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CCNT: Instrumentation, Electronics, and Electrical Safety

CCNT · Work through your exam, one topic at a time.

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Instrumentation, Electronics, and Electrical Safety

Domain 2 - Instrumentation, Electronics, and Electrical Safety

The exam probes how the amplifier, filters, and digital settings shape what you see, plus the electrical-safety rules that protect patients with and without intracardiac devices.

Core rules

  • Differential amplification. Output = gain × (G1 − G2). Biologic signal at G1 relative to G2 is amplified; identical (common-mode) noise is rejected. CMRR of 80–100 dB is expected; input impedance above 10 MΩ prevents high electrode impedance from shrinking the signal.
  • Polarity. When G1 is more negative than G2, the trace deflects upward - the convention used in EEG and most EP displays.
  • Sensitivity. Expressed in µV/mm (or µV/div). A smaller number means a larger deflection; it does not alter frequency content.
  • Filters. The low-frequency filter (high-pass) removes slow waves and sets the time constant: TC = 1/(2πf), so 1 Hz ≈ 0.16 s. The high-frequency filter (low-pass) removes fast activity; 70 Hz is standard for scalp EEG. A 60 Hz notch should be used only after other remedies.
  • Digital settings. Sampling must exceed twice the highest frequency of interest (Nyquist); a 70 Hz filter requires at least 140 Hz, and most systems sample at 200–256 Hz.
  • Electrical safety. Keep grounds intact, use isolation and GFCI protection, inspect leads, and never bypass a three-prong plug. Type CF applied parts are limited to about 10 µA patient leakage; as little as ~100 µA through an intracardiac catheter can cause ventricular fibrillation (microshock).

Worked example

A 60 Hz common-mode voltage of 10 V appears at the patient. With CMRR of 100 dB (100,000:1), the equivalent differential noise is 10 V ÷ 100,000 = 100 µV - enough to obscure EEG. The correct response is to lower electrode impedance, verify the ground and lead dress, and use the notch only if artifact persists.

Traps

Reversing the meaning of low- and high-frequency filters; raising sensitivity to hide artifact; assuming a grounded outlet guarantees safety; using the notch filter routinely and clipping real beta or muscle activity.