{
 "schema": "learn-deck/1",
 "module": "bioelec",
 "deck": "bioelec",
 "title": "Biosignal electronics",
 "lang": "en",
 "cards": [
  {
   "id": "be:resting-range",
   "type": "mcq",
   "q": "Resting potential of an excitable cell, inside relative to outside:",
   "choices": [
    "−40 to −90 mV",
    "+40 to +90 mV",
    "−0.4 to −0.9 mV",
    "0 mV"
   ],
   "a": 0,
   "ex": "At rest the membrane is 50–100× more permeable to K+ than Na+, so K+ leaving leaves the interior negative.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:nernst-k",
   "type": "flash",
   "q": "Nernst potential for K+ at 37 °C with [K]i = 140 mM and [K]o = 2.5 mM?",
   "a": "E = 0.0615 · log10(2.5/140) ≈ −108 mV.",
   "ex": "0.0615 V per decade at body temperature. Real cells sit less negative because Na+ leaks in (GHK equation).",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:ghk",
   "type": "tf",
   "q": "The Goldman–Hodgkin–Katz equation weights each ion's concentration ratio by its membrane permeability.",
   "a": true,
   "ex": "The resting potential is a permeability-weighted compromise, which is why it behaves like a potassium membrane.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:ap-amplitude",
   "type": "mcq",
   "q": "A nerve action potential swings roughly:",
   "choices": [
    "~120 mV in about 1 ms",
    "~12 mV in 10 ms",
    "~1 V in 1 µs",
    "~120 mV in 100 ms"
   ],
   "a": 0,
   "ex": "All-or-none: a stronger stimulus does not make it bigger. gNa rises first; a delayed gK rise repolarises and undershoots.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:refractory",
   "type": "flash",
   "q": "Why can a nerve fibre not fire faster than about 1000 impulses per second?",
   "a": "The absolute refractory period (~1 ms) blocks any second stimulus until the membrane recovers.",
   "ex": "It is followed by a relative refractory period, where only a strong stimulus succeeds.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:threshold-depol",
   "type": "mcq",
   "q": "Threshold for triggering an action potential lies about how far above rest?",
   "choices": [
    "~20 mV more positive",
    "~2 mV",
    "~100 mV",
    "At 0 mV"
   ],
   "a": 0,
   "ex": "Local circuit current ahead of the active region depolarises the next patch to threshold - that is how it propagates without attenuation.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:myelin-speed",
   "type": "tf",
   "q": "Myelination improves conduction velocity by roughly a factor of 20 for a given axon diameter.",
   "a": true,
   "ex": "Saltatory conduction: Na+ channels cluster at nodes of Ranvier 1–2 mm apart; the sheath cuts capacitance and leakage.",
   "tags": [
    "cells"
   ]
  },
  {
   "id": "be:extracellular-shape",
   "type": "mcq",
   "q": "An extracellular electrode near a propagating axon records the action potential as:",
   "choices": [
    "Triphasic, ~2nd derivative of the AP",
    "A monophasic copy of the AP",
    "An inverted copy of the AP",
    "A DC step"
   ],
   "a": 0,
   "ex": "Outward current ahead (+), inward sodium current opposite the electrode (−), outward again behind (+).",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:field-falloff",
   "type": "tf",
   "q": "The extracellular field of a single fibre is essentially zero beyond about 15 fibre radii.",
   "a": true,
   "ex": "The field potential falls off exponentially with radial distance, so surface signals need many synchronous fibres.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:volume-conductor",
   "type": "flash",
   "q": "Why does a higher-resistivity bath make the recorded field potential larger?",
   "a": "The active membrane behaves as a constant-current source, so V = I·R rises with the extracellular load.",
   "ex": "Same reason a smaller volume conductor gives bigger surface potentials.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:motor-unit",
   "type": "flash",
   "q": "What is a motor unit?",
   "a": "One motor neuron plus every muscle fibre it innervates - the smallest unit voluntary effort can recruit.",
