ADA4505-2/ADA4505-4

Rev. A | Page 16 of 20

APPLICATIONS INFORMATION

PULSE OXIMETER CURRENT SOURCE

A pulse oximeter is a noninvasive medical device used for

measuring continuously 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 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 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 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 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 to be improved, a more accurate

and low temperature coefficient drift voltage reference and

current source resistor should be used. 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.

8

4

6

7

5

C1

0.1µF

+5V

C3

22pF

R2

22Ω

R3

1kΩ

U1

1/2

ADA4505-2

62.5mA

CONNECT TO RED LED

R1

20Ω

0.1%

1/4 W 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%

1/2 W 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

ADA4505-2

V+

V–

Figure 52. Pulse Oximeter Red and Infrared Current Sources Using the

ADA4505-2 as a Buffer to the Voltage Reference Device