ECG, EEG and EMG instrumentation

In short

ECG, EEG and EMG instrumentation is the chain of electrodes, protection circuits, high-CMRR amplifiers, filters, isolation and digitisers that turns millivolt- to microvolt-level bioelectric potentials from the heart, brain and muscle into a trace or a data file. The three machines share the same architecture but differ hugely in amplitude, bandwidth and gain, which is exactly what examiners test. The topic carries steady marks in university Biomedical Instrumentation papers and appears in GATE BM (biomedical signal acquisition, amplifier specifications), and in interviews at Indian employers such as BPL Medical Technologies, Schiller Healthcare India, Allengers, Recorders & Medicare Systems (RMS), Nihon Kohden India, Philips/GE Healthcare Bengaluru, SCTIMST and AIIMS/PGI biomedical engineering departments. Field service, R&D, test-and-validation and hospital biomedical-engineering roles all need it, along with the safety standards (IEC 60601 series, adopted in India as IS 13450 parts and enforced through CDSCO licensing of medical devices).

Explained

Amplitude, bandwidth and gain - the three machines compared

• ECG (electrocardiogram): surface potential of cardiac depolarisation, 0.5-5 mV typical (lead II R-wave ~1 mV), diagnostic bandwidth 0.05-150 Hz (0.05-250 Hz for paediatric), gain ~1,000. Monitoring mode narrows to 0.5-40 Hz to suppress drift and noise.
• EEG (electroencephalogram): scalp potential of cortical synaptic activity, only 10-100 µV, bandwidth ~0.5-70 Hz (0.1-100 Hz for research), gain 10,000-100,000. Electrocorticogram from the exposed cortex is ~10 times larger.
• EMG (electromyogram): muscle fibre action potentials; surface EMG 50 µV-5 mV, needle EMG up to ~10 mV, bandwidth 10-500 Hz surface, 10 Hz-10 kHz needle (single-fibre to 20 kHz), gain 1,000-10,000.
• Why bandwidth differs: ECG needs a very low lower cut-off because the ST segment is nearly d.c.; EMG energy is concentrated at 50-150 Hz, so its lower cut-off is deliberately high to remove motion artefact.
• Electrode-skin potential (up to ±300 mV d.c. offset) and the ~50 Hz mains common-mode voltage (often 1-10 V on the body) are both far larger than the signal - hence a.c. coupling and high CMRR.

The common front end: electrodes, protection, amplifier

1. Electrodes: Ag/AgCl non-polarisable electrodes with conductive gel. ECG - limb plates/suction and disposable pre-gelled Ag/AgCl discs; EEG - cup/disc electrodes with collodion or paste, or a cap; EMG - surface bar electrodes, concentric needle, monopolar needle, single-fibre and fine-wire. Skin preparation to bring impedance below ~5 kΩ (EEG below 5 kΩ, ideally balanced between electrodes).
2. Defibrillation and ESU protection: series current-limiting resistors (~10 kΩ-1 MΩ), neon lamp/gas discharge tube or back-to-back diode clamps, spark gaps; protects the input from a 5 kV defibrillator discharge. Required by IEC 60601-2-25 (ECG) and -2-27 (monitors).
3. Lead-off / electrode-contact detection: a small high-frequency or d.c. current injected to flag a disconnected electrode.
4. Instrumentation amplifier: three-op-amp IA (or a dedicated chip - AD620/INA118/AD8232 ECG front end, or an integrated ADS1298 8-channel 24-bit AFE used in modern ECG boards). High input impedance (>10 MΩ, often >100 MΩ), CMRR > 100 dB (ECG ≥ 89 dB per standard; EEG 100-120 dB), low input noise (<1 µV p-p for EEG), input bias current in nA/pA.
5. Right-leg drive (driven right leg, DRL): the right-leg electrode is not grounded; the sensed common-mode voltage is inverted, amplified and fed back to the body, cutting effective common-mode interference by 10-40 dB while limiting fault current. Modern standard practice.
6. Filters: a.c.-coupling high-pass (0.05 Hz ECG; 0.5 Hz EEG) to block electrode d.c. offset; anti-aliasing low-pass before the ADC; a 50 Hz notch (India's mains frequency - 60 Hz in the Americas), used sparingly in diagnostic ECG because it distorts the QRS.
7. Isolation: optical or transformer-coupled isolation amplifier plus isolated d.c.-d.c. supply, giving a Type CF applied part (floating, defibrillation-proof, patient auxiliary current ≤10 µA normal / 50 µA single fault for CF). Protects against macro- and micro-shock.
8. Output: ADC (12-24 bit, 500-1,000 samples/s per channel for ECG; 250-512 for EEG; 2-10 kHz for EMG), DSP, display, thermal-array writer.

