What are the typical Bob Smith termination component values?

The classic network uses one 75-ohm resistor per pair (four resistors in a 10/100 design) connected to a common star node, and a single coupling capacitor of about 1000pF (1nF) rated to at least 2kV that links that node to chassis ground. The 75-ohm value approximates the common-mode impedance of the twisted pair; the 2kV capacitor blocks DC and survives hi-pot and surge stress.

Do integrated-magnetics RJ45 jacks (magjacks) include Bob Smith termination?

Yes. Integrated-magnetics RJ45 jacks such as VOOHU's integrated RJ45 series place the isolation transformer, common-mode choke, and the Bob Smith termination network inside the shielded jack housing. This guarantees repeatable common-mode termination, saves board area, and removes the risk of a designer fitting an under-rated coupling capacitor. You should not add a second external network to an integrated magjack.

Is 75 ohm the right resistor value, or should I use 52.3 ohm?

Both values appear in the field. The original Bob Smith scheme uses 75 ohm; a modified scheme uses 52.3 ohm. Published measurements show little practical difference in return loss between 75 ohm, 52.3 ohm, and even no resistor. What matters far more is a correctly rated 2kV coupling capacitor and a clean, short connection to chassis ground rather than digital ground.

Home › RJ45 Jack Resources › Bob Smith Termination in RJ45 Jacks

Bob Smith Termination in RJ45 Jacks: The 75Ω / 1000pF Common-Mode Network

Q: What is Bob Smith termination in an RJ45 jack?

Bob Smith termination is a small resistor-capacitor network that terminates the common-mode impedance of each twisted pair (including the two unused pairs in 10/100 Ethernet) and the transformer center taps. Each pair is tied through an approximately 75-ohm resistor to a common node, which connects to chassis ground through a high-voltage capacitor, typically 1000pF rated at least 2kV. It reduces common-mode EMI and gives common-mode noise a defined return path. In an integrated-magnetics RJ45 jack (magjack), this network lives inside the connector.

Updated July 15, 2026 · VOOHU Electronics · RJ45 Jack (母座) Design Series

An RJ45 jack that carries Ethernet does more than pass two or four differential pairs. It also has to deal with the common-mode currents riding on those pairs and on the transformer center taps — energy that would otherwise radiate as EMI or couple noise back into the PHY. Bob Smith termination is the small resistor-capacitor network that quietly handles this right at the jack. This guide explains what it does, the classic 75Ω/1000pF values, and why VOOHU's integrated-magnetics RJ45 jacks (母座) build it in.

Short answer. Bob Smith termination ties each twisted pair — including the two unused pairs in 10/100 Ethernet — and the transformer center taps through roughly 75Ω resistors to a common node, which connects to chassis ground through a high-voltage capacitor, typically 1000pF (1nF) rated ≥2kV. It reduces common-mode EMI emissions and gives common-mode noise a defined return path. In an integrated magjack, all of these parts live inside the connector housing, so the termination is fixed by the manufacturer rather than by your board layout.

Why an RJ45 jack needs common-mode termination

A standard RJ45 jack is the 8-position, 8-contact (8P8C) interface defined by IEC 60603-7, wired per ANSI/TIA-568 and carrying signals defined by IEEE 802.3. Each of its four twisted pairs has a differential (odd-mode) impedance of 100Ω, which the PHY and cabling already control. The problem is the even mode: common-mode currents that flow equally on both wires of a pair.

Two situations create common-mode energy at the jack:

Unused pairs. In 10BASE-T and 100BASE-TX only two pairs carry data (pins 1-2 and 3-6). The other two pairs (4-5 and 7-8) are electrically idle but still act as antennas — they pick up and re-radiate common-mode noise if left floating.
Transformer center taps. The isolation transformer inside the magnetics has center taps on the line side. Without a defined AC path, common-mode currents on those taps have nowhere controlled to go.

Bob Smith termination — named after Robert W. Smith, whose approach is described in U.S. Patent 5,321,372 (issued June 14, 1994, “Apparatus and method for terminating cables to minimize emissions and susceptibility”) — gives that energy a matched, high-voltage-isolated route to chassis ground. The payoff is measurably lower radiated emissions (helping designs pass CISPR 32 / EN 55032) and better common-mode noise immunity.

The classic Bob Smith network, component by component

The canonical implementation sits on the cable side of the magnetics, between the connector and chassis ground:

Element	Typical value	Function
Common-mode termination resistor (one per pair)	~75Ω, 1% tolerance	Approximates the common-mode impedance of the twisted pair; damps common-mode resonance on used and unused pairs
Resistor count (10/100 design)	4 (one per pair)	Terminates the Tx pair, Rx pair, and both idle pairs (4-5, 7-8)
Common / star node	—	Single point where all termination resistors meet before the capacitor
Coupling capacitor to chassis	~1000pF (1nF), rated ≥2kV	AC path for common-mode currents to chassis; blocks DC; provides high-voltage isolation and survives hi-pot / surge

The voltage rating of the capacitor is the part most often gotten wrong. Because the jack must withstand a 1500 Vrms isolation (hi-pot) test per IEEE 802.3, and can see cable-borne surge, the coupling cap is rated 2kV or higher — not a garden-variety 50V/100V part.

