The rail is never as
quiet as the schematic says
Notes and measurements from the awkward space between firmware and copper, plus the calculators that came out of re-deriving the same equations once too often. Every tool states the standard it implements and the assumptions it makes, so you can check it instead of trusting it.
Tools
- Trace widthHow wide a trace has to be to carry a current without cooking.IPC-2221
- Resistor decoderColour bands and SMD codes, with the nearest preferred values.IEC 60062 / 60063
- RC filterCorner frequency, time constant, and the rise time you get with it.First-order passive
- Microstrip impedanceSingle-ended and differential impedance, and propagation delay.IPC-2141
- Voltage dividerOutput, current and dissipation, with the nearest real part.Ohm's law
- Baud rate errorEvery standard rate at your clock, and which ones the UART can hold.Integer-divisor UART
- ViaCurrent, resistance and thermal resistance of a plated hole.IPC-2221 / IPC-6012
- I²C pull-upThe resistor window between sink current and rise time.NXP UM10204
- Crystal loadC1 and C2 for a Pierce oscillator once stray capacitance is counted.TI SWRA372; ST AN2867
- LED resistorSeries resistor, standard value, power, and the worst-case current.Ohm's law
- Capacitor codeThree-digit codes, tolerance letters, voltage prefixes; reactance and energy.IEC 60062 / EIA-198
- Battery runtimeLife from a sleep/wake profile, with self-discharge and cell presets.Time-weighted average
- DecouplingA real capacitor across frequency, the PDN target, and how many parts hold it.Series RLC / target impedance
- RS-485 busWhether to terminate, how long stubs can be, and the failsafe bias resistors.TIA-485 / TI SLLA272
- Ferrite bead filterAttenuation across frequency — and the resonance where a bead amplifies.Würth ANP045 / TI SLYT740
- 555 timerAstable frequency and duty, or one-shot width — and the resistors for a target.TI LM555 datasheet
- E-series finderThe closest stocked value, pair or divider — and what each one misses by.IEC 60063
- Wire gaugeDrop and resistance from the AWG definition, plus bundle derating and the thermal budget.AWG · NASA NESC
- RC snubberRinging frequency in, loop inductance and the snubber that damps it out.TI SLYT465
- ADC charge bucketWhether the SAR front end settles to half an LSB before the window shuts.TI SBAA286
- Op-amp DC errorOffset, drift and bias currents through the real network, worst-case.TI SBOA590
- TVS clampingWhether the ESD diode’s clamp actually lands under what the pin tolerates.TI SLLA305
- Crystal ppm budgetTolerance, stability and aging stacked against what the interface allows.TI SLLA122
- Fusing currentThe current that melts a trace in a given fault time, or the width that survives.Onderdonk (estimate)
- LDO thermalDissipation, junction temperature, and the copper a linear regulator needs.TI AN-2020
- Buck rippleInductor ripple current and the exact output ripple across a real capacitor.TI SLVA630 / SNVA559
- Buck inductorThe inductance for a ripple target at the worst-case input, then the peak against saturation, the RMS against heating, and the copper loss.TI SNVA559 / SLUP123
- Boost converterSLVA372's power stage: duty cycle with efficiency, inductor and ripple, the peak switch current, what the IC's limit allows, and the diode and output capacitor.TI SLVA372
- Capacitor charge timeThe RC time constant, the voltage at any moment while charging or discharging, the time to a threshold, and the current and energy on the way.Würth SN015
- Transformer turns ratioSecondary voltage, current and reflected impedance from equal volts per turn, and the primary turns a core needs for its flux swing.TI SLUP123
- Heat sinkThe sink rating a junction limit allows from SPRA953's chain, the interface from pad thickness and conductivity, and the junction temperature with the sink in hand.TI SPRA953
- Capacitance converterAny capacitance in pF, nF, µF and F at once, with the marking code a part of that value carries and the SI prefix rules behind the conversion.BIPM SI Brochure
- ReactanceCapacitive and inductive reactance at any frequency, or the frequency at which a part reaches a given reactance, with ESR, ESL and self-resonance.Würth ANP109 / ADI MT-101
- LC resonanceResonant frequency of an inductor and capacitor, or the L or C for a target frequency, with the characteristic impedance, Q and the filter's resonant peak.Würth ANP062 / TI SNVA871
- LM317Output voltage from R1 and R2 with the adjust-pin current, or R2 for a target voltage, with the headroom, dissipation and reference tolerance.TI LM317 datasheet
- Op amp gainInverting and non-inverting gain from the two resistors, or the resistor for a target gain, with the output swing and bandwidth it leaves.TI SBOA092 / SLOA011
- dB and dBmWatts to dBm and back, power and voltage ratios in dB, dBV and dBµV, with the reference each figure is quoted against.BIPM SI Brochure / Infineon AP24026
- PWM to voltageThe DC level a filtered PWM output settles to, the ripple a first- or second-order RC filter leaves, and the settling time it costs.TI SPRAA88