ECG instrumentation: leads and the recorder

• Einthoven's bipolar limb leads: I = VL − VR, II = VF − VR, III = VF − VL. Einthoven's law: II = I + III - a built-in check on recording integrity.
• Einthoven triangle: RA, LA and LL treated as vertices of an equilateral triangle with the heart's dipole at the centre; axes at 0° (I), +60° (II), +120° (III).
• Augmented unipolar (Goldberger) leads: aVR, aVL, aVF - each limb against the average of the other two; this "augmentation" raises amplitude by 50% compared with Wilson's unipolar leads. aVR + aVL + aVF = 0.
• Wilson central terminal (WCT): RA, LA, LL joined through equal 5 kΩ resistors to form the indifferent reference for the precordial leads.
• Precordial (chest) leads V1-V6: V1 - 4th intercostal space right sternal border; V2 - 4th ICS left sternal border; V3 - between V2 and V4; V4 - 5th ICS mid-clavicular line; V5 - anterior axillary line level with V4; V6 - mid-axillary line level with V4. Ten electrodes (4 limb + 6 chest) give 12 leads.
• Standardisation: paper speed 25 mm/s (50 mm/s for detail), sensitivity 10 mm/mV (1 mV = 10 mm deflection); grid 1 mm small square = 0.04 s × 0.1 mV, 5 mm large square = 0.2 s × 0.5 mV.
• Block diagram: patient cable → protection → lead selector (switch/multiplexer) → pre-amplifier (IA) → d.c. restorer/baseline reset → driver amplifier → ADC/DSP → thermal writer and display; plus calibration 1 mV pulse, DRL circuit, isolated supply.
• Normal waveform landmarks: P wave (atrial depolarisation, <0.12 s, <0.25 mV), PR interval 0.12-0.20 s, QRS 0.06-0.10 s, QT ~0.35-0.44 s, T wave (ventricular repolarisation).
• Artefacts: baseline wander (respiration, 0.15-0.3 Hz; electrode movement), 50 Hz pick-up, muscle tremor, motion artefact, incorrect lead reversal (RA/LA swap inverts lead I and makes aVR look normal).
• Variants: vectorcardiography (Frank lead system), Holter ambulatory monitoring (24-48 h), stress/treadmill ECG, foetal ECG (abdominal, needs very high CMRR and maternal-QRS cancellation), telemetry and 12-lead tele-ECG used in India's STEMI networks and Ayushman Bharat/eSanjeevani tele-cardiology.