   "ex": "Laryngeal muscles: 2–3 fibres per unit; gastrocnemius: several hundred. Recruitment and firing rate set force.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:smu-potential",
   "type": "mcq",
   "q": "A surface-recorded motor unit action potential is typically:",
   "choices": [
    "20–2000 µV, 3–15 ms, triphasic",
    "20–2000 mV, 1 ms",
    "1–5 µV, 100 ms",
    "0.1–1 V, 50 ms"
   ],
   "a": 0,
   "ex": "Firing at 6–30 per second. Strong contractions superimpose units into an interference pattern.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:emg-noise-look",
   "type": "tf",
   "q": "A strong surface EMG looks like noise because motor units fire asynchronously with random phases.",
   "a": true,
   "ex": "Deliberate: it produces smooth force. It is why EMG is characterised by RMS, envelope or spectrum rather than waveform.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:muscle-ap-speed",
   "type": "flash",
   "q": "Human skeletal muscle fibre action potential: conduction velocity and duration?",
   "a": "3–5 m/s, lasting 2–15 ms, from about −85 mV to about +30 mV.",
   "ex": "Much slower and longer than a nerve spike, which lasts 2.5–3 ms.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:ecg-waves",
   "type": "mcq",
   "q": "In the ECG, the QRS complex is produced by:",
   "choices": [
    "Ventricular depolarisation",
    "Atrial depolarisation",
    "Ventricular repolarisation",
    "Atrial repolarisation"
   ],
   "a": 0,
   "ex": "P = atrial depolarisation, T = ventricular repolarisation. Atrial repolarisation hides inside the QRS.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:ecg-amplitude",
   "type": "flash",
   "q": "Typical QRS amplitude on the body surface?",
   "a": "1–3 mV, ranging from about 400 µV to 2.5 mV with lead and body type.",
   "ex": "Ventricular cells rest at −85 mV with a 200–300 ms plateau; the surface sees the volume-conducted sum.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:eeg-amplitude",
   "type": "mcq",
   "q": "Scalp EEG amplitude compared with the same activity on the cortical surface:",
   "choices": [
    "~100 µV scalp, up to 10 mV cortex",
    "Both about 100 µV",
    "~10 mV scalp, 100 µV cortex",
    "About 1 V on the scalp"
   ],
   "a": 0,
   "ex": "Skull and scalp attenuate and spatially smear. The largest scalp potentials are about 150 µV peak.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:eeg-source",
   "type": "mcq",
   "q": "The main generator of scalp EEG is:",
   "choices": [
    "Summed pyramidal-cell PSPs",
    "Axonal action potentials",
    "Glial cell currents",
    "Blood-flow potentials"
   ],
   "a": 0,
   "ex": "Apical dendrites are parallel and vertical: an open field. Cells with radial dendrites cancel at a distance (closed field).",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:alpha",
   "type": "flash",
   "q": "Alpha rhythm: frequency, amplitude, where, and when?",
   "a": "8–13 Hz, 20–200 µV, occipital, awake and relaxed with eyes closed. Gone on eye opening or in sleep.",
   "ex": "Replaced by low-voltage desynchronised activity when attention engages.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:signal-ranges",
   "type": "flash",
   "q": "Order by amplitude: EEG, ECG, EMG, EOG.",
   "a": "EOG ~1 mV ≈ ECG 1–3 mV > surface EMG 20 µV–2 mV > EEG ~100 µV, all within 0.01 Hz–3 kHz.",
   "ex": "Spiky signals (ECG, EMG) need more bandwidth; slow ones (EEG, EOG) need low 1/f noise.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:bandwidths",
   "type": "flash",
   "q": "Standard amplifier passbands for ECG, EEG and EMG?",
   "a": "ECG 0.05–100 Hz; EEG about 0.2–100 Hz; clinical surface EMG about 100 Hz–3 kHz.",
   "ex": "Too narrow rounds off spikes; too wide adds noise. Facial EMG is often 15–500 Hz.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:eog-dc",
   "type": "tf",
   "q": "The EOG needs a DC-coupled amplifier, unlike most biopotentials.",
   "a": true,
