What is signal conditioning, and why is it needed before the ADC input?

Signal conditioning is the analog processing chain between a raw sensor output and the ADC input pin. It transforms the sensor signal into a form the ADC can accurately digitize. Without proper conditioning, even a perfect ADC produces garbage results because the raw signal violates one or more of the ADC's input requirements.

Voltage scaling and level shifting: Many sensors produce signals outside the ADC's input range (0 to V_REF). A 0-10V industrial sensor needs a resistive voltage divider to scale it to 0-3.3V. A thermocouple producing +/- 50 mV needs both amplification (gain of 60x to fill the ADC range) and level shifting (add a DC offset so the negative voltages map to positive ADC input). A 4-20 mA current loop sensor needs a precision sense resistor (e.g., 165 ohm for a 0.66-3.3V output) to convert current to voltage. Getting the scaling wrong can either clip the signal (losing data at the extremes) or under-utilize the ADC range (wasting resolution on unused voltage span).

Anti-aliasing filtering: A low-pass filter (passive RC or active with an op-amp) removes high-frequency noise and prevents aliasing, as required by the Nyquist theorem. The filter cutoff should be set below f_sample / 2, with enough attenuation at the Nyquist frequency to suppress aliases below 1 LSB.

Impedance matching: The ADC's sample-and-hold capacitor (typically 5-20 pF on STM32) must charge fully during the sampling window. If the source impedance is too high, the capacitor cannot settle to the correct voltage in time, causing measurement errors that worsen at higher sample rates. The STM32 datasheet specifies a maximum recommended source impedance (typically 10-50 kohm, depending on sampling time configuration). High-impedance sensors like thermistors or piezo elements need a buffer op-amp (unity-gain follower) to present a low-impedance source to the ADC. This is one of the most commonly overlooked design errors in student projects — the ADC "sort of works" but readings drift or depend on sample rate because the source impedance is marginal.