EEG instrumentation

• Rhythms: delta <4 Hz (deep sleep), theta 4-8 Hz, alpha 8-13 Hz (relaxed, eyes closed, occipital; blocked on eye opening), beta 13-30 Hz (alert, frontal), gamma >30 Hz.
• 10-20 electrode system (Jasper): electrodes at 10% and 20% of the nasion-inion and the preauricular distances. Labels - Fp, F, C, P, O, T with odd numbers left, even right, z for midline (Fz, Cz, Pz). 21 electrodes in the classic set; 10-10 and 10-5 systems for high-density arrays.
• Montages: bipolar (adjacent electrode pairs - localises by phase reversal), referential/unipolar (each electrode against an ear/mastoid or Cz reference), average reference, Laplacian (source derivation).
• Hardware chain: electrode board/jack box → head box with pre-amplifiers close to the patient (reduces cable pick-up) → montage selector → amplifiers with calibration (50 µV pulse) → high-pass, low-pass and 50 Hz notch → multi-channel 8/16/21/32/64-channel digitiser → display at 30 mm/s with 7 µV/mm sensitivity typical.
• Artefacts: eye blink/EOG (frontal, large and slow), ECG and pulse, EMG from frontalis and temporalis, glossokinetic, sweat (low-frequency), electrode pop, 50 Hz.
• Related recordings: evoked potentials - VEP, BAEP/ABR, SSEP - extracted from EEG by synchronous signal averaging (SNR improves as √N); polysomnography (EEG + EOG + EMG + ECG + respiration); intra-operative monitoring; BIS depth-of-anaesthesia index; EEG-based BCI research (IITs, IISc, NIMHANS, NBRC).

EMG instrumentation and nerve conduction

• Motor unit action potential (MUAP): sum of fibre potentials of one motor unit; needle EMG assesses amplitude, duration, phases and recruitment. Interference pattern at full effort.
• Electrodes: concentric needle (recording area ~0.07 mm², small pickup zone) for clinical EMG; monopolar needle; surface electrodes for kinesiology, ergonomics, prosthesis control and biofeedback; fine-wire for deep muscles; SENIAM recommendations guide surface electrode placement.
• Chain: needle/surface electrode → high-impedance differential pre-amplifier (gain 100-1,000, bandwidth 10 Hz-10 kHz) → further amplification → audio monitor (loudspeaker - diagnostically important, the clinician listens to the "crackle") → oscilloscope/digital display → integrator/RMS and frequency analysis.
• Processing: full-wave rectification + low-pass smoothing gives the linear envelope; IEMG (integrated EMG) and RMS EMG estimate effort; median/mean power frequency falls with fatigue.
• Stimulator for nerve conduction studies (NCS): isolated constant-current stimulator, 0-100 mA, 0.05-1 ms pulses, up to ~400 V compliance; measures distal latency, amplitude and conduction velocity; also F-wave, H-reflex and repetitive stimulation for neuromuscular junction disorders.
• Specific noise problems: stimulus artefact and shock saturation (blanking circuits), 50 Hz, crosstalk from neighbouring muscles, ECG contamination in trunk EMG.

Safety, standards and Indian regulation

• Leakage current limits (IEC 60601-1, 3rd ed. A1): patient leakage for a Type CF applied part ≤10 µA (normal condition) and ≤50 µA (single fault condition) for d.c./a.c.; Type BF ≤100 µA / 500 µA. Earth leakage ≤5 mA normal for general equipment. ECG, EEG and EMG applied parts are Type CF and defibrillation-proof where applicable.
• Micro-shock (current via an intracardiac catheter) is dangerous at tens of microamperes; macro-shock through intact skin needs milliamperes - ventricular fibrillation threshold around 80-100 mA at 50 Hz for a one-second surface contact. This is why floating CF inputs and equipotential earthing exist.
• Standards: IEC 60601-1 (general safety and essential performance), 60601-1-2 (EMC), 60601-2-25 (diagnostic ECG), -2-26 (EEG), -2-27 (ECG monitoring), -2-40 (EMG and evoked response), -2-4 (defibrillators); harmonised into Indian Standards as the IS 13450 series by BIS. Quote part numbers only where you have verified the current edition - several parts have been revised.
• Indian regulation: medical devices are regulated by CDSCO under the Medical Devices Rules, 2017 (framed under the Drugs and Cosmetics Act, 1940); ECG, EEG and EMG machines are notified devices requiring import/manufacturing licence (Form MD-5/MD-9/MD-15) with risk classification (ECG machine Class B-type, electrode gels and accessories lower class - verify the current classification list, which CDSCO updates). The ICMR National Ethical Guidelines (2017) govern human-subject recording in research. Hospital electrical safety also draws on the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 for installation earthing, and NABH accreditation standards for periodic biomedical equipment testing and calibration (traceable to NABL-accredited labs).

