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

Common-Mode Shield Driver

Differential 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/ECG 1/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 18 th 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