An integrated-magnetics RJ45 jack — widely called a magjack — is a female 8P8C modular socket (母座) with the Ethernet magnetics built inside the connector housing. Instead of placing a separate transformer module on the board and wiring it to a plain jack, the isolation transformer and common-mode choke live inside the RJ45 jack the cable plug mates into. This guide explains what is inside a magjack, how to read its datasheet, and how to choose the right one — with parameters sourced from IEC 60603-7, ANSI/TIA-568 and IEEE 802.3, and from VOOHU's own jack range.
Every BASE-T Ethernet port needs magnetics between the PHY (the Ethernet transceiver chip) and the RJ45 connector. Those magnetics do three jobs: they provide galvanic isolation between the cable and the equipment, they reject common-mode noise for EMC, and they block DC while passing the signal. IEEE 802.3 requires the port to survive an isolation test of 1500 Vrms; the magnetics are what deliver it.
There are two ways to place this magnetics stage on a board:
Both do the same electrical job. The magjack trades a little tuning flexibility for a much smaller footprint and a shorter, cleaner path between the PHY and the connector. If you are weighing which approach suits your board, see our companion guide on integrated-magnetics vs discrete RJ45 jacks.
Open the metal can of a VOOHU integrated-magnetics jack and you find the contact system of a normal RJ45 socket plus a complete magnetics network on a tiny internal carrier:
The table below uses VOOHU's Gigabit Tab-Up magjack SYT511Q218AC1A8D057 as a representative single-port (1×1) 1000BASE-T part. Every figure is taken from the product datasheet; the corresponding standard is named so you can see what each parameter governs.
| Parameter | Specified value | What it governs / reference |
|---|---|---|
| Turns ratio (@100 kHz) | 1CT:1CT, 1:1 ±2% | PHY matching; tighter tolerance than the ±5% common in the market |
| Open-circuit inductance (OCL) | 350 µH Min (@100 kHz, 100 mV, 8 mA DC bias) | Low-frequency droop / PoE bias headroom |
| Insertion loss | ≤ 1.0 dB (1–100 MHz) | Signal level into the cable |
| Return loss | ≥18 dB (1–30 M) / ≥14 dB (30–60 M) / ≥12 dB (60–80 M) / ≥10 dB (80–100 M) | Impedance match; ANSI/TIA-568 link budget |
| Crosstalk | ≥40 dB (1–30 M) / ≥35 dB (30–60 M) / ≥30 dB (60–100 M) | Pair-to-pair isolation |
| Common-mode rejection (CMR) | ≥30 dB (1–100 MHz) | EMI / emissions margin |
| Hi-Pot isolation (primary–secondary) | 2250 VDC (6 s, 1 mA) | Safety isolation; exceeds IEEE 802.3 1500 Vrms test |
| Durability | ≥ 750 mating cycles | IEC 60603-7 connector life |
| Mating force | ≤ 23 N | Insertion feel / panel design |
| Contacts / plating | Phosphor bronze C5210; gold on contact area (customizable) | Contact resistance & wear |
| Housing / shield | PBT UL94 V-0 body; C2680 nickel-plated brass shield | Flammability & EMI |
| Integrated LEDs | Yellow 585 nm / Green 568 nm, 20 mA | Link / activity indication |
| Operating temperature | −40 ~ +85 °C (industrial grade) | Environment; grades range 0~+70 to −40~+105 °C |
| Outline / panel cutout | 16.10 × 23.15 × 16.87 mm / 16.35 × 23.40 mm | Mechanical fit |
Values are representative of one VOOHU Gigabit magjack; always confirm against the specific product datasheet for your part number. Return-loss and crosstalk are specified in finer frequency bands than many commercial parts.
VOOHU builds magjacks as a configurable family, so a single part number encodes the choices below. Walk these axes top to bottom and you have specified the jack:
| Selection axis | Options |
|---|---|
| Ports / port array | 1×1, 1×N (1×2 … 1×8), 2×N (2×1 … 2×8) sink-type modules |
| Integrated magnetics | Yes (magjack) or No (plain jack, discrete magnetics on board) |
| Data rate | 10/100M, 100/1000M, 2.5GBASE-T, 5GBASE-T, 10GBASE-T (plus HDBaseT) |
| Tab orientation | Tab-Up or Tab-Down |
| PoE | non-PoE, or rated 350 mA / 600 mA / 720 mA / 850 mA / 900 mA / 1000 mA / 1.5 A per port |
| LED code | No LED, or green/yellow/orange/red combinations (e.g. G/Y, G/G, G/O, G/R) |
| Surge option | Standard, or "+SPD" integrated surge-protection variants |
| Mount | DIP (through-hole) or SMT |
| Shield tabs / EMI gasket | Yes / No |
| Temperature grade | 0~+70, −10~+85, −20~+70, −40~+85 or −40~+105 °C |
In a Power-over-Ethernet design, the DC power rides on the same pairs that carry data, and it reaches the powered device through the center taps of the magjack's integrated transformer. That means the transformer core must carry the PoE bias current without saturating — which is exactly why PoE magjacks are rated by per-port current rather than just by data rate. VOOHU offers PoE-capable parts from 350 mA up to 1.5 A per port; match the rating to your PoE type under IEEE 802.3af / 802.3at / 802.3bt (PoE/PoE+/PoE++). Under-rating the jack is a classic cause of hot connectors and link instability — see PoE failure prevention.
A magjack is a female 8P8C RJ45 jack (母座) with the Ethernet magnetics — the isolation/network transformer and usually a common-mode choke — built inside the shielded connector housing. One part replaces a plain RJ45 jack plus a separate discrete magnetics module. The generic engineering term is integrated-magnetics RJ45 or integrated connector module (ICM); MagJack is a registered trademark for the same idea.
In a discrete design the magnetics sit as a separate surface-mount module on the PCB and feed a plain RJ45 jack; in an integrated (magjack) design the transformer and choke live inside the jack itself. Integrated saves board area and shortens the PHY-to-connector trace; discrete gives more freedom to tune magnetics independently. The electrical job — galvanic isolation, common-mode rejection and DC blocking — is the same.
Yes. The center taps of the integrated transformer carry the PoE DC bias to the powered device, so the magnetics must not saturate under that current. PoE magjacks are therefore rated by per-port current — VOOHU offers grades from 350 mA up to 1.5 A — and should be matched to the PoE class (IEEE 802.3af/at/bt). Specifying a non-PoE magjack in a PoE design risks core saturation and overheating.
It can, because the magnetics are placed at the connector with short, controlled internal geometry and the metal housing provides a clean shield and ground return. VOOHU's Gigabit magjack is specified for ≤1.0 dB insertion loss, return loss ≥18 dB at low band, crosstalk ≥40 dB and common-mode rejection ≥30 dB across 1–100 MHz. Performance still depends on matching the part to the PHY and on good board grounding.