2. DEVELOPMENT OF AN ELETRO-OPTICAL
PHOTOPLETHYMOGRAPHY SYSTEM
(Laboratory Report )

3. Abstract
A simple and effective method of displaying changes in blood volume in peripheral vascular
bed from photoplethysmography has been developed using electro-optical
photoplethysmograph system. This method is developed to be incorporated as noninvasive
monitoring of blood volume from a patient in intensive care unit. This implementation
includes Infrared emitter s IRED light source, silicon photodiode as optical sensor to convert
light detected into voltage value, active and passive band pass filter circuits to reduce noise
levels in signal, amplifier designed configured for gain of 10 and digital oscilloscope in order
to observe the PPG signal generated due to systolic and diastolic volume change in atrial
blood. PPG waveform comprising of a pulsatile AC component synchronised to ach heart
beat superimposed on a slowly varying DC component. Proper high pass filter has been used
to reduce dominant low frequencies with no distortion in pulse shape. During experiment
pulses were measured by placing index finger between the light emitter and photodetector
and resultant visible pulse shape distortion were investigated
Introduction
A photoplethysmograph (PPG) is a device used to optically obtain a volumetric measurement
of an organ. It is an qualitative measurement. Commoly measured volume changes that are
caused by
 Breathing
 Blood forced into vessels such as arteries, veins, capillary
 Pumping of heart
Tissue or organ under investigation is bathed with a light of a suitable wavelength . Short
wavelength of light are strongly absorbed by melanin. Window in the absorption spectra of
water that allows visible red and near Infrared light to pass more easily, thereby facilitating
the measurement of blood volume at these wavelengths. Thus, red and near infrared
wavelengths are often chosen for PPG light sources. Illuminating light is reflected, absorbed
and scattered in tissue and blood. This light is measured with photodiode which produces
electrical signal. This received signal is assumed to be measure of volume changes due to
localised blood flow.
PPG can be operated in two modes; transmission mode and reflection mode.
Transmission mode: The light source is placed on one side of organ and detector is situated
on the opposite side of the organ. It is clearly illustrated in the following figure.

4. Fig.1. Photoplethysmographyprobe usedin Transmissionmode
Reflection mode: In this method, light source and photo detector are placed adjacent to each
other. Photodetector is placed to receive the light reflection from the tissues or arterial blood
.
Fig.2. Photoplethysmographyprobe used inReflectionmode
Signal generated by the phototodetector is amplified and used to generate corresponding
photoplethysmograph.
It has been proved that photoplethysmography can be a reliable tool to measure arterial
blood flow to the feet in people with diabetes(fig.3) which are particularly prone to peripheral
arterial disease.(C.Scanlon,2012) It is a non invasive technique to measure the arterial pulse
waveform from either arterial wall deflection or force at the surface of the skin above a
palpable vessel. Typically, sensors used in such technique are not directly calibrated in terms
of pressure , but ideally respond proportionally to pressure. Sensors can called as volume
sensors as they relies on the adjacent tissues surrounding the vessels as a non-deflecting

5. frame of reference. Skin deflections directly above the vessel are then measured relative to a
reference frame to represent arterial pressure.
Moreover, pulse oximetry use the technique of PPG to estimate the oxygen saturation values.
Incorporation of transreflectance PPG sensors has been reduced failure rate of pulse
oximeter which is more in the case of conventional PPG signals. (Shafiq.M, 2012) Key
factors that can that influence the amount of light received by photodetector such as blood
volume, blood vessel wall movement and orientation of red blood cells(RBCs). Following
diagram illustrates the arrangement of components to measure oxygen saturation using PPG
probe.
Fig.4 Block diagram of New multimode PPG processing system(Medical and biological
engineering and computing,2012)
Fig3.PPGfor measuringtoe bloodpressure

