Question

As compared to those employed for ECG recording, the electrodes and the preamplifier for EEG recording must have:

• A. Lower contact impedance and higher input impedance respectively
• B. Lower contact impedance and lower input impedance respectively
• C. Higher source impedance and higher input impedance respectively
• D. Higher source impedance and lower input impedance respectively

Hint:

To prevent the loading effect in any bio-potential signal conditioning chain: - Always ensure that the Input Impedance of the Preamplifier ($Z_{in}$) is significantly greater than the Source Impedance ($Z_s$). - EEG signals are roughly 100 times smaller in amplitude than ECG signals ($10\,\mu\text{V}$ vs. $1\text{ mV}$) and pass through highly resistive bone structures, requiring both a higher source impedance tolerance and a much higher preamplifier input impedance to maintain high common-mode rejection ratios (CMRR).

Answer 1

The Correct Option is C

Concept: Biomedical recording systems like Electrocardiography (ECG) and Electroencephalography (EEG) capture distinct physiological electrical activities from the human body.
• ECG (Electrocardiogram): Measures the electrical activity of the heart muscles. The signal amplitude typically ranges between $0.5\text{ mV}$ and $4\text{ mV}$, which is relatively large compared to other bioelectric potentials.
• EEG (Electroencephalogram): Measures the postsynaptic potentials of cortical neurons in the brain across the scalp surface. Due to attenuation through the cerebrospinal fluid (CSF), meninges, skull bone, and skin, the signals reaching the surface are incredibly faint, typically ranging between $10\,\mu\text{V}$ and $100\,\mu\text{V}$. Because of the highly resistive path through the skull and the small surface area of scalp electrodes, the source or skin-electrode contact impedance for EEG is inherently higher than that of ECG. To capture these minuscule, high-impedance voltage signals without loading effects or severe degradation, instrumentation preamplifiers must satisfy strict architectural constraints.

Step 1: Analyzing the Source/Contact Impedance of EEG vs. ECG.
ECG electrodes are placed on limbs or the chest wall, directly over soft, well-vascularized tissue or skin prepared with conductive gel. This layout yields a lower skin-electrode contact impedance. In contrast, EEG electrodes are applied to the human scalp, which contains a high density of hair follicles and relies on conduction through the thick, poorly-conductive bone of the skull. Therefore, the source impedance ($Z_s$) presented by the biological system to an EEG measurement setup is significantly higher than the source impedance found in ECG recording systems.

Step 2: Assessing the input impedance requirement of the preamplifier.
When a measurement system reads a voltage signal from a high-impedance source, it sets up a voltage divider configuration between the source impedance ($Z_s$) and the amplifier's internal input impedance ($Z_{in}$). The actual voltage sensed by the preamplifier ($V_{amp}$) is given by: \[ V_{amp} = V_{bio} \times \left( \frac{Z_{in}}{Z_s + Z_{in}} \right) \] To avoid attenuation and distortion caused by the voltage-divider loading effect, the input impedance of the preamplifier must be orders of magnitude larger than the source impedance: \[ Z_{in} \gg Z_s \quad \Rightarrow \quad \frac{Z_{in}}{Z_s + Z_{in}} \approx 1 \] Since the source/contact impedance of an EEG configuration is fundamentally higher than that of an ECG configuration, the EEG preamplifier must possess an exceptionally higher input impedance to guarantee optimal signal fidelity and prevent signal loading. Thus, the electrodes and preamplifier for EEG recording must have a higher source impedance and a higher input impedance respectively compared to an ECG setup. This matches Option (C).