3.3 Equipment Setup & Calibration

Why Equipment Setup and Calibration Are Tested

Calibration proves that the digital EEG instrument is faithfully reproducing brain activity. If gain, polarity, or filters are wrong, normal activity can look abnormal and abnormalities can be hidden. ABRET expects the technologist to set up the instrument correctly, verify it with calibration, document settings, and troubleshoot before the recording begins.

Instrument Setup

Modern EEG uses digital acquisition: scalp electrodes connect to a jackbox/electrode board, which feeds the amplifier/headbox, then an analog-to-digital converter and the acquisition computer. Setup steps:

• Confirm the system powers up and passes self-checks; verify the electrical safety of the equipment and that the patient is in a properly grounded, isolated environment.
• Map each electrode to the correct jackbox input so the recorded label matches the true scalp site.
• Connect the system reference and ground electrodes; never use the patient ground as a stimulating or therapy ground.
• Reduce environmental interference (move from unshielded mains sources where possible) before relying on the notch filter.
• Confirm an adequate sampling rate (routine digital EEG is commonly sampled at 256 Hz or higher) so waveforms are accurately represented.

Calibration Signals

Two complementary checks are performed and saved with every study.

In a square-wave calibration the deflection amplitude should be equal on all channels for a given sensitivity, and the waveform shape reflects the selected low- and high-frequency filters. The biocal is the final functional proof that the whole acquisition chain reproduces real signals identically.

Parameter Selection

The technologist selects display and filter parameters appropriate to the patient and clinical question. Common routine adult starting points:

Filters should never be used to hide poor electrode contact — the correct first action for 60 Hz interference is to re-check impedances and the environment, not to switch on the notch filter.

Documentation Before Recording

Documentation is part of data integrity and is examinable. Before the recording starts, the technologist records:

• Patient identifiers, age, and the clinical question/indication
• Montage(s) selected and the recording reference
• All parameter settings: sensitivity, LFF, HFF, notch status, timebase, sampling rate
• Electrode array and measured impedances
• Medications, last dose times, last seizure, and last meal
• Level of consciousness and baseline behavioral state
• Both calibration runs (square-wave and biocal) saved with the file

Throughout the study the technologist also annotates clinical events, activations, and state changes with accurate times.

Systematic Setup Troubleshooting

When the tracing looks wrong at setup, work from the patient outward rather than guessing:

1. Patient: movement, sweating, tension, or proximity to interference sources.
2. Electrodes: loose, dried, or high-impedance electrodes; re-prep and re-measure impedance.
3. Jackbox/cabling: wrong input, damaged lead, loose connector, mislabeled channel.
4. Amplifier/settings: verify with square-wave and biocal; check sensitivity, filters, and montage.

A single bad channel usually points to that electrode or its lead; a pattern affecting many channels usually points to the reference, ground, settings, or environment. Resolving the cause — not masking it with filters — preserves a diagnostic-quality study.

Time Constant and the Low-Frequency Filter

The time constant (TC) is the analog way of expressing the low-frequency filter and is tested directly (an ABRET sample question asks the appropriate TC for a low-voltage slow-wave focus). A longer time constant corresponds to a lower low-frequency-filter cutoff, which preserves slow activity; a short TC (higher cutoff) attenuates slow waves. A commonly cited routine TC is about 0.3 s (roughly a 0.5-1 Hz low-frequency filter). When the clinical question is a slow-wave focus, you want a longer TC / lower cutoff so the slow activity is not filtered away - the wrong setting can erase the very finding being sought.

The square-wave calibration pulse's decay rate visually reveals the TC: a slow decay = long TC, a fast decay = short TC.

Digital Advantages: Reformatting and Post-Acquisition Review

A major advantage of digital EEG is that the raw data is stored referentially and can be reformatted into any montage, and have its sensitivity and filters changed after acquisition during review (ACNS Guideline 4). This is why faithful acquisition - correct sampling rate, a clean system reference, accurate electrode labeling, and saved calibrations - matters so much: every post-hoc montage and remontage depends on the integrity of the originally recorded data. A mislabeled electrode or a contaminated reference at acquisition propagates into every reformatted view.