Near Field Notes

A field guide to circuit boards

See the one difference that makes a board work

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Step 1 of 2 · What is the board doing wrong?

Families

6 families · 27 notes
Note 4 · Gate ringing

Voltage on the FET

Fig. 4cV0488080 V rating0100200 ns1 A gate drive60 mAFig. 4cV0488080 V rating0100200 ns1 A gate drive60 mAFig. 4cV0488080 V rating0100200 ns1 A gate drive60 mA
  • 1 A
  • 60 mA

10 notes

Power

A FET rings, runs hot or a rail misbehaves

  • Note 4Gate ringing

    How hard should you drive a half bridge?

    DRV8323 · TPN19008QM · DRV8302 · moteus-c1

    81 Von an 80 V FET at full drive

  • Note 12Power inductors

    Does it matter how a 4.7 µH inductor is built?

    CD43NP-4R7MC · SRN4018BTA-4R7M · SRR4018-4R7Y · 2 more parts

    7.5 → 0.83 mVon a trace 1 mm from a 4.7 µH inductor, unshielded drum to molded powder

  • Note 13Capacitors

    Which kind of capacitor should feed a load?

    GRM21BR61E226ME44L · T520B107M006ATE040 · EEE-FK1C221P · 3 more parts

    1.1 s → 40 hsix parts that store charge, each running the same 1 mW load

  • Note 18LiPo vs LiFePO4

    How does a LiFePO4 cell differ from a LiPo?

    ANR26650M1 · ANR26650M1-B · LF280K

    42–71% vs 47–53%a half-full LiFe and LiPo cell read with the same 10 mV meter error

  • Note 22SIMO converters

    Can one inductor feed three rails?

    MAX77654 · TPS62840 · TPS7A02

    8.6 h in 61% of the areaa watch's three rails from one 300 mAh cell: one SIMO against three bucks at 8.8 h and three LDOs at 4.3 h

All 10 Power notes →

Note 5 · Shunt taps

What the taps read

Fig. 5cWhat 10 A reads as, A1012141601234567ShuntPadCopper10.1 A10.9 A14.1 AEach tap’s distance from the shunt’s center, mmFig. 5cWhat 10 A reads as, A1012141601234567ShuntPadCopper10.1 A10.9 A14.1 AEach tap’s distance from the shunt’s center, mmFig. 5cWhat 10 A reads as, A1012141601234567ShuntPadCopper10.1 A10.9 A14.1 AEach tap’s distance from the shunt’s center, mm

2 notes

Sensing

A current or voltage reads wrong

  • Note 5Shunt taps

    Where do you tap a 0.5 mΩ shunt?

    WSLF2512 · WSK2512 · moteus-x1

    14.1 Awhat 10 A reads as with the taps 2 mm out

  • Note 19Sense position

    Low side, high side or in line: where should a motor drive sense current?

    DRV8323 · INA240 · moteus-c1

    43.7 of 48 Vthe most the motor gets with all three low-side shunts read in a 3 µs window at 30 kHz
Note 1 · Ground cut

Solid ground and a pad-sized hole

Fig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedanceFig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedanceFig. 1b2 mm1 · Solid ground−5.6% impedance2 · Pad-sized hole−0.5% impedance

10 notes

Signal integrity

A fast signal fails at a connector

  • Note 1Ground cut

    Should you cut ground under connector pads?

    USB-C · HDMI · 0402

    −5.6% → −0.5%impedance error at the pads

  • Note 24I3C vs I2C

    What is I3C, and how does it differ from I2C?

    ICM-42688-P · BMI323 · SPD5118 · 2 more parts

    344 → 13.1 µsfor the same 12-byte read, I2C at 400 kHz to I3C

  • Note 26Line codes

    Why do fast serial links use line codes like 8b/10b?

    600 MB/s, not 750SATA’s 6 Gb/s after 8b/10b

  • Note 27Car camera coax

    How does one coax carry a car camera’s video, control and power?