   "ex": "It tracks a steady corneal–retinal dipole moving with gaze, roughly linear to ±30°. Electrode drift is the enemy.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:averaging",
   "type": "flash",
   "q": "How are evoked potentials extracted when their spectrum overlaps the background EEG?",
   "a": "Signal averaging over repeated stimuli - filtering cannot separate them.",
   "ex": "Works even at −60 dB input SNR; the gain grows with √N.",
   "tags": [
    "signals"
   ]
  },
  {
   "id": "be:electrode-transducer",
   "type": "tf",
   "q": "A biopotential electrode is a transducer: it turns ionic current in tissue into electron current in the wire.",
   "a": true,
   "ex": "Charge crosses the interface only through oxidation–reduction reactions, which is what creates half-cell potentials.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:half-cell-def",
   "type": "flash",
   "q": "What is a half-cell potential?",
   "a": "The potential between a metal and an electrolyte containing its ions, set by the metal, ion concentration and temperature.",
   "ex": "It cannot be measured alone; values are tabulated against the hydrogen electrode, defined as 0 V.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:halfcell-agagcl",
   "type": "mcq",
   "q": "Standard half-cell potential of Ag/AgCl?",
   "choices": [
    "+0.223 V",
    "+0.799 V",
    "−0.763 V",
    "0 V"
   ],
   "a": 0,
   "ex": "Pure Ag is +0.799 V, Zn −0.763 V, Au +1.42 V. Two different metals in one lead give a DC offset equal to their difference.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:overpotentials",
   "type": "mcq",
   "q": "The three components of electrode overpotential are:",
   "choices": [
    "Ohmic, concentration, activation",
    "Thermal, shot, flicker",
    "Series, shunt, contact",
    "Resistive, inductive, capacitive"
   ],
   "a": 0,
   "ex": "All appear only when current flows; a perfectly nonpolarisable electrode would have none.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:polarizable",
   "type": "mcq",
   "q": "A perfectly polarisable electrode passes current across the interface:",
   "choices": [
    "Only as displacement current",
    "Freely, with no overpotential",
    "Only in one direction",
    "Only above 1 V"
   ],
   "a": 0,
   "ex": "It acts as a capacitor. Noble metals (Pt) come closest: poor for slow signals. Ag/AgCl approaches the nonpolarisable ideal.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:agagcl-why",
   "type": "flash",
   "q": "Why is Ag/AgCl so stable in the body?",
   "a": "Its half-cell potential depends on Cl− activity, which is high and constant in body fluids and gel.",
   "ex": "AgCl is nearly insoluble (Ks ~1e-10), so the Ag+ term is fixed. It also has far less low-frequency noise than bare silver.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:sintered-vs-electrolytic",
   "type": "tf",
   "q": "Sintered Ag/AgCl pellets outlast electrolytically chlorided electrodes because deposited AgCl flakes off under stress.",
   "a": true,
   "ex": "Flaking exposes bare Ag: an unstable, noisy half-cell. Sintered pellets are the choice for reusable electrodes.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:chloriding-dose",
   "type": "mcq",
   "q": "Optimal electrolytic AgCl deposit for lowest impedance and defibrillation recovery:",
   "choices": [
    "~100–500 mC/cm²",
    "~1 mC/cm²",
    "~10 C/cm²",
    "Thinner is always better"
   ],
   "a": 0,
   "ex": "Below ~100 mA·s little changes; above it the impedance rises again. Das and Webster found 500 mC/cm² best for recovery.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:electrode-model",
   "type": "flash",
   "q": "Equivalent circuit of an electrode–electrolyte interface?",
   "a": "Half-cell voltage in series with Rd || Cd (the double layer) and a series Rs for electrolyte and interface resistance.",
   "ex": "Impedance is Rs + Rd at low frequency, Rs at high frequency, and depends on current density.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:electrode-corner",
