AD8506/AD8508

Rev. A | Page 15 of 20

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 45. 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 ½ 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 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 AD8506,

ADR1581, and ADG733 are 25 μA, 70 μA, and 1 μA, respectively,

making a total of 96 μA current consumption (480 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.

8

4

6

7

5

C1

0.1µF

+5V

C3

22pF

R2

22Ω

R3

1kΩ

U1

1/2

AD8506

62.5mA

CONNECT TO RED LED

R1

20Ω

0.1%

1/8W MIN

RED CURRENT

SOURCE

8

4

2

1

3

+5V

C4

22pF

R6

22Ω

R7

1kΩ

101mA

CONNECT TO INFRARED LED

R5

12.4Ω

0.1%

1/4W MIN

INFRARED CURRENT

SOURCE

Q2

IRLMS2002

Q1

IRLMS2002

S1A

S1B

D1

S2A

S2B

D2

S3A

S3B

D3

GND

A2

A1

A0

EN

I_BIT2

I_BIT1

I_BIT0

I_ENA

R4

53.6kΩ

U3

ADR1581

C2

0.1µF

+5V

U2

ADG733

+5V

14

15

4

16

8

12

13

2

1

5

3

9

10

11

6

7

V+

V–

U1

1/2

AD8506

V+

V–

Figure 45. Pulse Oximeter Red and Infrared Current Sources Using the

AD8506 as a Buffer to the Voltage Reference Device