Datasheet

ADA4505-1/ADA4505-2/ADA4505-4
Rev. D | Page 16 of 24
APPLICATIONS INFORMATION
PULSE OXIMETER CURRENT SOURCE
A pulse oximeter is a noninvasive medical device used for
continuously measuring the percentage of hemoglobin (Hb)
saturated with oxygen and the pulse rate of a patient. Hemo-
globin that is carrying oxygen (oxyhemoglobin) absorbs light in
the infrared (IR) region of the spectrum; hemoglobin that is not
carrying oxygen (deoxyhemoglobin) absorbs visible red (R) light.
In pulse oximetry, a clip containing two LEDs (sometimes more,
depending on the complexity of the measurement algorithm) and
the light sensor (photodiode) is placed on the finger or earlobe
of the patient. One LED emits red light (600 nm to 700 nm), and
the other emits light in the near IR (800 nm to 900 nm) region.
The clip is connected by a cable to a processor unit. The LEDs
are rapidly and sequentially excited by two current sources (one
for each LED) whose dc levels depend on the LED being driven,
based on manufacturer requirements; the detector is synchronized
to capture the light from each LED as it is transmitted through
the tissue.
An example design of a dc current source driving the red and
infrared LEDs is shown in Figure 57. These dc current sources
allow 62.5 mA and 101 mA to flow through the red and infrared
LEDs, respectively. First, to prolong battery life, the LEDs are
driven only when needed. One third of the ADG733 SPDT
analog switch is used to disconnect/connect the 1.25 V voltage
reference from/to each current circuit. When driving the LEDs,
the ADR1581 1.25 V voltage reference is buffered by one half of
the ADA4505-2; the presence of this voltage on the noninverting
input forces the output of the op amp (due to the negative feed-
back) to maintain a level that causes its inverting input to track
the noninverting pin. Therefore, the 1.25 V appears in parallel
with the 20 Ω R1 or 12.4  R5 current source resistor, creating
the flow of the 62.5 mA or 101 mA current through the red or
infrared LED as the output of the op amp turns on the Q1 or Q2
N-MOSFET IRLMS2002.
The maximum total quiescent currents for one half of the
ADA4505-2, the ADR1581, and the ADG733 are 15 µA, 70 µA,
and 1 µA, respectively, for a total of 86 µA current consumption
(430 µW power consumption) per circuit, which is good for a
system powered by a battery. If the accuracy and temperature
drift of the total design need improvement, use a more accurate
and low temperature coefficient drift voltage reference and current
source resistor. C3 and C4 are used to improve stabilization of U1;
R3 and R7 are used to provide some current limit into the U1
inverting pin; and R2 and R6 are used to slow the rise time of
the N-MOSFET when it turns on. These elements may not be
needed, or some bench adjustments may be required.
07416-047
8
4
6
7
5
C1
0.1µF
+5V
C3
22pF
R2
22
R3
1k
V
OUT1
U1
1/2
ADA4505-2
62.5mA
CONNECT TO RED LED
R1
20
0.1%
1/4 W MIN
RED CURRENT
SOURCE
8
4
2
1
3
+5V
C4
22pF
R6
22
R7
1k
V
OUT2
101mA
CONNECT TO INFRARED LED
R5
12.4
0.1%
1/2 W MIN
INFRARED CURRENT
SOURCE
Q2
IRLMS2002
Q1
IRLMS2002
S1A
S1B
D1
S2A
S2B
D2
S3A
S3B
D3
GND
A2
A1
A0
EN
V
SS
V
DD
I_BIT2
I_BIT1
I_BIT0
I_ENA
R4
53.6k
U3
ADR1581
C2
0.1µF
+5
V
U2
ADG733
V
REF
= 1.25V
+5V
14
15
4
16
8
12
13
2
1
5
3
9
10
11
6
7
V+
V–
U1
1/2
ADA4505-2
V+
V–
Figure 57. Pulse Oximeter Red and Infrared Current Sources Using the
ADA4505-2 as a Buffer to the Voltage Reference Device