   "type": "flash",
   "q": "Electrode: 30 kΩ below 50 Hz, 500 Ω above 20 kHz, corner at 100 Hz. Rs, Rd, Cd?",
   "a": "Rs = 500 Ω, Rd = 29.5 kΩ, Cd = 1/(2π · 100 · 29.5 kΩ) ≈ 54 nF.",
   "ex": "Webster example 5.4. The corner sits where 1/(ωCd) equals Rd.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:agagcl-impedance",
   "type": "mcq",
   "q": "Impedance at 10 Hz for 1 cm²: Ag/AgCl versus carbon-loaded silicone rubber?",
   "choices": [
    "<10 Ω vs ~30 kΩ",
    "~30 kΩ vs <10 Ω",
    "Both ~1 kΩ",
    "Both ~1 MΩ"
   ],
   "a": 0,
   "ex": "Low-frequency impedance is where the chemistry shows. Bare Ag is ~3× higher at 10 Hz than at 300 Hz.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:electrode-thermal-noise",
   "type": "flash",
   "q": "Thermal noise of an electrode whose real part is about 2 kΩ?",
   "a": "√(4kT · 2000) ≈ 5.8 nV/√Hz - comparable to a good amplifier's input noise.",
   "ex": "Drying gel raises |Z| and the noise with it.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:liquid-junction",
   "type": "tf",
   "q": "Two NaCl solutions whose activities differ 10× develop about 12 mV across their junction.",
   "a": true,
   "ex": "Liquid-junction potentials are tens of mV at most, but that is the size of some biopotentials.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:skin-impedance",
   "type": "mcq",
   "q": "Skin impedance for 1 cm² falls from about:",
   "choices": [
    "200 kΩ at 1 Hz to 200 Ω at 1 MHz",
    "2 kΩ at 1 Hz to 2 Ω at 1 MHz",
    "200 MΩ to 200 kΩ",
    "It is constant"
   ],
   "a": 0,
   "ex": "Almost all of it is the stratum corneum: a parallel RC with its own Nernst potential Ese. The dermis below is just resistive.",
   "tags": [
    "skin"
   ]
  },
  {
   "id": "be:abrasion",
   "type": "tf",
   "q": "Abrading the stratum corneum stabilises the signal, but the layer regrows in as little as 24 hours.",
   "a": true,
   "ex": "It shorts Ese, Ce and Re. Ten 0.5 mm punctures cut skin-stretch artifact from 5–10 mV to under 0.2 mV.",
   "tags": [
    "skin"
   ]
  },
  {
   "id": "be:motion-artifact",
   "type": "flash",
   "q": "Two sources of electrode motion artifact?",
   "a": "Disturbance of the double-layer charge at a polarisable interface, and change in skin potential Ese when the skin stretches.",
   "ex": "Nonpolarisable Ag/AgCl fixes the first; abrasion or a recessed gel cavity fixes the second.",
   "tags": [
    "skin"
   ]
  },
  {
   "id": "be:motion-lowfreq",
   "type": "mcq",
   "q": "Motion artifact is mostly low-frequency, so it harms which signals most?",
   "choices": [
    "ECG, EEG and EOG",
    "EMG and axon spikes",
    "Only signals above 1 kHz",
    "None - it is broadband"
   ],
   "a": 0,
   "ex": "For EMG a high-pass filter removes it cheaply; for EEG the same filter would remove the signal.",
   "tags": [
    "skin"
   ]
  },
  {
   "id": "be:floating-electrode",
   "type": "flash",
   "q": "How does a recessed (floating) electrode reduce motion artifact?",
   "a": "The metal sits in a gel-filled cavity and never touches skin, so the gel cannot move relative to the double layer.",
   "ex": "Disposable versions use a gel-soaked foam disc bonded to the metal.",
   "tags": [
    "skin"
   ]
  },
  {
   "id": "be:electrode-standard",
   "type": "flash",
   "q": "Bench limits for pregelled disposable ECG electrodes?",
   "a": "Offset <100 mV, noise <150 µV, 10 Hz impedance <2 kΩ, defibrillation recovery <100 mV after four 2 mC charges.",
   "ex": "Plus bias-current tolerance: 200 nA for 8 h must leave under 100 mV of offset.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:same-electrode-trick",
   "type": "flash",
   "q": "Why measure a mucosal potential by moving one electrode between two sites instead of using two electrodes?",
   "a": "Two Ag/AgCl electrodes can differ by mV to tens of mV; one electrode's half-cell cancels exactly.",
   "ex": "Webster example 5.3.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:emg-electrode-metal",
   "type": "tf",
