Figure 9-11: Variable Gain Circuit - Mallinckrodt Nellcor N-20PA Service Manual

Portable pulse oximeter

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Table Of Contents

Table of Contents

Technical Supplement

9.5.3

Signal Gain

9.5.3.1 Variable Gain Circuit

9-10

If the instantaneous average photocurrent (DC offset) is excessive and U1D

cannot bring it to VREF, the PHOTOI line to the CPU (HSI0) is activated. This

action is an indication of excess ambient light into the photosensor, or the

occurrence of excess noise in the input circuit. It also serves as a warning to the

instrument that the sensor signal may be contaminated and causes the software to

send an error message. After about 3 seconds of continuous photocurrent signal,

pulse search annunciation will begin. After about 10 seconds of continuous

photocurrent signal, zeros will be displayed.

The separated IR and red signals are amplified so that their DC values are within

the range of the A/D converter. Because the received IR and red signals are

typically at different current levels, the signal gain circuits provide independent

amplification for each signal as needed. The gain in these circuits is adjusted by

means of the PWM lines.

After the IR and red signals are amplified, they are filtered to improve the

signal-to-noise ratio and clamped to a reference voltage to prevent the combined

AC and DC signals from exceeding an acceptable input voltage from the A/D

converter.

The variable gain circuits are illustrated in Figure 9-11.

-5V

VCC

C35

.1UF

RED