AN2944

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Doc ID 15467 Rev 1

9/12

4

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An example of the pulse wave measurement as recorded by the plethysmograph is shown in

Figure 7. The experiment was carried out on a healthy subject at rest because movement of

the finger causes additional compression of the blood volume which varies the DC

component of the measurement. The peaks in the signals correspond to a maximum in the

blood flow because the greater the voltage, the lesser the light which is received by the

phototransistor and the greater the voltage between the phototransistor collector and

ground.

The time interval between two consecutive peaks can be easily measured by combining the

A/D converter with a triggered timer of the microcontroller which computes this interval.

Figure 7.

Five seconds recording by plethysmograph of a healthy subject