   "q": "Stainless steel or gold discs are used for EMG because they resist reacting with sweat, at the cost of being polarisable.",
   "a": true,
   "ex": "Fine for EMG, where motion artifact filters out; poor for EEG and ECG, where the low-frequency artifact would stay.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:micropipette",
   "type": "flash",
   "q": "Why does a glass micropipette need an electrometer amplifier?",
   "a": "Tip resistance is 20 MΩ–1 GΩ, so input bias must be ~10 fA and input resistance ~10^15 Ω.",
   "ex": "A 200 MΩ tip makes ~100 µV rms Johnson noise over 3 kHz and, with 1 pF tip capacitance, rolls off near 800 Hz.",
   "tags": [
    "electrodes"
   ]
  },
  {
   "id": "be:amp-requirements",
   "type": "flash",
   "q": "Four basic requirements of a biopotential amplifier?",
   "a": "Input impedance ≥10 MΩ; patient protection and isolation; gain ~1000 in calibrated steps; bandwidth limited to the signal.",
   "ex": "Plus high CMRR, because bipolar electrodes ride on a common-mode voltage far larger than the signal.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:input-impedance-why",
   "type": "mcq",
   "q": "Why does a biopotential amplifier need very high input impedance?",
   "choices": [
    "No electrode loading, better CMRR",
    "To boost gain",
    "To lower noise",
    "To speed settling"
   ],
   "a": 0,
   "ex": "Loading distorts electrode behaviour, and vcm converts to a differential error via (Z2 − Z1)/Zin.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:einthoven",
   "type": "flash",
   "q": "Einthoven's law for the limb leads?",
   "a": "I − II + III = 0, with I = LA − RA, II = LL − RA, III = LL − LA.",
   "ex": "Lead vectors at 0°, 60° and 120° form the frontal-plane triangle.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:wilson-terminal",
   "type": "mcq",
   "q": "Wilson's central terminal is:",
   "choices": [
    "Mean of RA, LA, LL via equal resistors",
    "The right-leg electrode",
    "Chest lead V1",
    "Earth ground"
   ],
   "a": 0,
   "ex": "The reference for unipolar leads. Resistors of at least 5 MΩ, or buffers, keep the loading small.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:augmented-leads",
   "type": "tf",
   "q": "Augmented leads aVR, aVL and aVF are 50% larger than VR, VL and VF, with the same direction.",
   "a": true,
   "ex": "Dropping the measured limb from the central terminal removes the shunting resistor. All six frontal leads then sit 30° apart.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:ecg-spec",
   "type": "flash",
   "q": "Key ECG standard numbers: upper cutoff, input impedance, DC lead current, risk current?",
   "a": "3 dB point at 150 Hz; ≥2.5 MΩ at 10 Hz per lead; ≤0.1 µA DC into any lead; risk current ≤10 µA.",
   "ex": "Gains of 5, 10 and 20 mm/mV. CMRR test: 20 V at 60 Hz with 51 kΩ imbalance must give under 10 mm of noise.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:lf-distortion",
   "type": "mcq",
   "q": "An ECG amplifier with a 1 Hz high-pass corner shows what distortion?",
   "choices": [
    "Wandering baseline, biphasic waves",
    "Rounded QRS peaks",
    "Clipped R waves",
    "No visible change"
   ],
   "a": 0,
   "ex": "High-frequency roll-off rounds corners and shrinks the QRS instead. That is why the standard low cutoff is 0.05 Hz.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:transient-recovery",
   "type": "flash",
   "q": "16 s time constant, 10 mV transient, 1 mV R wave, ±2 mV display: how long until the whole ECG is visible?",
   "a": "10 · e^(−t/16) = 1 mV, so t = 16 · ln 10 ≈ 37 s.",
   "ex": "Long low-frequency time constants make transient recovery slow. Baseline-restore circuits shorten it.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:protection-diodes",
   "type": "mcq",
   "q": "Breakdown voltage of a parallel silicon-diode input limiter?",
   "choices": [
    "~600 mV",
    "~6 V",
    "~60 V",
    "~0.6 mV"
   ],
   "a": 0,