    MAX96717 · MAX96724 · PFL1609-471 · 3 more parts · Antmicro GMSL Serializer · Antmicro GMSL Deserializer

    86% vs 36%of ADI’s band at 1 kΩ, four coils vs one

  • Note 28CRCs on fast links

    How does a CRC catch bad bits, and what do fast links do when it fails?

    1 in 4.06 billion4-bit errors that pass CRC-32 in a 1518-byte frame

All 10 Signal integrity notes →

Note 6 · Spread spectrum

The drop at each harmonic

Fig. 6eDrop, dB5101500.41 MHz10301 dB10 dB12 dBFig. 6eDrop, dB5101500.411030 MHz1 dB10 dB12 dBFig. 6eDrop, dB5101500.411030 MHz1 dB10 dB12 dB
  • TI’s pattern
  • Triangle only

2 notes

EMC

It fails an emissions scan or an ESD zap

  • Note 2USB-C shell

    Should a USB-C shell go straight to ground, or through an RC?

    USB-C · GCT USB4105 · CM5 IO board

    249 → 0 Vleft on the shell after an 8 kV zap

  • Note 6Spread spectrum

    What does spread spectrum do to a buck’s EMI test?

    LM62460 · MNT Reform

    −10 dBat 4 MHz, quasi-peak
Note 3 · Antenna ground

Four boards

Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%

2 notes

RF

A radio’s range comes up short

  • Note 3Antenna ground

    How much ground does a 2.4 GHz PCB antenna need?

    TI AN043 · CC2511 · CC2511 dongle

    85%radiated with 40 mm of ground

  • Note 25Trace or U.FL jumper

    When does a U.FL jumper beat a trace to the antenna?

    U.FL · MHF I · 1.13 mm coax

    83% vs 60%jumper vs trace, 10 cm at 5.5 GHz
Note 7 · Thermal relief

From above

Fig. 7SolidCopper all the way roundRelief4 spokes, 0.5 mm wide2 mmFig. 7SolidCopper all the way roundRelief4 spokes, 0.5 mm wide2 mmFig. 7SolidCopper all the way roundRelief4 spokes, 0.5 mm wide2 mm

1 note

Board and fab

A joint won’t solder

  • Note 7Thermal relief

    Should a ground pin tie into the plane through a thermal relief?

    536 → 402 °Ciron to melt the far side in 3 s

Logistics

1 note
Note 9 · Chip journeys

Six chips

Fig. 9c060120180240 daysTI INA24071–129 d7–17 dMicron LPDDR4X91–150 d7–15 dNVIDIA B300134–231 d6–18 dST L298N36–78 d9–20 dInfineon IPT015N10N536–87 d7–17 dEspressif ESP32133–168 d9–19 dFig. 9c060120180240 daysTI INA240 current-sense amp71–129 d7–17 dMicron LPDDR4X on the 8 GB Raspberry Pi 591–150 d7–15 dNVIDIA B300 GPU, die path134–231 d6–18 dST L298N dual H-bridge36–78 d9–20 dInfineon IPT015N10N5 100 V MOSFET36–87 d7–17 dEspressif ESP32 in a WROOM-32E module133–168 d9–19 dFig. 9c060120180240 daysTI INA240 current-sense amp71–129 d7–17 dMicron LPDDR4X on the 8 GB Raspberry Pi 591–150 d7–15 dNVIDIA B300 GPU, die path134–231 d6–18 dST L298N dual H-bridge36–78 d9–20 dInfineon IPT015N10N5 100 V MOSFET36–87 d7–17 dEspressif ESP32 in a WROOM-32E module133–168 d9–19 d
  • In factories
  • Moving

How parts reach a board: where they are made, how far they travel and how long it takes.

  • Note 9Chip journeys

    Where does a chip travel before it reaches your board?

    INA240 · LPDDR4X · B300 · 3 more parts · Raspberry Pi 5

    31,100 kmfor a chip made 61 km from Mouser

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