Sensor Interfacing

NTC linearization — Adding Rp in parallel pulls the steep NTC tail down and straightens the transfer curve around T0. The price is reduced sensitivity and a resistance offset — accept it in firmware calibration.

Noise floor — The sensor, its interface resistance and the amplifier bandwidth together set the minimum detectable signal. Widening bandwidth or gain also multiplies noise — trade them against the required resolution and response speed.

Parallel-plate sensor — Capacitance C = ε₀·εr·A/d changes with the electrode overlap A, the gap d or the dielectric constant εr. Read it as an RC time, an oscillator frequency or a bridge imbalance — measuring a change rather than an absolute value makes stray capacitance far less important.

Sensor signal chain — Excitation energizes the element, conditioning turns its small change into a clean voltage, the amplifier scales it and an anti-alias filter rejects out-of-band noise before the ADC. Design the chain end to end — gain, offset, and filter each decide how many counts map to the measurand.

Wheatstone bridge — Four arms, one of which changes with the measurand (strain, pressure). Balanced arms give zero differential output; imbalance produces Vdiff proportional to ΔR/R, amplified by an instrumentation amp.

3-wire RTD — Excitation and both sense lines share one lead, so its resistance drops out of the measurement. This removes lead error without needing the more expensive four-wire connection.

Photodiode TIA — The photocurrent flows through Rf, and the op-amp holds the photodiode at zero reverse bias, converting Iph to a clean voltage Vout = Iph·Rf with Rf/Cf setting the bandwidth.

Thermistor divider — Rth and R1 form a voltage divider whose output feeds the ADC. A small capacitor across the ADC pin plus a series resistor filters sampling glitches and RF noise.

Thermocouple & cold junction — Two dissimilar metals join at the hot junction. The volt-µV signal is the difference between hot and the cold (reference) junction on the board; an on-board temperature sensor feeds the cold-junction compensation so the reading reflects T1 alone.

Load-cell bridge to ADC — Excite the strain-gauge bridge, amplify its small differential signal with an instrumentation amplifier, then convert with a delta-sigma ADC. The gain and ADC resolution together set how many counts one bit of load maps to.

4-wire Kelvin & self-heat — Two wires force the current, two more sense the voltage so lead resistance drops out of the reading. The bias current still heats the element by I²·R·θ, so keep excitation low and pulse it to trim the self-heating error.

Ratiometric excitation & ADC — A bridge is usually excited from the same supply that later becomes the ADC reference. Both the bridge output and the reference scale with the supply, so a slow droop or ripple on Vexc cancels out of the final code — the reading stays proportional to ΔR/R, not to the supply rail, for less sensitivity to noise on the power line.

Hall switch & bias — A current-biased Hall plate develops a small voltage proportional to the perpendicular field (Vh ∝ B·I). An amplifier raises it, then a comparator with hysteresis converts the smooth analogue field into a clean digital on/off as the magnet sweeps by — the hysteresis band stops contact bounce and threshold chatter from stuttering the output.