   "ex": "Back-to-back Zeners give 2–20 V; gas-discharge tubes 50–90 V, always with a series resistor to limit input current.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:ina-3opamp",
   "type": "tf",
   "q": "A home-made three-op-amp instrumentation amplifier needs resistors matched to about 0.02% to reach a commercial IA's CMRR.",
   "a": true,
   "ex": "That is the argument for buying an AD620-class part.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:ad620",
   "type": "flash",
   "q": "AD620-class IA: gain range, bandwidth at G = 1000, noise, CMRR?",
   "a": "Gain 1–1000 by one resistor; ~10 kHz at G = 1000; 9 nV/√Hz and 100 fA/√Hz; CMRR 90 dB at G = 1 to 130 dB at G = 1000.",
   "ex": "Input bias 0.5 nA, inputs 10 GΩ || 2 pF. CMRR improves with gain.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:ads1299",
   "type": "flash",
   "q": "What does an ADS1299-style front end give an EEG design?",
   "a": "Eight simultaneous 24-bit delta-sigma channels with built-in PGA and reference, up to 16 kSPS, over SPI.",
   "ex": "One chip replaces the instrumentation-amplifier plus ADC chain.",
   "tags": [
    "amplifier"
   ]
  },
  {
   "id": "be:efield-coupling",
   "type": "flash",
   "q": "How does power-line electric-field coupling into lead wires become a differential signal?",
   "a": "Displacement current (~6 nA per lead) flows through unequal skin–electrode impedances: 6 nA × 20 kΩ = 120 µV.",
   "ex": "Fix: shield the leads and ground the shield at the amplifier; lower the electrode impedances.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:vcm-origin",
   "type": "flash",
   "q": "Where does the body's common-mode voltage come from?",
   "a": "Displacement current into the body (~0.2 µA) through the ground-electrode impedance: 0.2 µA × 50 kΩ = 10 mV.",
   "ex": "In poor environments idb exceeds 1 µA and vcm exceeds 50 mV.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:cm-to-dm",
   "type": "mcq",
   "q": "vcm = 10 mV, electrode imbalance 20 kΩ, Zin = 5 MΩ. Differential error?",
   "choices": [
    "40 µV",
    "4 mV",
    "0.4 µV",
    "10 mV"
   ],
   "a": 0,
   "ex": "vA − vB = vcm · (Z2 − Z1)/Zin. Tolerable on an ECG, ruinous on a 100 µV EEG. Raise Zin or balance the electrodes.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:cmrr-80db",
   "type": "flash",
   "q": "A CMRR of 80 dB means the differential gain exceeds the common-mode gain by:",
   "a": "10^4, a factor of ten thousand.",
   "ex": "12 µV of EEG against 1 mV common-mode is 12:1000 at the input and 120:1 at the output.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:magnetic-pickup",
   "type": "mcq",
   "q": "Cheapest fix for magnetically induced power-line interference?",
   "choices": [
    "Twist the lead wires together",
    "A mu-metal room",
    "Raise the input impedance",
    "Add a 60 Hz notch"
   ],
   "a": 0,
   "ex": "Induced voltage scales with the loop area formed by patient, leads and amplifier. Twisting shrinks it.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:rf-shunt",
   "type": "flash",
   "q": "Why put ~200 pF across the amplifier inputs?",
   "a": "It shorts RF picked up by the leads before p–n junctions rectify it, while barely loading the inputs at ECG frequencies.",
   "ex": "Modern IAs such as the INA188 build EMI filtering in.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:drl-principle",
   "type": "flash",
   "q": "How does the driven-right-leg circuit work?",
   "a": "It senses vcm through averaging resistors, inverts and amplifies it, and drives it back into the body so feedback nulls vcm.",
   "ex": "The patient is not grounded; displacement current returns through the op-amp output instead.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:drl-numbers",
   "type": "mcq",
   "q": "DRL with Ra = 25 kΩ, Rf = 5 MΩ, electrode 100 kΩ, id = 0.2 µA. Effective right-leg resistance and vcm?",
   "choices": [
    "~249 Ω and 50 µV",
    "100 kΩ and 20 mV",
    "25 kΩ and 5 mV",
    "5 MΩ and 1 V"
   ],
   "a": 0,
