Figure 9-12: Filtering Circuit - Mallinckrodt Nellcor N-20 Service Manual

Portable pulse oximeter

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Table of Contents

RED

9.9.8

AC Ranging

In order to measure a specified level of oxygen saturation and to still use a standard-type combined

CPU and A/D converter, the DC offset is subtracted from each signal. Because the DC portion of the

signal can be on the order of one thousand times the AC modulation, 16 bits of A/D conversion would

otherwise be required to accurately compare the IR and red modulations between the combined AC

and DC signals. The DC offsets are subtracted by using an analog switch to set the mean signal value

to the mean of the range of the A/D converter whenever necessary. The AC modulation is then

superimposed upon that DC level. This is also known as AC ranging.

Each AC signal is subsequently amplified such that its peak-to-peak values span one-fifth of the range

of the A/D converter. The amplified AC signals are then filtered to remove the residual effects of the

PWM modulations and, finally, are input to the CPU. The combined AC and DC signals for both IR

and red signals are separately input to the A/D converter.

9.9.8.1 Offset Subtraction Circuits

The AC variable gain control circuit is illustrated in Figure 9-13 (at the end of this section). Voltage

dividers R22 and R41 (red), and R31 and R5 (IR), which are located between VREF and ground,

establish a baseline voltage of 2.75 V at the input of the unity gain amplifiers U7C (red) and U7D

(IR).

C16

100K

.12UF

U4D

OP490SO

100K

C15

.068UF

+12V

C18

R11

100K

.12UF

U4A

R10

OP490SO

100K

C17

.068UF

-5V