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VREF
R63
2.0K
R160
51.1K
R23
1M
VREF
C40
390PF
R17
3.32K
R58
11
9
3.32K
10
U6C
LTC201
TP44
SENSOR
INPUT
P1
L1
5
9
4
8
3
7
2
6
1
DB9F
Figure 9-9: Differential Synchronous Demodulation Circuit
The same line that controls the on/off pulsing of the LEDs controls U6D, a
single-pole-single-throw (SPST) analog switch. When either of the LEDs is on
(the line is low and the switch is closed), U35 is used as a noninverting
amplifier. When the LEDs are both off, U35 is used as an inverting amplifier.
The signal at the output of amplifier U35 is then demultiplexed.
The CPU HSO lines SAMPRED and SAMPIR, which are both active low,
control SPST analog switches U6A and U6B respectively. Switch U6A is closed
to sample the red signal; switch U6B is closed to sample the IR signal. The
sampling rate for both switches is 10 kHz. Switching is coordinated with the
LED transmission so that the IR and red signals are each sampled twice per
cycle; that is, once when the LED is off (signal inverted), and once when the
LED is on (signal not inverted). The filtering circuit that follows has a long time
constant, thereby acting as an averaging circuit.
A simplified N-20PA HSO timing diagram is illustrated in Figure 9-10, at the
end of Section 9.
R159
88.7K
+12V
.1%
7
5
2
U35
6
3
LT1097
R158
4
1
8
88.7K
.1%
-5V
8
6
7
U6D
LTC201
U1D
12
14
13
LF444
R2
1M
C1
TP49
18PF
+12V
R19
R1
280K
4
C59
182K
.047UF
3
U1A
1
2
LF444
R157
511K
1
1
C22
-5V
.047UF
R56
15.8K
Technical Supplement
TP78
SAMPRED
SAMPIR
OFF/ON
1
R26
3
2
U6A
3.32K
LTC201
C24
.47UF
VREF
1
6
R27
14
15
U6B
3.32K
LTC201
C25
.47UF
VREF
R57
40.2K
3
4
TP79
82.5K
RED
VREF
IR
VREF
U9B
5
PHOTOI
LT1013
9-9
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