Key points

• Amplitude ladder - EEG ~50 µV, EMG surface ~1 mV, ECG ~1 mV; gain therefore rises EEG > EMG ≈ ECG.
• Bandwidth ladder - ECG 0.05-150 Hz, EEG 0.5-70 Hz, EMG 10 Hz-10 kHz (needle).
• CMRR - ability to reject the ~50 Hz common-mode voltage on the body; needs matched gains and high input impedance; >100 dB in practice.
• Driven right leg - feeds inverted common-mode signal back to the patient instead of earthing them; improves rejection and limits fault current.
• Einthoven's law - lead II = lead I + lead III; a quick validity check on a recording.
• Wilson central terminal - RA+LA+LL through 5 kΩ each, the reference for V1-V6.
• Augmented leads - aVR, aVL, aVF are 50% larger than Wilson's unipolar limb leads; they sum to zero.
• 12-lead ECG from 10 electrodes - 3 bipolar + 3 augmented + 6 precordial.
• ECG calibration - 10 mm/mV at 25 mm/s; one small square = 0.04 s and 0.1 mV.
• 10-20 system - EEG electrode positions at 10%/20% fractions of nasion-inion and preauricular arcs; z = midline, odd = left, even = right.
• Montage - bipolar localises by phase reversal; referential shows true amplitude but contaminates with a noisy reference.
• Signal averaging - evoked potentials are recovered because SNR improves as √N for N time-locked sweeps.
• Isolation amplifier - optical or transformer coupling breaks the galvanic path, giving a floating Type CF applied part.
• Defibrillator protection - series resistors plus neon/spark-gap or diode clamps withstand 5 kV.
• Type CF leakage - 10 µA normal, 50 µA single fault; micro-shock risk is in microamperes, macro-shock in milliamperes.
• MUAP and interference pattern - the diagnostic content of needle EMG; an audio monitor is part of the instrument.
• Linear envelope - rectified and low-pass filtered EMG, used for prosthesis control and gait analysis.
• Indian regulation - CDSCO under the Medical Devices Rules, 2017; IEC 60601 series adopted as IS 13450 parts.

Formulas and examples

Example 1 - CMRR requirement for ECG. A patient picks up a 50 Hz common-mode voltage of 2 V. The ECG signal is 1 mV and the allowable 50 Hz artefact at the output, referred to input, is 10 µV (1% of signal).

• Required common-mode rejection ratio (as a number) = 2 V / 10 µV = 200,000.
• CMRR = 20 log10 (200,000) = 20 × 5.301 = 106 dB.
• A single op-amp differential stage with 1% resistors gives only ~45-50 dB; hence a dedicated IA plus right-leg drive.

Example 2 - ECG heart rate and interval reading. A lead II trace at 25 mm/s, 10 mm/mV shows R-R = 20 mm, QRS width 2.5 mm, R-wave height 13 mm.

• Time per mm = 1/25 = 0.04 s, so R-R = 20 × 0.04 = 0.8 s → HR = 60/0.8 = 75 bpm (check: 1500/20 = 75).
• QRS duration = 2.5 × 0.04 = 0.10 s - upper limit of normal.
• R amplitude = 13 mm / (10 mm/mV) = 1.3 mV.
• If paper speed were wrongly set to 50 mm/s and read as 25 mm/s, the computed rate would be halved - 37.5 bpm - a classic lab error.

Example 3 - Instrumentation amplifier gain and a.c. coupling for EEG. Design for a 50 µV peak EEG signal to give ±2.5 V at the ADC input, with a 0.5 Hz high-pass.

