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Signal conditioning

Raw sensor signals are often tiny or noisy. Conditioning amplifies and filters them so the ADC can measure them well.

Core

Read firstWhat a sensor actually isAnalog-to-digital conversion
From a strain gauge to the ADC: bridge, amplifier, filter A strain gauge sits in a Wheatstone bridge, which turns a tiny change in resistance into a small differential voltage. An amplifier raises it, and a low-pass filter removes frequencies above half the sampling rate before the ADC converts it. Wheatstone bridge 5 V excitation 0 V strain gauge millivolts gain Amplifier Low-pass filter ADC Tiny signal A tiny resistance change becomes a small voltage centred near zero. Load cells: 1 to 3 mV/V 5 V excitation gives about 5 to 15 mV at full load Amplify first an instrumentation amplifier, or the ADC's programmable gain, before conversion Anti-aliasing filter Cuts frequencies above half the sampling rate. Left in, they fold back as false lower-frequency signals.
A Wheatstone bridge turns a strain gauge's tiny resistance change into millivolts, which are amplified and low-pass filtered before the ADC.

Key points

  1. Resistive sensors such as strain gauges are usually wired as a Wheatstone bridge, which turns a tiny change in resistance into a small differential voltage centred near zero.1
  2. Load cells specify rated output in millivolts per volt of excitation, typically 1 to 3 mV/V, so a 5 V excitation gives only about 5 to 15 mV at full load, which needs amplification (an instrumentation amplifier or the ADC's programmable gain) before conversion.1
  3. An analog low-pass (anti-aliasing) filter before the ADC attenuates frequencies above half the sampling rate, which would otherwise fold back into the measurement as false lower-frequency signals.2

Where to go next

Sources

  1. A Basic Guide to Bridge Measurements (SBAA532A) · Texas Instruments · Reference
  2. TI Precision Labs: Aliasing and Anti-aliasing Filters (TIPL 4304) slides · Texas Instruments · Reference