- Supercapacitor backupThe capacitance a power backup needs, the constant-power hold-up time between two voltages, and the ESR drop at the first instant.Abracon, Designing with Supercapacitors
- Transistor base resistorThe base resistor that drives a BJT switch into saturation at the worst-case gain, and the base current, V_CE(sat) and dissipation it gives.Nexperia BJT Application Handbook
- Smoothing capacitorThe reservoir capacitor after a bridge or half-wave rectifier: capacitance for a valley voltage, or the ripple a given capacitor leaves, with hold-up and voltage rating.TI UCC28740, TIDUB51
- Coplanar waveguideImpedance and effective permittivity of a coplanar waveguide with or without a ground plane below, with copper thickness, and the width for a target impedance.Sandia SAND2017-12410J
- Comparator hysteresisThe resistors that set the upper and lower thresholds of a comparator, inverting or non-inverting, and the hysteresis they give.TI SBOA313, SNOA997
- MOSFET power lossConduction, switching and gate-drive loss of a MOSFET in a buck converter from its datasheet values, and the junction temperature they lead to.TI SLYT664
- Transimpedance amplifierThe feedback resistor and capacitor of a photodiode amplifier for an output swing and bandwidth, and the op amp bandwidth it needs to stay stable.TI SBOA268, SBOA122
- Slew rateThe slew rate a sine or step output needs, the full-power bandwidth a given slew rate allows, and the rise time it gives.TI SLOA011
- Johnson noiseThe thermal noise of a resistor over a bandwidth, shot noise, and how op amp voltage and current noise add to it at the output.TI SLVA043
- Buck-boost converterDuty cycle, inductor and currents of a 4-switch buck-boost converter in both buck and boost modes, sized for the worst of the two.TI SLVA535
- Air-core inductorThe inductance of a single-layer air-core coil from its diameter, length and turns, or the turns for a target inductance, with the Nagaoka correction for short coils.NBS Scientific Paper 169
- ThermocoupleThermocouple voltage to temperature and back for type K and the other letter types, with the cold-junction compensation, from the NIST ITS-90 reference functions.NIST Monograph 175
- PT100 RTDPT100 and PT1000 resistance to temperature and back from the Callendar-Van Dusen equation, with tolerance class and the error of lead resistance.TI SBAA275, IEC 60751
- 4–20 mA scalingScale a 4–20 mA loop to engineering units and back, the voltage across a sense resistor, and the loop supply a transmitter needs.TI SLAA866, SLYT847
- Wheatstone bridgeBridge output voltage for a resistance change, quarter, half and full bridges, strain gauge and load cell mV/V, and the nonlinearity of each.TI SLOA034
- Attenuator padResistor values for a matched pi, T or bridged-T attenuator at any attenuation and impedance, and the minimum-loss pad between two impedances.Skyworks 200312E, Maxim AN972
- VSWR and return lossConvert between reflection coefficient, VSWR, return loss and mismatch loss, from a load impedance or a measurement, with the mismatch uncertainty of two ports.Keysight AN 1449-3, Huber+Suhner
- Coax impedanceCharacteristic impedance of a coaxial line from its conductor diameters and dielectric, with velocity factor, capacitance per metre and the TE11 cutoff.Huber+Suhner, R&S 1MA99
- Skin depthSkin depth in copper and other metals at a frequency, and the AC resistance of a round wire or a PCB trace it leads to.TI SLUP197, NBS SP 169
- Noise figureNoise figure, noise factor and noise temperature, and the Friis cascade of amplifiers, cables and attenuators into a system noise figure.Keysight AN 57-1
- Instrumentation amplifier gainThe gain resistor for an instrumentation amplifier gain, or the gain a resistor gives, for common in-amps, with the nearest 1 % value, drift and the common-mode limit.TI INA128, ADI AD620
- PLL loop filterPassive second- and third-order PLL loop filter components for a loop bandwidth and phase margin, from the charge-pump gain, VCO gain and divide ratio.TI SNAA106C (Banerjee)
- Op-amp integratorAn op-amp integrator or differentiator: the 0 dB frequency, the gain-limiting resistor, the usable band and the op-amp bandwidth it needs.TI SBOA275, SBOA276
- RC oscillatorFrequency and gain condition of a Wien-bridge or phase-shift oscillator from its resistors and capacitors, or the parts for a target frequency.TI SLOA060, AN-31
- Summing amplifierOutput of an inverting or non-inverting op-amp summer from its inputs and resistors, and the resistors for a weighted sum and output range.TI SBOA272, AN-31
- AliasingWhere a signal above half the sample rate lands after sampling, its Nyquist zone, and the antialiasing filter needed to keep it out.ADI MT-002
- Charge pumpOutput voltage, ripple and efficiency of a switched-capacitor doubler or inverter from its output resistance, capacitors and switching frequency.TI LM2662
- Constant current sourceSense resistor, compliance voltage and dissipation of an op-amp or LM317 constant current source, and the Howland current pump.TI SBOA325, SBOA436, LM317
