ADA4505-2ARMZ-RL Analog Devices Inc, ADA4505-2ARMZ-RL Datasheet - Page 16

• Manufacturer Part Number: ADA4505-2ARMZ-RL
• Description: DUAL LV 10uA CMOS OPAmp AD8506derivative
• Manufacturer: Analog Devices Inc

Specifications of ADA4505-2ARMZ-RL

• Design Resources: Precision Pulse Oximeter LED Current Sinks Using ADA4505-2, ADR1581, and ADG1636 (CN0125)
• Amplifier Type: Voltage Feedback
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 0.006 V/µs
• Gain Bandwidth Product: 50kHz
• Current - Input Bias: 0.5pA
• Voltage - Input Offset: 500µV
• Current - Supply: 7µA
• Current - Output / Channel: 40mA
• Voltage - Supply, Single/dual (±): 1.8 V ~ 5 V, ±0.9 V ~ 2.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ADA4505-2ARMZ-RLTR

ADA4505-2
APPLICATIONS INFORMATION
PULSE OXIMETER CURRENT SOURCE
A pulse oximeter is a noninvasive medical device used for measur-
ing continuously 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 52. 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
analog switch is used to disconnect/connect the 1.25 V voltage
reference from/to each current circuit. When driving the LEDs,
the
ADA4505-2; 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 makes 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 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 the ½ ADA4505-2,
ADR1581, and ADG733 are 15 μA, 70 μA, and 1 μA, respectively,
making 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 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.
ADR1581
1.25 V voltage reference is buffered by ½ of the
ADG733
SPDT
Rev. 0 | Page 16 of 20
Figure 52. Pulse Oximeter Red and Infrared Current Sources Using the
Q1
IRLMS2002
Q2
IRLMS2002
CONNECT TO INFRARED LED
CONNECT TO RED LED
R5
12.4Ω
0.1%
1/4 W MIN
R1
20Ω
0.1%
1/8 W MIN
62.5mA
101mA
22Ω
1kΩ
22Ω
1kΩ
R2
R3
R7
R6
ADA4505-2 as a Buffer to the Voltage Reference Device
V
V
OUT1
ADA4505-2
OUT2
INFRARED CURRENT
RED CURRENT
22pF
7
C3
U1
1/2
22pF
1
C4
SOURCE
0.1µF
C1
SOURCE
+5V
V+
V–
8
4
+5V
V+
V–
8
4
ADA4505-2
1/2
U1
5
6
3
2
14
15
4
8
0.1µF
C2
D1
D2
D3
GND
16
V
V
+5V
DD
SS
7
S1A
S1B
S2A
S2B
S3A
S3B
EN
A2
A1
A0
U2
ADG733
12
13
2
1
5
3
9
10
11
6
+5V
V
R4
53.6kΩ
U3
ADR1581
REF
I_BIT2
I_BIT1
I_BIT0
I_ENA
= 1.25V