How it works
The INA128 instrumentation amplifier amplifies the differential signal from a bridge or thermocouple while rejecting common-mode noise; gain G = 1 + 50kΩ/RG, so a 51.1 Ω resistor gives G ≈ 980. A first-order low-pass RC filter (R = 10 kΩ, C = 100 nF) cuts off at fc = 1/(2πRC) = 159 Hz, removing 50 Hz interference and high-frequency noise. For industrial applications, galvanic isolation using an optocoupler (4N25) or isolation amplifier (ISO124) breaks ground loops that would otherwise inject millivolts of error into a microvolt-level signal.
Key points to remember
Signal conditioning blocks in order: sensor → amplifier → filter → isolation → ADC. Anti-aliasing filter must have fc < fs/2 (Nyquist) before the ADC input. The CMRR of an instrumentation amplifier should exceed 80 dB at 50 Hz to reject power-line interference; the INA128 achieves 120 dB CMRR. Offset voltage and drift are critical for DC measurements — an OP07 has Vos < 75 µV and drift < 1.3 µV/°C. Sample-and-hold circuits freeze the analogue value during the ADC conversion period to prevent aperture error on fast-changing signals.
Exam tip
Every Anna University instrumentation paper has a question on the INA128 gain formula — write G = 1 + 50k/RG and calculate RG for a required gain, since examiners often give the gain and ask for the resistor value.