Lecture 8
Dimitar Stefanov
Design and Control of Devices for Human-Movement Assistance
Recapping
Nonpolarized electrodes – allow to current to pass freely in electrode-electrolyte
interface. Their HCP is near to 0.
Silver-silver chloride electrodes (Ag/AgCl)
Equivalent circuit of a
biopotential electrode:
Scheme of the contour
“electrode-amplifier”
The input signal to the amplifier consists of 5 components: (1)Desired biopotential, (2) Undesired
biopotentials, (3) A power line interference signal and its harmonics, (4) Interference signals
generated by the tissue-electrode interface, (5) Noise.
Block diagram of a biopotential amplifier
FET transistors
Galvanic
decoupling of the
patient
Motion artifacts – the contact between the electrode and the tissue changes during
the relative motions between the electrodes and the tissue.
How to decrease the motion artifacts?
•High input resistance of the amplifier
•Usage of non-polarized electrodes (Ag/AgCl)
•Reduction of the source impedance by usage of electrode gel.
Special circuits which built the biopotential amplifier
Instrumentation amplifiers
DC instrumentation amplifiers
AC instrumentation amplifiers
AC amplifiers eliminate the electrode offset potential, permit high gain and permits
higher CMRR.
The capacitors between the electrodes and the input stage of the amplifier cause
charging effects from the input bias current.
Isolation amplifier
Isolation is realized in the following technologies:
•Transformer isolation
•Opto-isolation.
Complete galvanic separation between the input stage (patient) and the remaining
part of the measure equipment.
Surge protection of the bioamplifiers
Protection of the amplifier from damage due to surge input potentials.
•Diodes
•Zener diodes
•Gas-discharge tubes
Input guarding
Technique for increase both the input impedance of the amplifier of biopotentials
and the CMRR
Instrumentation amplifier providing input guarding
Driven-right-leg circuit
reducing common-mode
interference.
Example for practical design of biosignal amplifier
AD620 – a low cost, low power instrumentation amplifier in 8-Lead Plastic Mini-DIP
Gain Set with One External Resistor (Gain Range 1 to 1000); 50mV Input Offset
Voltage; 100 dB min Common-Mode Rejection Ratio (G = 10); low noise; 120 kHz
Bandwidth (G = 100).
Gain equation:
Make vs. Buy
ECG Monitor Circuit
Input guarding of AD620
Differential Shield Driver
Common-Mode Shield Driver
Circuit to measure
ECG
Standard limb leads
In clinical ECG measurements, four electrodes are attached to the four limbs: left
arm (LA), left leg (LL), right arm (RA) and right leg (RL).
The electrode on RL is usually grounded while the voltage drop between any two of
other three electrodes are measured.
In Lead I configuration, ECG is measured as the voltage drop from LA to RA.
In other words, LA is connected to the positive input of the amplifier and RA is
connected to the negative input of the amplifier.
In Lead II configuration, ECG is measured as the voltage drop from LL to RA, and
in Lead III configuration, ECG is measured as the voltage drop from LL to LA.
More information on the same topic:
http://www.analog.com/pdf/AD620_e.pdf and
http://www.cs.wright.edu/EGR199/PH/ECG1/Ecg_1.htm#anchor4
5mm X 4mm X 1.75 mm !
Techniques which eliminate` the influence of the capacitance of
the connective electrode wires
Best elimination of the capacitance – if no wires are used.
Bipolar Concentric ring sensor for surface Laplacian ECG
(University of Miami)
Double-sided 13x13
mm PC board
The contact area of the outer ring and the inner dot are equal.
More information: M.Talero, C.C.Lu, Active Bipolar Concentric Ring Sensor for surface Laplacian ECG, in the
Proc. First BMES/EMBS conference on Serving Humanity, Advancing Technology, Oct. 13-16, 1999, Atlanta,
GA, USA
Tripolar electrode sensor for Laplacian
cardiograms
(University of Miami, Department of Biomedical Engineering)
•The sensor contains three closely spaced rings.
•The width of each ring is 1.0 mm with diameters of 36 mm for
the outer ring and 18 mm for the middle ring.
•The ring/dot in the center is 2.0 mm in diameter.
•An instrumentation amplifier with an input impedance of 10
Gohms is used.
Further information:
http://sbec.abe.msstate.edu/abstracts/lu.htm
Wireless electrodes for surface electromyography
(Keio University – Japan)
Electrode part (Ag/AgCl electrodes) + amplifier + high-pass filter + built-in
transmitter + battery
Instrumentation amplifier AD620BR; FM transmitter; five button
battery cells; 20 m distance between the electrodes and the
receiver; 15 hours operation with one set batteries.
Before
transmission
After transmission
More information: M. Ohyama, Y. Tomita, S. Honda, H. Uchida, and N. Matsuo, Active wireless electrodes for
surface electromyography, Proc. Of the 18th Annual International Conference of the IEEE EMBS, Amsterdam,
1996, pp. 295 – 296.
Micro system for sensing of biological
parameters (Waseda University – Japan)
•There are no wire lines between the sensors and the transmitter. One
transmitter, located on the wrist, is used for transmission of the data from
all sensors.
•Between the detector part and the transmitter, the signals are sent as a
AC micro current flow through the tissue of the body.
ECG monitoring system
(The distance between the electrodes is 5 cm)
Block diagram of the ECG detector - transmitter
Sampling frequency – 900Hz
Carrier frequency – 70 kHz
(sinusoidal signal)
Block diagram of the relay transmitter
Transmission of the signals between the ECG transmitter and the
relay transmitter (transmission of the signal in the human body):
Tissue equivalent circuit
Equivalent circuit of the tissue-electrodes contour
Frequency characteristic
Distance between the electrodes (Rf
and T) for applying the signals – 7
cm.
Distance between the electrodes (B,
S and T) for detection the signal – 3
cm.
In case of two channels, two carrier frequencies are chosen: 50 kHz and 70
kHz.
More information: T. Handa, S. Shoji, S. Ike, S. Takeda, and T. Sekiguchi, A Very low-power consumption wireless
ECG monitoring system using body as a signal transmission medium, Proc. Transducer’97- Int. Conf. On solid-state
sensors and actuators, Chicago, June 16-19, 1997
Eiji Takeda, Takashi Handa, Shuichi Shoji, Akihiko Uchiyama, STUDIES ON BIO-SENSING MICROSYSTEMS
FOR HEALTH CARE, XIV International Symposium on Biotelemetry, Marburg, Germany April 6 - 11, 1997,
http://baby.indstate.edu/isb/publications/abstracts/session3-6.htm