   "ex": "RRL/(1 + 2Rf/Ra) = 100 kΩ/401. When the op amp saturates only Rf || Ro limits current - hence the large values.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:ground-loop",
   "type": "flash",
   "q": "What is a ground loop in patient monitoring?",
   "a": "Two grounded machines on the patient, plugged into outlets whose grounds differ slightly, so current flows through the patient.",
   "ex": "It raises the body's common-mode voltage and is a safety issue. A single patient-grounding point fixes it.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:emg-in-ecg",
   "type": "tf",
   "q": "EMG interference on an ECG can be told from electromagnetic interference by watching the patient rather than the trace.",
   "a": true,
   "ex": "Any muscle between the electrodes contributes. Both look like hash on the recording.",
   "tags": [
    "interference"
   ]
  },
  {
   "id": "be:thermal-noise",
   "type": "flash",
   "q": "Johnson noise of a resistor?",
   "a": "White and Gaussian, PDS = 4kTR V²/Hz; over a bandwidth B the rms value is √(4kTRB).",
   "ex": "1 kΩ at room temperature is about 4 nV/√Hz.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:one-over-f",
   "type": "mcq",
   "q": "Passing DC through a resistor adds which noise?",
   "choices": [
    "1/f noise, corner at A·I²/(4kTR)",
    "Shot noise only",
    "Nothing",
    "The white noise doubles"
   ],
   "a": 0,
   "ex": "Carbon composition is worst; metal film is nearly free of it. Nine resistors in series–parallel cut the 1/f term nine-fold.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:two-source-model",
   "type": "flash",
   "q": "How is an amplifier's noise specified?",
   "a": "Two input-referred sources: ena in V/√Hz in series and ina in A/√Hz in shunt; the amplifier itself is then noiseless.",
   "ex": "Both are flat at midband and rise at high frequency; ena also has a 1/f region at low frequency.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:noise-factor",
   "type": "flash",
   "q": "Noise factor of an amplifier driven from a source resistance Rs?",
   "a": "F = 1 + (ena² + ina² · Rs²)/(4kT · Rs), and NF = 10 · log10 F.",
   "ex": "Minimum at an optimum Rs where ena and ina·Rs balance. Bandwidth cancels out.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:noise-bandwidth",
   "type": "mcq",
   "q": "Equivalent noise bandwidth of a one-pole low-pass with time constant τ?",
   "choices": [
    "1/(4τ) - π/2 times the −3 dB point",
    "1/(2πτ)",
    "1/τ",
    "2πτ"
   ],
   "a": 0,
   "ex": "White noise through |H|² integrates to gain² · 1/(4τ). Use it to turn nV/√Hz into µV rms.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:shot-noise",
   "type": "flash",
   "q": "Shot noise of a DC current I?",
   "a": "PDS = 2qI A²/Hz. A 2 pA JFET gate leakage gives about 0.8 fA/√Hz.",
   "ex": "BJTs have shot noise on base and collector currents, so their best-noise bias point depends on Rs.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:jfet-ena",
   "type": "flash",
   "q": "Thermal noise of a JFET channel, referred to the input?",
   "a": "ena² ≈ 4kT/gm, plus a 1/f term: (4kT/gm)(1 + fc/f).",
   "ex": "Higher gm is quieter and cooling helps. MOSFETs lack the gate diode, so little 1/f in ina.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:noise-through-filter",
   "type": "tf",
   "q": "Gaussian noise through a linear filter stays Gaussian, with output PDS = input PDS × |H(f)|².",
   "a": true,
   "ex": "So an amplifier chain can be summed stage by stage in mean-square terms.",
   "tags": [
    "noise"
   ]
  },
  {
   "id": "be:perception-threshold",
   "type": "mcq",
   "q": "Threshold of perception for 60 Hz current through the hands?",
   "choices": [
    "~0.5 mA minimum, mean ~1 mA",
    "~50 mA",
    "~5 µA",
    "~5 A"
   ],
   "a": 0,
   "ex": "Through ECG gel electrodes it averages only 83 µA. DC thresholds are 2–10 mA.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:let-go",
   "type": "mcq",
   "q": "Minimum let-go current at 60 Hz?",
   "choices": [
    "~6 mA",
    "~60 mA",
    "~0.6 mA",
    "~600 mA"
   ],
   "a": 0,
