Instrumentation, Amplifiers, Filters, Sampling, and Digital Signals

1. Acquisition, impedance, and amplification

Signal acquisition is detecting the small physiological voltages at the electrode-skin interface and passing them to the amplifier. Electrode impedance is the resistance at that interface; if too high, the signal is degraded. Input impedance is the resistance the amplifier presents to the signal source; it must be high relative to electrode impedance so it does not load down and shrink the small signal. The differential amplifier amplifies the difference between two inputs while rejecting what they share. A common mode signal is one present at both inputs (such as 50/60 Hz interference); the common-mode rejection ratio (CMRR) quantifies how well the amplifier rejects it — higher is better. Gain is the amplification factor; sensitivity describes how much voltage is represented per unit of display deflection (e.g., µV/mm). In older analog systems, pen deflection described how far a pen moved per unit signal. Signal calibration applies a known input to verify the amplifier's response, so amplitudes are trustworthy.

2. AC vs. DC coupling and filters

An AC-coupled amplifier blocks slow or steady (DC) components and passes the faster changes typical of EEG, EOG, EMG, and ECG; a DC-coupled amplifier preserves slow or sustained changes and is used for respiratory effort, oximetry, or position, where the slow baseline carries meaning. Filters shape which frequencies pass. A high-frequency/low-pass filter attenuates frequencies above its setting (removing fast artifact); a low-frequency/high-pass filter attenuates frequencies below its setting (removing slow drift). The time constant is a related way of expressing the low-frequency filter — longer time constants pass slower activity. A notch filter removes a narrow band centered on the 50/60 Hz power-line frequency. Filters are necessary but can cause filter distortion — altering waveform shape or amplitude near the cutoff — so technologists must recognize when a filter, not physiology, is shaping the tracing.

3. Sampling, digital resolution, and aliasing

Modern systems digitize the analog signal. The sampling rate is the number of samples per second (Hz), and digital resolution is the number of bits representing each sample's amplitude (more bits = finer steps). The Nyquist theorem states the sampling rate must be at least twice the highest frequency of interest; otherwise aliasing occurs, making high-frequency activity appear falsely as a lower frequency. Technologists verify settings meet technical specifications (per current AASM and manufacturer requirements) and understand that display settings vs. underlying signal can differ: on-screen filters and gain determine what is seen during review, while the stored raw signal may contain more than the display shows.

How it works

1. Physiological voltages are acquired at the electrode-skin interface.
2. A differential amplifier amplifies the difference and rejects common-mode noise (CMRR).
3. Coupling (AC vs. DC) and filters (high-pass, low-pass, notch) shape the frequency content.
4. Calibration verifies gain and sensitivity with a known input.
5. The signal is digitized at a rate and resolution satisfying Nyquist to avoid aliasing.
6. Display filters and gain determine what is seen, distinct from the stored raw signal.

Common confusions

Memory aids

Signal chain: "A-C-F-S-D" — Acquire, amplify (CMRR rejects common mode), Filter, Sample (Nyquist), Display. Filters: "Low-pass kills the highs; high-pass kills the lows; notch kills the hum." Aliasing: "Sample too slow, highs look low."

Quick review

Topic Recap

• The signal chain is acquisition → differential amplification → filtering → sampling → display.
• Differential amplifiers reject common-mode noise (CMRR); input impedance must be high.
• AC coupling suits fast signals; DC coupling preserves slow baselines.
• Low-pass/high-pass/notch filters shape frequency content; time constant relates to the low-frequency filter.
• Sampling rate and resolution must satisfy Nyquist to prevent aliasing.
• Display settings differ from the underlying stored signal and must be understood during review.

Knowledge Check

1. Why is a differential amplifier used for physiological signals?
2. When is DC coupling used instead of AC coupling?
3. What does the notch filter remove, and at what frequency?
4. State the Nyquist theorem and what happens when it is violated.
5. Why must a technologist distinguish display settings from the underlying stored signal?

Answers and Rationales

1. Because it amplifies the difference between inputs while rejecting common-mode noise (CMRR). Why: Physiological signals are small, and interference arrives at both inputs.
2. For slow or sustained signals — effort, oximetry, position — where the baseline carries meaning. Why: AC coupling would block that slow information.
3. A narrow band centered on the 50/60 Hz power-line frequency. Why: It is the most common environmental interference.
4. The sampling rate must be at least twice the highest frequency of interest; otherwise aliasing makes high frequencies appear lower. Why: Undersampling cannot faithfully represent fast activity.
5. Because on-screen filters and gain determine what is viewed, while the stored raw signal may contain more. Why: Otherwise display settings could be mistaken for physiology.