6. Orientation effect has been demonstrated by recording pulsatile waveforms from dental pulp
and in a glass tube where volumetric changes should not possible and more recently by Nas
lund (2006) who detected pulsatile waveform in bone. As published by the K. Hamunen
(2012),photoplethysmographic pulse wave amplitude (PPGA) and heart rate (HR) can be
used to measure cold, nociception-induced autonomic responses. Principle states that
correlation between intensity of experimental pain to changes in physiological variables
represents the autonomic nervous system response to pain.
Fig 5. Changes in PPGA after 43°C and 48°C heat stimuli in 29 volunteers.
Thus,PPG based technology can be found in wide range of commercially available medical
devices for measuring oxygen saturation,blood pressure,cardiac output,assessing autonomic
function and also detecting peripheral vascular diseases.
Methods
Systematic arrangement of the electrical and optical components to build the complete
Infrared Photoplethysmography system (PPG) system is shown in following fig.6.
The complete circuitry consist of optical components, electrical filter building components
and digital display devices to analyse data.

7. Fig.6. Block diagram of the Infrared PPG system
Sensor: Photodetector used to detect the scattered light from IRED is silicon photodiode. It
works on the photoelectric effect and capable of converting light into current or voltage
depending on mode of operation. The photodetector used in this experiment is a 5.2 mm²
silicon photodiode with transimpedance amplifier in the form of a 8- pin TO99 metal
package.
(a) (b)
Fig.7. (a)Symbol and outlook of silicone photodiode
(b) Activation of diode by current source
I-V amplifier: Transimpedance amplifier is used in connection with the photo detector to
convert current generated by photodetector into voltage for further processing. Ideally it
should have low input impedance to prevent any large value of voltage. In this experiment we
have used 8- pin TO99 metal package as an I-V amplifier.
Fig.8. Transimpedance I-V amplifier in inverting configuration

8. Infrared emitter current driver circuit: It has been designed for constant current source
using input operational amplifier in non -inverting configuration (TL 084) and a series NPN
transistor. The output intensity is depends on forward current (40mA) flowing through
IRED.
Fig.9. IRED constant current source driver circuit
Ideal Operational amplifier is characterised by :
Parameter Ideal Op-amp Practical Op-amp
Calculations: Circuit was mounted on breadboard as indicated in fig. Voltage of 9V is
supplied with voltage source and voltage across R3 is taken as 5V assuming that voltage
divides following voltage divider circuit and remaining 4 V passed through the op-amp. By
Ohm’s law;
R3 = V3 / I3
=5/ (40*10^-3

9. R3 = 125 ohms
By potential divider circuit,
Voltage across R2; V2= (R1/R1+R2)*Vin
5V=(R1/R1+R2)*9V
.....(1)
Voltage acrossR1; V1=(R2/R1+R2)*Vin
4V= (R2/R1+R2)*9V .....(2)
Solving equations (1) and (2) simultaneously for R1 and R2 we get;
R1=2KOhms ; R2=2.5K Ohms
So by proper selection of resistor values ensures the current of 40 mA flowing through the
IRED which important for the further accurate performance.
Filter Design: A circuit capable of selectively filtering one frequency or range of
frequencies out of a mixture of different frequencies is called filters is designed. It only
allows the fundamental waveform frequency to pass through, blocking all higher frequencies.
Active filter built with active components such as transistors , op-amp with resistors and
capacitors where passive filter includes passive components as resistors , capacitors ,
inductors. Filters have been used to split the AC and DC component of the photodetector I-V
amplifier circuit.
Fig.10. Band PassFilter circuit design with active and passive components

10. 2-pole Low pass active Butterworth filter: Importance of this filter is to extract the AC
component from combined ACand DC PPG signal. Frequency response of filter is depends
on Gain /bandwidth product of the op-amp being used.
(a) (b)
Fig.11. (a) circuit designed for active low pass filter (b) frequency response
(http://www.electronics-tutorials.ws/filter/filter_8.html)
High pass passive filter: It is designed in order to block the DC component of the output.
Selection of resistor and capacitor determines the breakpoint frequency;
Calculations :
High pass filter: It is given that C1=2.2μF, R1= 150K Ohm,
Hence; cut –off frequency can be given as
fc= 1/ (1/ (2п*R1*C1)= 0.482 Hz
Cut-off frequency of high pass filter= 0.482 Hz
Low pass active filter:
Cut off frequency for low pass active filter is given by
fc= 1/2 𝜋RC
= 1/2 𝜋* 2.2Mohm*0.47uF= 0.15Hz
Cut off frequency of low pass active filter= 0.15Hz

