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

Technical Supplement
The two variable gain circuits are functionally equivalent. The gain of each circuit is contingent upon
the signals received level and is controlled to bring each signal to approximately 3.5 V. Each circuit
uses an amplifier and one switch in the triple SPDT analog multiplexing unit U2.
The gain in each of the circuits is accomplished by means of a feedback loop, which includes one of
the SPDT switches in U2. The PWMs control whether the feedback loop is connected to ground or to
the amplifier output. The feedback is then averaged by C33/R25 (red), and C34/R24 (IR). The higher
the value of PWM2, the greater the IR gain; the higher the value of PWM1, the greater the red gain.
9.9.7.2 Filtering Circuits
The filtering circuits are illustrated in Figure 9-12.
These circuits consist of two cascaded second-order filters with a break frequency of 10 Hz. Pairs of
diodes (D1/D3 and D2/D4), that are located between VREF and ground at the positive inputs of the
second amplifiers, maintain the voltage output within the range of the A/D converter.
9-10
-5V
OFF/ON
VCC U2
16
VDD C
B
C35
7
VEE A
INH
.1UF
8
VSS
Z1
Z0
4
Z Y1
Y0
15
Y
X1
14
X X0
4053
R25
82.5K
R26
RED
3.32K
C24
.47UF
VREF
R24
82.5K
IR

Figure 9-11: Variable Gain Circuit

VREF
C122
.1UF
9
10
R39
11
6
47.5K
3
5
1
C126
2
220PF
Q6
VREF
C33
13
2N3906
12
1NF
9 U1C
10
LF444
C127
220PF
Q7
VREF
2N3906
C34
1NF
U1B
6
5
LF444
To LED control
PWM2
PWM1
IR LED/AV
RED
8
TP51
IR
7
TP52