction
applied to the sensor LEDs.
SATIN from micro controller output CMSR3. SATIN controls the inverting
amplifier through U10B and U14A.
Analog to Digital Conversion
Analog to digital conversion (figure 3-5) is accomplished by a dual-channel
device (U20) that produces a multiplexed serial output. Channel 1 (AIN1)
accommodates either VCAL1 voltage or the analog pulse waveform from the
red filter and amplifier channel. Channel 2 (AIN2) accommodates either
VCAL2 voltage or the analog pulse waveform from the IR Filter and amplifier
channel. These selections are determined by the configuration of quad field
effect transistor (FET) switch U24A, B, C, and D. Switching is controlled by
logic signal RESISTORS.
Logic signal ADCCHN determines the output bit stream multiplexing from
device U20. In turn, bit stream multiplexing determines the channel present
on the ADCDATA line. The ADCDATA line is coupled directly to micro
controller U5 for conditioning prior to transmission to the monitor display
processing circuits.
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