• Required gain = 2.5 V / 50 µV = 50,000. Split it: pre-amp 500, post-amp 100 (keeps the pre-amp out of saturation from electrode offset).
• Using an AD620-type pre-amp: G = 1 + 49.4 kΩ/RG = 500 → RG = 49.4 kΩ/499 = 99 Ω (use 100 Ω, G = 495).
• High-pass between the stages: fc = 1/(2πRC) = 0.5 Hz. With C = 1 µF, R = 1/(2π × 0.5 × 1×10⁻⁶) = 318 kΩ (use 330 kΩ → fc = 0.48 Hz).
• Input noise budget: for a 70 Hz bandwidth and 10 kΩ electrodes, thermal noise ≈ √(4kTRB) = √(4 × 1.38×10⁻²³ × 300 × 10⁴ × 70) ≈ 0.11 µV rms - negligible against the 50 µV signal, so amplifier noise dominates.

Example 4 - Evoked potential averaging. A visual evoked potential of 5 µV is buried in 50 µV of background EEG (SNR = 0.1, i.e. −20 dB).

• To reach SNR = 1, need √N = 10 → N = 100 sweeps.
• To reach SNR = 3 (comfortably readable), √N = 30 → N = 900 sweeps.
• At 2 stimuli per second, 900 sweeps take 450 s = 7.5 minutes - why clinical VEP runs use 100-200 sweeps per block, averaged across blocks.

Example 5 - Nerve conduction velocity and sampling. Median nerve stimulated at the wrist (distal latency 3.4 ms) and the elbow (proximal latency 7.6 ms); the two sites are 24 cm apart. EMG recorded at 10 kHz, 12-bit ADC, ±5 V full scale.

• NCV = 0.24 m / (7.6 − 3.4) ms = 0.24 / 0.0042 = 57.1 m/s (normal; median motor NCV is roughly 50-65 m/s).
• Nyquist check: for a needle EMG bandwidth of 10 kHz, fs must be ≥ 20 kHz - 10 kHz is inadequate; use 20-40 kHz or restrict bandwidth to 5 kHz by the anti-alias filter.
• ADC step q = 10 V / 2¹² = 10/4096 = 2.44 mV at the ADC; referred to the input with gain 1,000, that is 2.44 µV per LSB - fine for a 10 mV MUAP, marginal for 20 µV single-fibre work (needs more bits or more gain).

Where it is used

• Medical device manufacturing and R&D: BPL Medical Technologies, Schiller Healthcare India, Allengers (Chandigarh), RMS (Chandigarh), Nihon Kohden, Philips Innovation Campus and GE Healthcare (Bengaluru) - analog front-end design, firmware, electrode and cable engineering, type testing to IEC 60601.
• Hospital biomedical engineering departments: AIIMS, PGIMER, CMC Vellore, Apollo, Fortis - installation, preventive maintenance, electrical safety testing (leakage current, defibrillator sync), calibration against NABL-traceable simulators, NABH documentation.
• Diagnostics and neurophysiology labs: 12-lead and Holter ECG, EEG for epilepsy and sleep studies, EMG/NCS for carpal tunnel syndrome, neuropathies and motor neuron disease; polysomnography.
• Rehabilitation and assistive devices: myoelectric prostheses and orthoses (surface EMG control), biofeedback, gait laboratories; work at SCTIMST Thiruvananthapuram, IIT Madras TTK Centre, ALIMCO.
• Tele-health and wearables: tele-ECG for STEMI triage under Ayushman Bharat and eSanjeevani, single-lead patch ECG and smartwatch ECG startups, ambulatory EEG; ICMR/DST-funded projects.
• Research and BCI: EEG-based brain-computer interfaces and neuro-ergonomics at IITs, IISc, NIMHANS, NBRC; sports and occupational EMG for ergonomics assessment.