   "ex": "Means are 16 mA for men and 10.5 mA for women. 50–60 Hz is the worst frequency; let-go rises below 10 Hz and above a few hundred Hz.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:vf-threshold",
   "type": "flash",
   "q": "Macroshock current range that fibrillates the ventricles?",
   "a": "About 75–400 mA at 60 Hz for a 70 kg adult, hand to hand, for 1–3 s.",
   "ex": "Respiratory arrest at 18–22 mA; sustained myocardial contraction at 1–6 A. VF does not stop when the current is removed.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:microshock",
   "type": "mcq",
   "q": "Accepted safe limit for current applied directly to the heart (microshock)?",
   "choices": [
    "10 µA",
    "10 mA",
    "100 mA",
    "1 mA"
   ],
   "a": 0,
   "ex": "VF seen at 20 µA in dogs and 49 µA in patients through pacing catheters. Only isolated patient leads may touch the heart.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:skin-resistance",
   "type": "flash",
   "q": "Dry versus wet skin resistance, and what it means at 120 V?",
   "a": "Dry: 15 kΩ–1 MΩ per cm², about 2 mA. Wet: ~1% of that, ~500 Ω, about 240 mA - above the VF threshold.",
   "ex": "Internal body resistance is only ~500 Ω limb to limb. Gel electrodes and catheters bypass the skin.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:leakage-limits",
   "type": "flash",
   "q": "IEC 60601 leakage limits, normal / single fault?",
   "a": "Chassis 100 / 500 µA; non-isolated patient leads 10 / 100 µA; isolated patient leads 10 / 50 µA.",
   "ex": "Up to 100 µA of patient auxiliary current at ≥0.1 Hz is allowed for bias and impedance measurement.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:isolation-amp",
   "type": "flash",
   "q": "Three defining features of an isolation amplifier?",
   "a": "Over 10 MΩ of ohmic isolation, over 1 kV isolation-mode voltage, over 100 dB common-mode rejection.",
   "ex": "Built with transformers, optocouplers or ~1 pF capacitive barriers; IMRR rates the rejection of vISO.",
   "tags": [
    "isolation"
   ]
  },
  {
   "id": "be:isolation-capacitance",
   "type": "mcq",
   "q": "Barrier capacitance that limits 120 V, 60 Hz leakage to 10 µA?",
   "choices": [
    "~220 pF",
    "~22 nF",
    "~2.2 pF",
    "~2.2 µF"
   ],
   "a": 0,
   "ex": "Z = 120 V / 10 µA = 12 MΩ, C = 1/(2π · 60 · 12 MΩ). Real barriers aim for single picofarads.",
   "tags": [
    "isolation"
   ]
  },
  {
   "id": "be:isolation-types",
   "type": "flash",
   "q": "Isolation-amplifier architectures?",
   "a": "Transformer (modulated carrier, 50–500 kHz), optical (LED–photodiode matched pair), capacitive (1 pF pair, duty-cycle modulated).",
   "ex": "Plus GMR-bridge and flying-capacitor variants. All need an isolated supply for the patient side.",
   "tags": [
    "isolation"
   ]
  },
  {
   "id": "be:gfci",
   "type": "tf",
   "q": "A GFCI trips at about 6 mA of hot–neutral imbalance, so it protects against macroshock but not microshock.",
   "a": true,
   "ex": "Not used in patient-care circuits, where losing life-support power is the greater risk.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:critical-care-40mv",
   "type": "flash",
   "q": "Allowed potential between exposed conductive surfaces near a critical-care patient?",
   "a": "40 mV (500 mV in general care), with every ground tied to one patient-equipment grounding point at under 0.15 Ω.",
   "ex": "A 5 A ground fault through 0.1 Ω of ground wire makes 500 mV - enough for microshock via a catheter.",
   "tags": [
    "safety"
   ]
  },
  {
   "id": "be:isolated-power",
   "type": "flash",
   "q": "What does an isolated power system with a line-isolation monitor do?",
   "a": "Neither conductor is grounded, so a single ground fault passes little current; the LIM alarms at 3.7–5 mA of hazard current.",
   "ex": "Mainly for wet locations and flammable anaesthetics. It does not bring leakage below the 10 µA microshock limit.",
   "tags": [
    "safety"
   ]
  }
 ],
 "version": "1.0.0",
 "updated": "2026-09-14T14:57:54Z"
}