11. 2-pole low pass Butterworth filter:
Cut off frequency is given by
fc = 1/(2 𝜋 √ 𝑅2𝑅3𝐶2𝐶3) = 19.40Hz
Cut-off frequency of low pass Butterworth filter = 19.40 Hz
Calculation of Gain of 2-pole low pass butterworth filter:
Gain = (R5 + R4 ) / (R5) = 1.57 dB
Gain of Butterworth 2 pole active low pass filter = 1.57 dB
Results: The complete circuit consists of current driver , I-V amplifier and band pass filters is
mounted on breadboard and supplied with corresponding values of voltages. Output is
observed on the two channel digital oscilloscope .
Fig. 12. Photograph of final circuit mounted on breadboard for PPG system
The readings of output voltage and corresponding frequency in Hz are noted to plot the
frequency response of the filter circuit as shown in fig.11.
IRED constant
currentdriver
PhotodetectorandI-V
amplifier
Highpass and 2 pole
ButterworthLow Passfilter ,
Active Low Pass filter
Amplifier designed for
10x amplification

12. Cut-off frequency is the frequency either above or below which power output of the circuit
has fallen to a given proportion of power in passband. Most frequently this proportion is one
half the passband power, also referred to as the 3 dB point since a fall of 3 dB corresponds
approximately to half power.
Cut off frequencies of band pass and low pass filters were found to be 0.459 Hz and 19.54 Hz
respectively. Output from the filter is appears to be small so it is made pass through amplifier
with gain 10 and amplified signal is sent to the oscilloscope. When the finger is placed in
between emitter and diode waveform shows peaks at definite intervals as follows
Fig.13. Typical PPG signal waveform observed on digital oscilloscope
Discussion: PPG signal has been successfully obtained and our findings are in correlation
with the previously published experimental measurements(Hertzman and Randall,1948).
Pulse wave can be characterised in two phases; anacrotic phase being rising edge of pulse
-8
-6
-4
-2
0
2
4
6
0 5 10 15 20 25 30 35 40
Gain(dB)
Frequency (Hertz)
Frequency Response

13. concerned with systole and catacrotic phase being falling edge of the pulse concerned with
diastole and wave reflections from periphery.(Hertzman and Spealman,1937).
Phtoplethysmograph can be useful to diagnose cardiovascular patients. It becomes a non-
invasive technique to measure relative blood volume changes in blood vessels close to
skin.PPG signal of 10 patients was recorded from ear lobe. Five of them were ill patients with
2-Atrial flutter ,3 Post mayocardial infarction and remaining 5 were normal. By spectral
analysis content of PPG signal found to be different for normal patient and subjects with
abnormalities.
(a) (b)
Fig.12. PPG signal for (a) Atrial flutter patient (b) Post Mayocardial infraction patient
It has shown that in normal subjects,PPG signal peaks coincide with the R peaks of ECG.This
result states PPG can act as indicator of cardiac activity and R-R interval variability in
abnormal subjects.
It is important to maintain the temperature of the finger at 31 °C while measuring the finger
PPG waveform . If temperature is less than 28°C to 30°C then false results may appear as
output , in such case finger is warmed in hot water.(P.Lanzer) Distance between emitter and
detector is maintained at 4 to 5mm as it provides best sensitivity in terms of detecting
adequately large pulsatile photoplethysmographic signals.(Mandelson and Ochs,1988).
Use of optical fibre for data communication can reduce noise level in output. However during
lab experiment, in electronic design of filter noise can be reduced by severe approaches such
as
 Capacitive coupling – which proper selection of capacitor values
 Avoiding grounding loops in circuit – this can be achieved by bringing all ground
wires to same potential in a ground bus
 Twisted pair wiring – which reduces electromagnetic dramatically.
For proper selection of filters it is important to consider

14.  Voltage and current that will pass through the filter
 Frequency range specified to adequately reject all undesirable frequencies
 Insertion loss determines degree of attenuation provided by filter
 Pass-band impedance, capacitive – inductive input and output.
Thus , by considering ambient temperature effects in addition to motion artefacts more
precise PPG signal can obtained followed by accurate signal conditioning.

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