Biomedical Instrumentation – Vocabulary Review

Active Filter

An electronic filter that uses active components (e.g., op-amps) to shape frequency response and can provide signal gain.

Passive Filter

A filter built only from resistors, capacitors, and inductors; it cannot amplify the input signal.

EEG (Electroencephalography)

A technique that records the brain’s electrical activity from the scalp.

PPG (Photoplethysmography)

An optical method that measures blood-volume changes in skin tissue using light absorption.

High-Frequency ECG

ECG signal components above ~100 Hz used to detect subtle cardiac changes.

Medical Device Class I

Lowest-risk FDA class; general controls are sufficient for safety and effectiveness.

Medical Device Class II

Moderate-risk devices requiring special controls in addition to general controls.

Medical Device Class III

Highest-risk devices; require pre-market approval and extensive evidence of safety.

Analog-to-Digital Converter (ADC)

Circuit that transforms a continuous analog signal into a discrete digital representation.

Food and Drug Administration (FDA)

U.S. authority that regulates food, drugs, and medical devices for safety and efficacy.

Flash ADC

Ultra-fast ADC that compares the input simultaneously against 2ⁿ – 1 reference levels.

Comparator

Device that outputs a digital level indicating whether an analog input exceeds a reference voltage.

Eindhoven’s Triangle

Triangle formed by ECG limb leads I, II, and III for assessing the heart’s electrical axis.

Direct Measurement

Technique that senses the desired physical quantity itself.

Indirect Measurement

Technique that measures a related variable and computes the desired quantity.

Augmented Leads

ECG limb leads aVR, aVL, and aVF that amplify voltage from one limb relative to the others.

Typical ECG Amplitude

Approximate peak-to-peak value of 1 mV in a normal adult ECG.

Atrial Fibrillation Indicator

Absence of P-waves and irregular RR intervals with fine f-waves on an ECG.

Common-Mode Rejection Ratio (CMRR)

A measure of an amplifier’s ability to reject signals common to both inputs; higher is better.

P-Wave

ECG waveform segment representing atrial depolarization.

Piezoelectric Sensor

Transducer that converts mechanical stress into an electric charge via the piezoelectric effect.

Sample-and-Hold Circuit

Circuit that captures an analog voltage and maintains it steady for accurate ADC conversion.

Implantable Loop Recorder

Subcutaneous device that continuously monitors and stores long-term cardiac rhythm.

SAR ADC

Successive-Approximation Register converter that resolves bits sequentially from MSB to LSB.

Delta-Sigma ADC

High-resolution ADC that oversamples and noise-shapes the input before digital decimation.

ECG Noise Sources

Interference from muscle activity, power-line hum, motion artifacts, poor electrode contact, grounding issues, and EMI.

EMG (Electromyography)

Measurement of electrical activity produced by skeletal muscles.

Delta Waves (EEG)

0.5–4 Hz brain waves dominant in deep sleep and some pathologic awake states.

Theta Waves (EEG)

4–8 Hz waves associated with light sleep and relaxation.

Alpha Waves (EEG)

8–13 Hz rhythm seen during relaxed, eyes-closed wakefulness.

Beta Waves (EEG)

13–30 Hz waves linked to active thinking and alertness.

Baseline Wander

Low-frequency drift in ECG caused by respiration or motion.

IIR Filter

Infinite-Impulse-Response filter with feedback paths; efficient but may be unstable.

FIR Filter

Finite-Impulse-Response filter with strictly feed-forward paths; always stable and can provide linear phase.

Notch Filter

Very narrow band-stop filter designed to suppress a single frequency, e.g., 50/60 Hz mains hum.

Nyquist Criterion

Requirement that sampling rate be at least twice the highest signal frequency to avoid aliasing.

Aliasing

Distortion that causes high-frequency components to appear as lower frequencies when sampled too slowly.

Instrumentation Amplifier

Precision differential amplifier with high input impedance and high CMRR, ideal for biopotential signals.

Differential Amplifier

Circuit that amplifies the voltage difference between two inputs while rejecting common signals.

Gain

Ratio by which an amplifier increases signal amplitude.

Signal-to-Noise Ratio (SNR)

Measure comparing desired signal power to background noise power; higher values indicate clearer signals.

Bioimpedance

Electrical impedance of biological tissue, used for body composition and fluid status assessment.

Galvanic Skin Response (GSR)

Variation in skin conductance reflecting sweat gland activity and emotional arousal.

Spirometer

Device that measures lung volumes and airflow to assess pulmonary function.

Electrode Impedance

Total opposition an electrode presents to current flow; low values improve signal quality.

Ag/AgCl Electrode

Silver/silver-chloride electrode widely used for biopotential recording due to stability and low noise.

Artifact (Physiological Signal)

Any undesired component in a biosignal not originating from the target physiology.

Calibration

Adjustment process ensuring an instrument’s output matches known reference values.

Inv asive Measurement

Technique requiring entry into the body, such as catheterization.

Non-Invasive Measurement

Technique performed outside the body, e.g., ECG or pulse oximetry.

BF Device

Body-floating medical device safe for general patient contact but not directly with the heart.

CF Device

Cardiac-floating device designed to be safe even when connected directly to the heart.

Sampling Rate

Number of samples taken per second from an analog signal, expressed in hertz (Hz).

Transducer

Device that converts one form of energy (e.g., pressure) into an electrical signal.

Opto-Isolator

Component that electrically isolates two circuits using light to transfer signals, enhancing patient safety.

Electrode Potential

DC voltage generated at the electrode–electrolyte interface due to chemical reactions.

Thermistor

Temperature sensor whose resistance changes predictably with temperature; NTC types decrease resistance as temperature rises.

Capacitive Respiration Measurement

Technique that tracks chest expansion by detecting capacitance changes between electrodes.

Impedance Spectroscopy

Measurement of tissue impedance over a range of frequencies to characterize its electrical properties.

Hall Sensor

Device that converts a magnetic field into a proportional electrical signal, used for flow or position sensing.

Lead-Off Detection

System that identifies when an electrode becomes detached from the patient.

Active Electrode

Electrode that integrates a pre-amplifier at the sensing site to reduce noise pickup.