Interview questions

• Q: Why does an ECG machine need a CMRR above 100 dB? A: The mains-induced common-mode voltage on the body can be volts, while the signal is about a millivolt; rejecting the interference to under 1% of the signal needs roughly 10⁵ rejection, i.e. about 100 dB.
• Q: Why is the right leg driven rather than earthed? A: Driven right leg feeds the inverted common-mode signal back to the body, actively cancelling interference and improving effective CMRR, while keeping the patient floating so fault current is limited.
• Q: How are 12 leads obtained from only 10 electrodes? A: Four limb electrodes give three bipolar (I, II, III) and three augmented unipolar leads (aVR, aVL, aVF) by arithmetic combination, and six chest electrodes give V1-V6 against the Wilson central terminal.
• Q: State Einthoven's law and its practical use. A: Lead II = lead I + lead III; if the recorded leads do not satisfy it, an electrode is misplaced, reversed or disconnected.
• Q: Why is the ECG high-pass cut-off as low as 0.05 Hz? A: The ST segment and T wave carry near-d.c. information, so a higher cut-off would distort ST elevation or depression and could mislead infarction diagnosis.
• Q: Why is a 50 Hz notch filter discouraged in diagnostic ECG? A: The notch has a long impulse response and attenuates genuine QRS spectral content, causing ringing and amplitude distortion; good shielding, DRL and electrode preparation are preferred.
• Q: What is the 10-20 system? A: The standard EEG electrode placement scheme in which positions are set at 10% and 20% of the nasion-inion and preauricular distances, labelled Fp, F, C, P, O, T with odd for left, even for right and z for midline.
• Q: How is a 5 µV evoked potential recovered from 50 µV EEG? A: By averaging many stimulus-locked sweeps - the time-locked response adds coherently while random EEG partially cancels, so SNR improves as √N.
• Q: Why is EMG bandwidth so much wider than EEG? A: Muscle fibre action potentials are brief (a few milliseconds) spikes with energy up to several kilohertz, especially with needle electrodes, whereas EEG is a filtered, spatially summed signal dominated by rhythms below 70 Hz.
• Q: What does a Type CF applied part mean and what is its leakage limit? A: It is a floating applied part safe for direct cardiac connection; patient leakage current is limited to 10 µA in normal condition and 50 µA in single-fault condition under IEC 60601-1.
• Q: How is an ECG amplifier protected during defibrillation? A: Series current-limiting resistors with voltage clamps - neon lamps, gas discharge tubes or back-to-back diodes - divert and limit the 5 kV pulse, and the machine must recover within the time the standard allows.
• Q: If a baseline drifts slowly and the trace leaves the paper, what do you check first? A: Electrode contact and skin preparation - a drying gel, loose electrode or sweat creates large low-frequency offset; then check the high-pass/baseline-restore setting and patient movement or respiration.
• Q: A sampling rate of 500 Hz is used for ECG but 1,000 Hz for pacemaker-patient ECG - why? A: Narrow pacemaker spikes are only a fraction of a millisecond wide and would be missed or aliased at the lower rate, so a higher sampling rate and a dedicated pace-pulse detector are used.

Common mistakes

• Confusing amplitude scales - writing EEG in millivolts or ECG in microvolts, which wrecks the gain calculation.
• Using a plain op-amp difference amplifier for biopotentials: its input impedance is low and resistor mismatch limits CMRR to about 50 dB.
• Earthing the right leg in a block diagram instead of showing the driven-right-leg feedback, and omitting the isolation barrier altogether.
• Reading ECG intervals without first checking paper speed and the 1 mV calibration pulse, or mixing 25 mm/s with 50 mm/s.
• Applying a 50 Hz notch and a narrow 0.5-40 Hz monitoring band to a diagnostic ECG, then interpreting the distorted ST segment.
• Setting the anti-alias filter above fs/2, or sampling needle EMG at 10 kHz when its bandwidth is 10 kHz.
• Mixing up aVR/aVL/aVF derivation with Wilson's unipolar leads, and forgetting that augmented leads are 1.5 times larger.
• Quoting leakage current limits for Type B or BF when the applied part is Type CF, or stating micro-shock thresholds in milliamperes.

Related topics

Bioelectric signals and electrodes · Signal conditioning and data acquisition · Biomedical signal processing · Digital signal processing: filters and FFT · Patient monitoring and life-support equipment · Medical device safety standards and electrical safety testing