1. Biomedical Electrodes, Sensors, and Transducers
from: Chaterjee, Biomedical Instrumentation, chapter 6
© D. J. McMahon rev

2. Definition of Biomedical Electrodes, Sensors, and Transducers
Solid electric conductor
Detects electrical activity
Transducer:
Converts energy types
Sensor:
Detects change in physical stimulus
Turns it into a signal

3. FIGURE 6-1 Various sensors

4. FIGURE 6-3 A block diagram of human-machine interface

5. Biomedical Electrodes
Conductor
Makes contact with a nonmetal
Electrochemical cell
Electrode is anode or cathode
FIGURE 6-4 Charge density in the muscle-electrode interface

6. Theory of Electrodes Electrochemical cell is divided into:
Voltaic cell
Generates electrical current
Chemical reaction: two half reactions
Electrolytic cell
Uses the electrical current

7. FIGURE 6-7 An electrolytic cell
FIGURE 6-6 A voltaic cell

8. FIGURE 6-8 A table of half-cell potentials

10. Silver/Silver Chloride Reference Electrode
Stable reference electrode
FIGURE 6-12a Silver/silver chloride electrode

11. FIGURE 6-12c A photo of silver/silver chloride
button electrodes
FIGURE 6-12b A view of a silver/silver chloride electrode

14. Provides a low-offset, low impedance interface with
the monitor with minimal effect on the patient’s skin.

17. Types of Electrodes Include: Polarizable or nonpolarizable
Electrode shapes: button or bar types
Electrode configurations: monopolar or bipolar
Skin-surface electrodes
Needle electrodes
Cup electrodes

18. FIGURE 6-13a A surface electrode
FIGURE 6-13b A needle electrode.
FIGURE 6-13c A cup electrode

19. Classification of Sensors and Transducers
If based upon stimulus:
Mechanical
Electric
Magnetic
Optical
Thermal
Acoustic

20. Performance Characteristics of Electrodes, Sensors, and Transducers
Sensitivity
Output change divided by input change
Static and dynamic error
Difference between measured and actual value
Static error: measured value doesn’t change
Dynamic error: measured value changes

21. Performance Characteristics (cont’d.)
Accuracy
How close output is to true value
Hysteresis
Unable to repeat data in opposite direction
Frequency response
Range of frequencies
Bandwidth

22. Performance Characteristics (cont’d.)
Reproducibility
Ability to produce identical output values
At different times under the same conditions
Resolution
Smallest step of meaningful output
Measurable over range

23. Pressure Transducers Pressure Types of pressure transducers:
Force per unit area
Types of pressure transducers:
Metal strain gauge
Linear variable differential transformer (LVDT)

24. FIGURE 6-15a A simple metal gauge
FIGURE 6-15b Backing material of a metal gauge

25. FIGURE 6-16a A simple LVDT transducer
FIGURE 6-15c A balanced bridge circuit with a metal gauge

26. FIGURE 6-16c LVDT in an amplifier circuit
FIGURE 6-16b Output voltage in a LVDT transducer

27. Flow Transducers Measure flow of gases, liquids, or solids Given as:
Q = (area) (velocity)
FIGURE 6-17 A simple flow transducer

28. Blood flow detection by ultrasound & Doppler shift:

29. Temperature Transducers
Types include:
Resistance temperature detector (RTD)
Metal wire resistance increases with temperature
Thermistor
Semiconductor device
Negative or positive temperature coefficient
Thermocouple
Converts temperature reading into voltage reading
Uses two dissimilar metals: Seebeck effect

30. FIGURE 6-19a A simple thermocouple transducer
FIGURE 6-19b A thermocouple with ice bath

31. Optical Transducers Don’t require direct physical contact
Avoid disturbances
Photoconductive cell
Common type
Uses reflection of light
Detects distance or level

32. Summary Acquiring biomedical signals Electrode Sensor senses stimulus
Electrode or transducer is essential
Electrode
Solid electric conductor
Sensor senses stimulus
Strain gauge and LVDT
Pressure sensors
RTD, thermistor, and thermocouple
Temperature sensors

33. Summary (cont’d.) Strain gauge LVDT RTDs or thermistors Thermocouples
Determines amount of strain
Most common: metal foil gauges
LVDT
Provides output voltage proportional to displacement
RTDs or thermistors
Resistance varies with temperature
Thermocouples
Voltage varies with temperature