Download USB BASED DUAL CHANNEL DATA ACQISITION SYSTEM FOR

USB BASED DUAL CHANNEL DATA ACQISITION SYSTEM
FOR CAPTURING AND PROCESSING OF
PHOTOPLETHESMOGRAPHY AND
ELECTROCARDIOGRAPHY SIGNALS
Vibhuti Bagga*1, Tanvi Kalra2 , Abhinav3 and Sneh Anand3
1
2
3
Faculty of Science, Jamia Hamdard, New Delhi
Department of Biomedical Engineering, Rayat & Bahra Institute of Engineering & BioTechnology, Punjab
Centre for Biomedical Engineering, Indian Institute of Technology – Delhi
Abstract
This paper deals with the development of a USB based dual channel data acquisition system to
capture photoplethesmography (PPG) and Electrocardiography (ECG) signals to monitor Pulse
Transit Time, Heart Rate Variability and detection of arrhythmias using signal processing
algorithm on MATLAB. It can operate with input signal ranging from +/-0.5 V to +/-16V. Using
PIC 18F2550 the analog signals were converted to digital value and was plotted using a
software was developed on VISUAL BASICS 6.0.
Keywords: PPG, ECG, PPT, PIC 18f2550, USB, visual basic 6.0.
Introduction
Heart rate variability refers to the regulation of the sinoatrial node the natural pacemaker of the
heart by the sympathetic and parasympathetic branches of the autonomic nervous system. The
beat-to-beat fluctuations in the rhythm of the heart provide an indirect measure of heart health,
as defined by the degree of balance in sympathetic and vagus nerve activity.
RSA is the natural cycle of arrhythmia that occurs through the influence of breathing on the flow
of sympathetic and vagus impulses to the sinoatrial node. HRV is important because it provides
a window to observe the heart's ability to respond to normal regulatory impulses that affect its
rhythm. Photoplethysmography (PPG) is the electro-optic technique of measuring the
cardiovascular pulse wave. This pulse wave is caused by the periodic pulsations of arterial
blood volume and is measured by the changing optical absorption which this induces. Pulse
Transit Time (PTT) is the time it takes the pulse pressure waveform to propagate through a
length of the arterial tree. The pulse pressure waveform results from the ejection of blood from
the left ventricle and moves with a velocity much greater than the forward movement of the
blood itself.
To measure HRV and Pulse transit time ECG and PPG signals need to be measured. The
proposed dual channel USB based data acquisition system is capable of acquiring the ECG and
PPG signals and plot it on any computer using a user friendly visual basic based plotting
software. The small size lends an added advantage of portability. The use of USB 2.0 offers
high speed data transfer and plug-n-play capability.
METHODOLOGY
Parameter measuring module Here a three electrode ECG amplifier is used to measure
ECG in lead II configuration.
This three leads ECG system includes:
1. 3-Electrode: are placed at Right Arm (RA), Left Arm (LA) & left leg (LL). Hence taking
the heart signal from LEAD II of the Einthoven triangle.
2. Pre-amplifier stage: the signal from the RA electrode and LL electrode is buffered & then
is pre-amplified (A=5).
3. Differential amplifier: after the amplification, differential output of the signal is taken
keeping it in reference with the third electrode (LA).
4. Filter: the signal is then filtered using 2nd order Butterworth band-pass filter allowing the
frequency range of 1.2 Hz – 40 Hz to pass.
5. Output stage: the filtered signal is then inverted by putting it on inverting terminal,
amplified and output is given with respect to a reference voltage.
Acquisition Module:
USB
At the heart of this acquisition system lies a USB 2.0 compliant PIC 18f2550 microcontroller
resulting in a compact unit. The acquisition system is USB driven, which precludes the use of
any additional power supply. USB 2.0 with a design data rate of 480 megabits/second [1]
facilitates higher speed of data transfer.
18F2550 is capable of executing 12 MIPS, has a 10 bit ADC module with upto 13 – channel
multiplexer. It has a wide operating voltage range (2.0V-5.5V).
The ECG & PPG signals are transmitted to programmable gain amplifiers well suited to driving
ADCs. The gain of the amplifiers can be selected from a range of 1:1-32:1.These programmable
gain amplifiers are controlled by the PIC through a simple three wire serial peripheral interface.
These amplifiers are designed such that they don’t exhibit phase inversion when input signals
exceed the supply voltage. Vref should lie between Vss – Vdd. But these amplifiers accept only
positive values which precludes the use of voltage shifting operational amplifier such as LF353
which shifts the input voltage to a 0-5V range.
Graphical User Interface
The graphical user interface has been developed as a visual basic windows based application.
Visual basic offers many features which are useful in designing the graphical plotting software.
It reads data from the USB port in bulk and displays it on the monitor.
The features include :



