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geoffery Bob
geoffery Bob

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The RJ45 Selection Decision-Making Process

Now that we’ve looked at the design of the RJ45, let’s discuss the steps involved in selecting the right model.

Step 1: Determine the speed rating
The speed category determines the transmission bandwidth that an RJ45 connector must support, and directly affects the connector’s insertion loss and return loss specifications. The following selection recommendations apply:

  • 10/100M → Select an RJ45 connector that supports a bandwidth of 100 MHz
  • 1G (Gigabit) → Select an RJ45 connector that supports a bandwidth of 100 MHz and meets the return loss requirements of IEEE 802.3ab
  • 2.5G/5G → Select an RJ45 connector that supports a bandwidth of 200 MHz/400 MHz
  • 10G → Select an RJ45 connector that supports a bandwidth of 500 MHz

Step 2: Determine the PHY driver type

  • Refer to the PHY data sheet to confirm whether it is voltage-mode or current-mode.

  • Plan the centre-tap connection according to the drive type:

  • Current-mode → Connect the centre tap to VDD (3.3V/2.5V/1.8V).

  • Voltage-mode → Connect the centre tap to earth via a 100 nF capacitor.

  • If the PHY data sheet does not explicitly specify the drive type, you can make an informed judgement by measuring the DC voltage between the TXD± pins and ground: for voltage-mode, this is approximately VDD/2; for current-mode, it is close to 0 V.

Step 3: Determine PoE power requirements

  • No PoE required → Select a standard model (non-PoE).
  • PoE (15.4W) → Select a model supporting 350mA (compliant with IEEE 802.3af).
  • PoE+ (30W) → Select a model supporting 720mA (compliant with IEEE 802.3at; WoHu models with the suffix ‘DP’).
  • PoE++ (60W/90W) → Select a model supporting 1.2A/2A (compliant with IEEE 802.3bt; WoHu models with the suffix ‘PG’ or ‘PTG’).

Step 4: Determine the mounting method

  • DIP (Through-hole) → High mechanical strength; suitable for industrial equipment and vibration-prone environments; requires space on both sides of the PCB.
  • SMT (Surface Mount Technology) → Suitable for automated reflow soldering production; lower profile, suitable for high-density equipment; mechanical strength is lower than that of DIP.
  • Low-Profile → Part of the housing is recessed into a slot in the PCB; height can be as low as 8 mm; suitable for 1U/half-U slim-line equipment.
  • Wire-Bonded (Cable Type) → Wires are soldered directly; suitable for non-board-mounted applications.

Step 5: Determine the environmental rating

  • Commercial grade (0–70 °C): Offices, homes, controlled environments.
  • Industrial grade (-40–85 °C): Factories, outdoor environments, in-vehicle applications, rail transport.
  • Automotive grade (-40–105 °C): Extreme temperature environments such as engine compartments.

Step 6: Identify any special requirements

  • Waterproof and dustproof: Is an IP67/IP68 rating required (e.g. for outdoor base stations or marine equipment)?
  • Vibration-resistant locking: Is a metal spring, Cannon self-locking mechanism or threaded locking required (e.g. for rail transport or vehicle-mounted applications)?
  • LED indicators: Are specific colours (green/orange/yellow), brightness levels or common-cathode/common-anode configurations required?
  • Shielding requirements: Is a fully shielded enclosure required (for EMI-sensitive environments)?

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