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HKIE Professional Assessment - Biomedical Discipline study materials
HKIE Professional Assessment - Biomedical Discipline study resources

HKIE Professional Assessment - Biomedical Discipline: Medical Devices and Instrumentation: Design, Sensors, Signal Acquisition and Safety

HKIE Professional Assessment - Biomedical Discipline · Work through your exam, one topic at a time.

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Medical Devices and Instrumentation: Design, Sensors, Signal Acquisition and Safety

This domain tests the chain from transducer to digitised signal, and whether you can design and fault-find an instrument that is both accurate and electrically safe for a patient. Expect biopotential front-end questions, sampling arithmetic and IEC 60601-1 leakage current limits.

The acquisition chain

  • Electrode–skin interface: half-cell offset potentials of tens to hundreds of millivolts sit in series with the biopotential, so the front end needs AC coupling or DC servo restoration plus high input impedance (typically >100 MΩ) to avoid loading the source.
  • Differential amplification with high CMRR rejects mains pick-up; typical ECG CMRR is 80–100 dB.
  • Anti-alias filtering must precede sampling; Nyquist requires fs > 2 × fmax.
  • ADC resolution sets the smallest resolvable step: LSB = FSR / 2^n. For a 10 mV ECG on a ±5 mV range with 12 bits, LSB ≈ 2.4 µV.

Safety architecture

  • Class I relies on protective earth; Class II on double or reinforced insulation; BF and CF applied parts differ in allowable patient leakage.
  • IEC 60601-1 limits (typical, normal condition): earth leakage 500 µA, touch current 100 µA, patient leakage 100 µA AC, patient auxiliary current 100 µA; single-fault-condition limits are higher (commonly 500 µA, earth leakage 1000 µA).
  • Defibrillation-proof applied parts need protection networks that survive the discharge without failing safe-critical functions.

Worked example

A bedside monitor sees 1 V of 50 Hz common-mode interference and must keep the differential error below 0.5 mV. Required CMRR = 20 log10(1 / 0.0005) = 66 dB. An 80 dB amplifier leaves 0.1 mV error, so the specification is met with margin. If the same amplifier is fed by mismatched electrode impedances, gain error degrades effective CMRR, so balance the electrodes as well as specifying the op-amp.

Traps that cost marks

  • Using 20 log10 for a voltage ratio in one place and 10 log10 in another.
  • Ignoring electrode offset saturation when specifying high gain.
  • Quoting single-fault limits as normal-condition limits.
  • Assuming a Class II device needs no earth reference for EMC purposes.
  • Confusing sampling rate with bandwidth: a 500 Hz sampler cannot faithfully capture a 150 Hz ECG component without a proper anti-alias filter.