Two selectable channels
No of samples :10-500
selectable time base: 10 us – 1000 ms

Data can be stored as a text file and processed as desired.
Before acquiring data the acquisition module must be first calibrated also the gain error should
be adjusted through the software. The necessary parameters like the channels being used, the
number of samples desired, the time base should also be set as per the signal being plotted.
The analog input is sampled and digitized by the controllers ADC. A series of commands are
given by the software to open the USB lines for reading the digital data and then close data
lines once all data has been acquired. All the data so read is stored and displayed together. If
needed the signal plotted can be smoothened, averaged and inverted.
Discussion
The acquisition system and the GUI successfully acquire and plot the ECG signal with clarity
and accuracy. This system can be further improved by adding the feature of user selectable
acquisition duration so as to display a continuous and dynamic plot as long as the user desires.
The various data processing features have to be further added.
References
1. VisualECG: a Bluetooth based software for ECG monitoring on personal digital assistants
(PDAs) Fernandez, J.; Ruiz, J.; Led, S.; Serrano, L.; Cabeza, R.;
Computer Architectures for Machine Perception, 2003 IEEE International Workshop on
Digital Object Identifier: 10.1109/ISSMD.2004.1689560 Publication Year: 2004 , Page(s): 57 - 62
2. A fundamental data acquisition saving block Loureiro, C.F.M.; Martins, V.M.G.; Clemencio,
F.M.C.; Correia, C.M.B.A.; Nuclear Science Symposium Conference Record, 2005 IEEE
Volume: 2 Digital Object Identifier: 10.1109/NSSMIC.2005.1596351 Publication Year: 2005 ,
Page(s): 685 - 686
3. A preliminary study on the correlation of frequency components between finger PPG and
radial arterial BP waveforms Hsin Hsiu; Chia-Liang Hsu; Tsung-Lin Wu; Biomedical and
Pharmaceutical Engineering, 2009. ICBPE '09. International Conference on Digital Object
Identifier: 10.1109/ICBPE.2009.5384104 Publication Year: 2009 , Page(s): 1 - 4
4. Multichannel Reflective PPG Earpiece Sensor With Passive Motion Cancellation Lei Wang;
Lo, B.P.L.; Guang-Zhong Yang; Biomedical Circuits and Systems, IEEE Transactions on
Volume: 1 , Issue: 4
Digital Object Identifier: 10.1109/TBCAS.2007.910900 Publication Year: 2007 , Page(s): 235
- 241
5. QT variability and HRV interactions in ECG: quantification and reliability Almeida, R.;
Gouveia, S.; Rocha, A.P.; Pueyo, E.; Martinez, J.P.; Laguna, P.; Biomedical Engineering,
IEEE Transactions on
Volume: 53 , Issue: 7 Digital Object Identifier: 10.1109/TBME.2006.873682 Publication Year:
2006 , Page(s): 1317 - 1329
6. An evaluation of object-based programming with Visual Basic Dukovic, J.M.; Joyce, D.T.;
Computers and Communications, 1995. Conference Proceedings of the 1995 IEEE
Fourteenth Annual International Phoenix Conference on Digital Object Identifier:
10.1109/PCCC.1995.472470
Publication Year: 1995 , Page(s): 346 - 351
7. Evaluation of arterial compliance on pulse transit time using photoplethysmography
Byeong Cheol Choi; Hee Jeong Lee; Soo Young Ye; Dong Keun Jung; Gi Ryon Kim;
Kwang Nyon Kim; Gye Rock Jeon;
Industrial Electronics Society, 2004. IECON 2004. 30th Annual Conference of IEEE Volume:
3 Digital Object Identifier: 10.1109/IECON.2004.1432329 Publication Year: 2004 , Page(s):
3219 - 3222 Vol. 3