AD8505ARJZ-R2 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8505ARJZ-R2
Analog Devices
AD8505ARJZ-R2 Datasheet PDF : 20 Pages
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APPLICATIONS INFORMATION
PULSE OXIMETER CURRENT SOURCE
A pulse oximeter is a noninvasive medical device used for con-
tinuously measuring the percentage of hemoglobin (Hb) saturated
with oxygen and the pulse rate of a patient. Hemoglobin 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, and the detector is synchro-
nized 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 49. 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 or connect the 1.25 V voltage
reference from or to each current circuit. When driving the LEDs,
the ADR1581 1.25 V voltage reference is buffered by half of the
AD8506; the presence of this voltage on the noninverting input
forces the output of the op amp (due to the negative feedback)
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 Ω R5 current source resistor, creating the flow
of 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 the AD8506 (that is,
half of the AD8506), ADR1581, and ADG733 are 25 μA, 70 μA,
and 1 μA, respectively, resulting in a total of 96 μA current con-
sumption (480 μW power consumption) per circuit, which is good
for a system powered by a battery. If the accuracy and temperature
AD8505/AD8506/AD8508
drift of the total design need to be improved, then a more accurate
and low temperature coefficient drift voltage reference and current
source resistor should be utilized. 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
down the rise time of the N-MOSFET when it turns on. These
elements may not be needed, or some bench adjustments may
be required.
+5V
CONNECT TO RED LED
62.5mA
+5V
C1
0.1µF
U1
1/2
AD8506
C2
0.1µF
16
VDD
U2
ADG733
+5V
R2
22Ω
VOUT1
8
5
V+
7
V–
Q1
6
4
IRLMS2002 C3
22pF
R3
1kΩ
R1
20Ω
0.1%
1/4W MIN
RED CURRENT
SOURCE
14 D1
S1A 12
S1B 13
15 D2
S2A 2
S2B 1
4 D3
S3A 5
S3B 3
8
GND
A2 9
A1 10
A0 11
EN 6
R4
53.6kΩ
VREF = 1.25V
U3
ADR1581
VSS
CONNECT TO INFRARED LED
7
101mA
U1
+5V
1/2
AD8506
R6
22Ω VOUT2
8
3
V+
1
V–
Q2
2
4
IRLMS2002
C4
22pF
R7
1kΩ
R5
12.4Ω
0.1%
INFRARED CURRENT
SOURCE
1/2W MIN
I_BIT2
I_BIT1
I_BIT0
I_ENA
Figure 49. Pulse Oximeter Red and Infrared Current Sources Using the
AD8506 as a Buffer to the Voltage Reference Device
Rev. E | Page 15 of 20
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