114

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4 Measurement of Biosignals and Analog Signal Processing

Tab. 4.3: Characteristic values of commercially available LEDs for photoplethysmography and

SpO2measurement; λpeak is the wavelength of maximum emission, TK is the temperature coefficient

at λpeak.

LED type

λpeak in nm

Bandwidth in nm

TK in nm/K

infrared

950

42

0,25

red

660

17

0,13

green

525

34

0,03

maximum emission λpeak changes with temperature. From this point of view, short

pulse times are preferable because then, at the same power, the absorbed electrical

energy remains low and the LED does not heat up much. A positive side effect of short

pulse times is the lower power consumption, which is important for battery-powered

sensors. On the other hand, shorter pulse times reduce the signal-to-noise ratio, which

in turn can be compensated by higher power. In the control scheme, both aspects

must always be weighed against each other. The LED is usually controlled by a mi-

crocontroller. A typical value for the pulse width is 250 μs and for the repetition rate

500 Hz. A phase without illumination is usually inserted between the LED pulses to

determine the ambient light offset. Alternatives to semiconductor LEDs are organic

LEDs (OLED) [42] or semiconductor laser diodes. OLEDs are based on flexible poly-

mer films that can conform to the body surface. Semiconductor laser diodes have the

advantage of very small bandwidth.

Photodiodes are used as detectors. These convert the incident light into a photo-

current which is proportional to the light intensity. Silicon-based photodiodes have

a spectral response of about 400 nm to 1100 nm with a maximum at about 920 nm.

Thus, the radiation from the LED listed in Table 4.3 can be detected by a Si photodi-

ode, but with quite different sensitivities in each case. For the SpO2 determination,

however, this circumstance does not cause any problems, since the spectral sensitiv-

ity is not included in the calculation of the oxygen saturation index, as will be shown

in subsection 6.3.2. A transimpedance converter is connected after the photodiode to

convert the photocurrent to a voltage (cf. Figure 4.22, middle). The resistor R1 also

provides amplification, and the capacitor C1 provides band-limiting and compens-

ation for the photodiode’s junction capacitance. When designing the low-pass filter

formed by R1 and C1, it must be ensured that the short light pulses of the LED are

transmitted without distortion.

SpO2 devices most commonly use finger clip sensors that work with transmission.

Light sources and detector are located on opposite sides of the finger (cf. Figure 4.22,

left). The finger clip is connected to a small control unit and integrated display, but

finger clips with integrated electronics and display are also available. For continuous

monitoring of newborns, sensor bands tied around the foot are often found instead

of finger clips. In reflection sensors, the LED and photodiode are on one side (cf. Fig-

ure 4.22, right). This offers the advantage that areas of the body such as the wrist or