Design note — 75Ω vs 52.3Ω. There is a long-running debate about the “right” resistor value: the original Bob Smith network uses 75Ω, while a well-known modified version uses 52.3Ω. Published bench measurements have shown little practical difference in return loss between 75Ω, 52.3Ω, and even no termination resistor at all. The capacitor's voltage rating and a clean chassis-ground connection matter far more than the exact ohms.

Discrete network vs integrated magjack (母座)

You can realise Bob Smith termination two ways.

1. Discrete magnetics + external network

PHY → common-mode choke and transformer module → RJ45 jack, with the 75Ω resistors and the 2kV capacitor placed as discrete parts near the connector. This is the lowest raw-BOM path, but it multiplies the ways to get EMC wrong: an under-rated cap, resistors tied to digital ground instead of chassis, or long, noisy traces to the star node.

2. Integrated-magnetics RJ45 jack (magjack)

An integrated-magnetics RJ45 jack puts the isolation transformer, the common-mode choke, and the Bob Smith termination network inside the shielded metal jack housing. VOOHU's integrated RJ45 series is offered in 1×1, 1×N and 2×N sink-type modules, Tab-Up and Tab-Down, spanning 100M, 1000M, 2.5G, 5G, 10G and HDBaseT. Because the termination is fixed at the factory, every board gets the same, correctly rated common-mode network — a real advantage for industrial and PoE ports where EMC margin is tight.

Real VOOHU integrated-magjack specifications

These are published parameters for a VOOHU single-port integrated jack; the center-tap / Bob Smith termination is internal to the part.

Parameter	VOOHU SYT511Q293AC1A8D057 (Gigabit integrated jack)
Type / orientation	Single-port (1×1), Tab-Up integrated-magnetics RJ45 jack
Data rate	10/100/1000BASE-T
Turns ratio	1CT : 1CT
OCL (open-circuit inductance)	350µH
Isolation (Hi-pot)	2250 VDC
Insertion loss	≤1.0 dB
Common-mode rejection	−30 dB
Operating temperature	−40°C to +85°C (industrial)
Indicators	Green / Yellow dual-color LEDs

VOOHU also offers 100Base 1×1 Tab-Down integrated jacks such as the industrial SYT111B002CA2A1D and SYT111B002FA2A1D (−40°C to +85°C), pin-compatible drop-ins for common commercial magjacks — all with the magnetics and common-mode termination integrated inside the socket.

Choosing your approach

Reach for a discrete transformer plus external Bob Smith network when you are squeezing the last cents out of a very high-volume consumer board, already own the magnetics module, and have full control of the layout and chassis-ground stitching.

Reach for a VOOHU integrated magjack when you want guaranteed, repeatable common-mode termination, a smaller footprint, fewer placement mistakes, and a shielded housing that helps EMC — the usual choice for industrial gateways, PoE endpoints, IP cameras and anything that must pass emissions with margin.

Common mistakes

Using a low-voltage coupling capacitor. The chassis cap must be rated ≥2kV. A 50V/100V part will fail hi-pot and can rupture under surge.
Tying the star node to digital/signal ground. The Bob Smith node belongs on chassis ground; connecting it to logic ground injects noise and defeats the EMI benefit.
Leaving the unused pairs (4-5, 7-8) unterminated in a 10/100 design — they still radiate common-mode energy.
Adding an external network to an integrated magjack. The termination is already inside; doubling up mis-terminates the port.
Over-optimising the resistor value. 75Ω vs 52.3Ω barely moves return loss; spend the effort on cap rating and grounding.

Frequently asked questions

What is Bob Smith termination in an RJ45 jack?

It is a resistor-capacitor network that terminates the common-mode impedance of each pair (including the unused 10/100 pairs) and the transformer center taps. Each pair goes through about a 75Ω resistor to a common node, which reaches chassis ground through a high-voltage capacitor (typically 1000pF, ≥2kV). It lowers common-mode EMI and gives noise a defined path. In a magjack the network is inside the connector.

What are the typical component values?

One 75Ω resistor per pair (four in a 10/100 design) to a shared star node, plus a single ~1000pF (1nF) capacitor rated at least 2kV linking that node to chassis ground.

Do integrated-magnetics RJ45 jacks include Bob Smith termination?

Yes. VOOHU integrated RJ45 jacks house the transformer, common-mode choke and Bob Smith network inside the shielded socket, so the termination is fixed and repeatable. Do not add a second external network.

Should I use 75Ω or 52.3Ω resistors?

Either works. Measurements show little return-loss difference between 75Ω, 52.3Ω and no resistor. A correctly rated 2kV capacitor and a short, clean chassis-ground connection matter far more.

Related VOOHU guides

Bob Smith termination RJ45 jack integrated magnetics common-mode EMI VOOHU