Decoupling caps where the datasheet actually wants them

Read the power-supply section first

Every IC datasheet has a power-supply paragraph that lists recommended decoupling. Most people skim it, place a 100nF near the chip, and call it done. That works until your microcontroller resets during a motor pulse or your ADC reads noise instead of signal.

The datasheet tells you what the silicon needs. A microcontroller might want 100nF close plus 10µF bulk a few millimeters away. An op-amp might specify 1µF ceramic right at the pins. A switching regulator usually asks for low-ESR electrolytics at specific points in the feedback loop. These aren't suggestions—they're the conditions under which the chip was characterized.

Ignore them and you're testing a different circuit than the one the vendor validated.

Placement matters more than value

A 100nF capacitor six inches from the power pin is a 100nF capacitor plus several nanohenries of trace inductance. At high frequencies that inductance dominates and your bypass stops bypassing. The current loop from the IC, through the cap, and back to ground needs to be tight.

I place decoupling caps on the same side of the board as the IC, as close to the power pin as routing allows. If I have to choose between a slightly longer trace to the cap or a longer trace in the return path, I shorten the return. Ground matters just as much as VCC.

For multi-rail chips—anything with separate analog and digital supplies—each rail gets its own local decoupling. Don't share capacitors between AVCC and DVCC even if they're both 3.3V. The whole point is to isolate the noise each section generates.

Bulk caps handle the slow stuff

Small ceramics respond fast but store almost no energy. When your load steps from 10mA to 200mA the local 100nF caps provide the first few microseconds of current, but after that you need bulk storage. That's where the 10µF or 47µF electrolytics come in.

I put bulk caps close to the point of load but not necessarily right at the IC pins. They handle lower-frequency droop—millisecond-scale transients—so a centimeter or two of trace is fine. What matters is that they're electrically close to both the load and the main power supply, not stuck at the edge of the board because that's where the connector landed.

If you're driving motors or solenoids from the same supply as your logic, add extra bulk right at the driver stage. A DC motor starting pulls tens of amps for a few milliseconds. Your microcontroller's 10µF cap won't save it; the motor driver needs its own reservoir.

Check the ESR for switching supplies

Switching regulators are fussier. The feedback loop depends on specific output impedance, and that means the decoupling cap's ESR matters. A low-ESR ceramic might oscillate where a slightly higher-ESR tantalum or polymer electrolytic stabilizes the loop.

The datasheet will specify this. Buck converters often call out a maximum ESR for the output cap—usually tens of milliohms. If you substitute a different capacitor type or value, simulate or measure the loop response. I've seen perfectly good boards go unstable because someone swapped a 22µF tantalum for a 22µF X7R ceramic without checking.

When in doubt, follow the reference design exactly for the first build. Once you've verified it works you can experiment with alternates.

Test under real load

Scope the power rails while the circuit does actual work. If you're reading sensors, trigger a burst of conversions. If you're running motors, start and stop them. Look for voltage sag, ringing, or high-frequency noise that wasn't there at idle.

A clean rail at 10mA tells you nothing about what happens at 200mA. If you see problems, check your decoupling first—location, value, and type. Nine times out of ten that's where the fix lives.

I keep a small assortment of ceramics and electrolytics on the bench so I can tack extras onto a prototype without spinning a new board. Sometimes the datasheet is wrong or your layout adds unexpected inductance. Having a 47µF to hand-solder onto the back of a board saves a week of turnaround.

Decoupling isn't glamorous. It's also not optional. Place the caps where the datasheet says, route the return paths short, and verify it with a scope. Your rails will stay flat and your circuits will work the first time.

For more on keeping weekend projects powered correctly, see Power the weekend like it matters.