November 25, 2025
Laplace PTSD vs RC Zen
The 101 of Analog Signal Filtering
Engineers cheer a no-calculus filter explainer — purists say “read the app notes”
TLDR: A friendly guide explains how simple parts tame signals without drowning readers in calculus. Comments split between relief at the plain-English approach and calls for “real” engineering—use app notes, simulate in LTSpice, and dive into Sallen–Key—showing the appetite for both approachable intros and deeper follow-ups.
A blog breaks down analog filtering—the way electronics smooth out messy signals—using a friendly resistor-and-capacitor story (think faucet-and-bucket) instead of scary math. The crowd went from Laplace PTSD to RC Zen in seconds. One commenter admitted, “Learning calculus in mid-life is tough,” while others begged, “please keep it simple.” Meanwhile, the purists rolled in, waving manuals: “TI App Notes and LTSpice are your friends,” invoking legend Bob Pease like an EE patron saint.
Fans loved the demo: constant current makes a straight-line rise; constant voltage makes that classic curve. No heavy jargon, just vibes. “rc filtering is rarely explained this intuitively,” cheered one reader, sounding like they’d just escaped a textbook. But drama brewed: the No-Math Squad vs. the Real Engineers Club. In the “more please” camp, a reader asked for a deeper cut on Sallen–Key filters, linking a diagram and basically saying, “okay, now level us up” (image).
Meta-mod dang dropped the HN thread like a receipt, proving this topic always stirs feelings. Verdict: a rare post that makes signal filtering feel like a story instead of a struggle, with the comments serving equal parts therapy, tool wars, and “teach me more.”
Key Points
- •Analog signal filters are common across electronics, but typical explanations rely on Laplace transforms and transfer functions.
- •A simple RC circuit under constant-voltage charging shows initial current I = V/R decaying as the capacitor voltage rises to the supply level.
- •The resistor’s voltage drop reduces current over time per Ohm’s law, creating an exponential-like charging behavior.
- •Under constant-current charging, the capacitor voltage increases linearly with time, following Vcap(t) = (Isupply · t)/C.
- •Examples include a 100 µF capacitor through a 10 kΩ resistor at 48 V, and a ~600 µA constant-current case producing a straight-line voltage ramp.