ST AN2944 APPLICATION NOTE

AN2944

Application note

Plethysmograph based on the TS507

Introduction

This application note provides a method to make an analog front-end plethysmograph (from
the ancient greek plethysmos, which means increase), which is an instrument for measuring
changes in volume within an organ or whole body, usually resulting from fluctuations in the
amount of blood or air it contains. In this context, we refer in particular to the fluctuations in
the quantity of blood in blood vessels.

January 2010 Doc ID 15467 Rev 1 1/12

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Contents AN2944

Contents

1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Model for creating a plethysmograph . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Analog front-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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AN2944 List of figures

List of figures

Figure 1. Light absorption by hemoglobin at different wavelengths. . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 2. Schematic representation of the pulse wave transit time (PWTT) . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Sensing system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Schematic diagram for the analog front-end plethysmograph . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Analog front-end demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Entire system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 7. Five seconds recording by plethysmograph of a healthy subject . . . . . . . . . . . . . . . . . . . . . 9

Doc ID 15467 Rev 1 3/12

Description AN2944

1 Description

1.1 Purpose

This application note describes a demonstration board which is designed for demonstration
purposes only, and shall not be used as a medical instrument, nor for domestic installation.
The technical data included in this document shall be taken as a guideline.

1.2 Theoretical background

The contraction of the heart causes a pressure wave which moves along the arteries
producing, as a consequence, their expansion during the positive peak. The wave is faster
than the blood flow and its speed reaches a few meters per second. The pulse wave can be
sensed at a limb as well as the wrist or a finger.

The two possibilities for sensing the pulse wave are via a pressure sensor or through an
optoelectronic plethysmograph which uses the physical mechanism of light absorption.
Hemoglobin present in the blood absorbs the light emitted in a particular wavelength range
(see figure below). In this system infrared light can be used with no distinction between
oxyhemoglobin and deoxyhemoglobin.

Figure 1. Light absorption by hemoglobin at different wavelengths

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For this reason, the light which is able to pass through the body at a wavelength of
600-900 nm depends on the quantity of hemoglobin flowing in the blood vessels. Therefore,
since the quantity of hemoglobin at a given time is proportional to the pulse wave at that
time, it is possible to calculate the pulse wave from the transmitted light.

The information from the pulse wave is often used in conjuntion with a three-lead
electrocardiogram (ECG or EKC) with the differential electrodes placed on the thorax. In
fact, from the two measurements, it is possible to calculate the PWTT (pulse wave transit
time) which is the time interval between the R wave peak of the ECG and the positive peak
of the plethysmography (see Figure 2) and can be correlated with blood pressure.

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