Recipe

Size an RC filter for PWM

Use when: a PWM pin must give a nearly steady voltage, for a reference, a slow analog input, or a sensor test

Goal: a filter whose ripple is small enough and whose response is fast enough, at the highest PWM frequency available

Taught in From PWM to analog

1. 1

   Decide the largest ripple you can accept and the slowest response you can tolerate.

   Example

   Ripple under 50 mV; settle within 100 ms.

   Why: the filter is a trade, and you need both ends before you can choose.

2. 2

   Response ≈ 5τ, so τ ≤ response / 5.

   Example

   τ ≤ 100/5 = 20 ms.

   Why: a filter slower than this misses the deadline whatever its ripple.

3. 3

   Use V_H \times 0.25 \times T/\tau, the worst case.

   Example

   At 490 Hz: 5 \times 0.25 \times 2.04/20 \approx 0.13 V, too much. At 5 kHz: 5 \times 0.25 \times 0.2/20 = 12.5 mV. Done.

   • under the limit — done; choose R and C for that τ.
   • over it — raise the PWM frequency, then recheck.

   Why: raising the frequency is the only change that cuts ripple without slowing the response.

4. 4

   Pick R of a few kΩ to tens of kΩ and the C that gives τ; make sure whatever reads the output has a resistance far above R.

   Example

   τ = 20 ms: 20 kΩ and 1 μF.

   Why: a load comparable to R divides the output down.