- TL431 shunt regulatorOutput voltage of a TL431 from its divider, the divider for a target voltage, and the cathode resistor that keeps it in regulation across the load.TI TL431 datasheet
- RF link budgetFree-space path loss, EIRP and received power for a radio link from frequency, distance, transmit power and antenna gains, and the range for a given sensitivity.ITU-R P.525
- Impedance matchingL-network and pi-network component values to match a source to a load at one frequency, with the loaded Q and bandwidth they give.Silicon Labs AN1275
- Toroid inductorTurns on a ferrite or powder toroid for a target inductance from its AL value, and the inductance a given number of turns gives.TDK (EPCOS) ferrites
- Capacitor ESRESR from the dissipation factor of a capacitor and back, the heat a ripple current generates, and impedance across frequency.Vishay, TDK, KEMET
- Wire inductanceSelf-inductance of a straight round wire, a flat strip or PCB trace, and a pair of parallel wires, from the classic Bureau of Standards formulas.NBS Scientific Paper 169
- Mutual inductanceMutual inductance and coupling factor between two parallel wires or two coaxial circular loops.NBS Scientific Paper 169
- Critical lengthHow far a trace can run unterminated before reflections outlive the edge.TI AN-807 / IPC-2141
- Active filterSallen-Key or MFB second-order low-pass from fc, shape and one capacitor.TI SLOA049
- Gate resistorSeries gate resistor from the turn-on dv/dt wanted, the dv/dt the FET survives held off, and where the drive power goes.TI SLUA618 / SLUP170
- ADC noise floorQuantization noise, SNR, process gain, the FFT floor, SINAD, ENOB and the jitter limit.ADI MT-001 / MT-003
- Current dividerHow a current splits between parallel branches, and the resistor that sets one branch.Kirchhoff / Ohm's law
- Relay coil suppressionFlyback diode, diode + Zener or resistor across a coil: switch voltage, diode ratings, release delay.TE 13C3264 / Omron
- Inrush current limiterNTC cold resistance, energy and steady-state ratings from the peak the fuse allows; or a MOSFET soft start.Ametherm 24002 / TDK / TI SLVA156
- Switch debounceRC debounce into a Schmitt input: press and release delays against the bounce time, with real threshold limits.Würth SN015 / SN74HC14
- LDO dropoutDatasheet dropout scaled to the real load, headroom at the input floor, dissipation and efficiency across the range.TI SLVA207 / SLVA079
- Pull-up / pull-downThe resistor range from leakage and the reader's threshold at the top and the driver's I_OL at the bottom, with edge time and power.TI SLVA485 / SDAA246
- CAN bus terminationTermination, node count, length against bit rate, capacitance budget and stub limit from TI SLLA486 and SLLA270.TI SLLA486 / SLLA270
- MOSFET level shifterAN10441's one-FET bidirectional shifter checked state by state: gate drive, body diode, sink current, low levels, rise times.Nexperia AN10441 / UM10204
- Probe loadingGround-lead ring frequency against the scope bandwidth, tip loading on the node, and the rise time the screen shows.Tektronix ABCs of Probes
- Op amp capacitive loadThe pole a load capacitor makes with R_O, the phase margin it leaves, and SBOA418's optimal isolation resistor with margin, overshoot and drop.TI SBOA418 / SBOA626
- Bypass placementThe inductance a capacitor's lands, traces and vias add, the resonance that leaves it with, and XAPP623's λ/40 placement radius against where it sits.Xilinx XAPP623 / TI SLOA069
- Exposed pad stencilThe windowed stencil for a QFN or PowerPAD thermal pad: window size, IPC-7525 ratios, paste volume, via array, and what voiding costs in θJA.TI SLUA271 / SNVA183
- ADC / DAC resolutionBits to volts: the LSB, SLAA013's four static errors in volts and % FSR, the total and the bits it leaves, quantization SNR, aperture limit and Nyquist rate.TI SLAA013
- Bootstrap capacitorSLUA887's two sizes for a high-side driver's bootstrap capacitor, the droop per cycle against UVLO, the bypass, the diode peak and the recharge window.TI SLUA887
- Current sense resistorValue from current and sense voltage, power and self-heating, TCR at the element temperature, and the copper a two-terminal connection puts in the reading.TI SBOA197 / Vishay 30405
- Reverse polaritySchottky, P-channel MOSFET or ideal diode: drop, power, loss and heat at your current, the ideal diode's crossover, the gate checks, and the hot diode's reverse leakage.TI SLVAE57B
- Li-ion chargeFast-charge, precharge and termination currents from capacity and C-rate, the ±50 mV window, an estimate of the CC and CV times, and a linear charger's heat.TI SLAA287 / SSZTA38
- NTC thermistorR at temperature from R25 and B, the tolerance butterfly as degrees, the divider's output and sensitivity, and the self-heating against Vishay's 50 % budget.TDK / Vishay NTC notes
- Flyback transformerSNVA866's CCM design: turns for a target duty cycle, primary inductance for a ripple ratio, ripple and peak current, reflected voltage and the drain and rectifier stresses.TI SNVA866 / SLUP254