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ADC front end: reference and anti-alias filter

Embedded (ESP32) · Advanced · about 20 minutes · part of Electronics Engineering

Sampling at 1 kHz can only represent signals below 500 Hz; anything faster (here 3 kHz interference) folds down and looks like a false slow signal. An RC low-pass in front of the ADC removes it first. A TL431 2.495 V reference on a second pin lets the sketch correct the ESP32’s inaccurate ADC.

ADC front end: reference and anti-alias filter: the circuit on the simulator's bench

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How it works

Corner f_c = 1 ÷ (2π·R·C) = 159 Hz. At 3 kHz the filter passes 1 ÷ √(1 + (f ÷ f_c)²) = 0.053 of the interference.

The TL431 regulates its cathode to 2.495 V; the ESP32’s ADC reads it a few percent off, and the sketch divides that error out.

Try it on the bench

  1. Set the interference to 100 Hz: it passes almost untouched, as a real signal should.

Parts on the bench

In the course

Electronics Engineering, module “Op-amps and feedback”.

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