Lesson 7/1070%
MODULE 07

Differential Signaling

Master differential pair routing, impedance control, common-mode noise rejection, skew management, and mode conversion in high-speed serial links.

45-60 min
Intermediate
6 Sections

Differential vs Single-Ended

Differential signaling transmits data as the voltage difference between two complementary traces (D+ and D-). The receiver detects only Vdiff = V+ - V-, rejecting any noise that couples equally into both lines. This provides 6-20 dB better noise immunity than single-ended signaling.

All modern high-speed interfaces use differential signaling: PCIe, USB 3.x, SATA, HDMI, DisplayPort, Ethernet (LVDS), DDR5 DQ (via pseudo-differential). It enables multi-Gbps data rates over PCB traces and cables.

Key Advantages

Common-Mode Noise Rejection
EMI, ground bounce, and supply noise couple equally — subtracted at receiver
Lower Voltage Swing
200-400mV swing vs 1.5-3.3V single-ended — lower EMI, less power
Reduced EMI Radiation
Equal and opposite currents cancel far-field radiation
No Ground Reference Needed
Signal is between traces, not trace-to-ground — works across connectors

Interactive: Differential vs Single-Ended Comparison

LIVE SIMULATION

Observe how differential signaling rejects common-mode noise. Increase noise level to see that the differential signal remains clean while single-ended signals are corrupted.

Signal
Frequency (GHz)2.5
Amplitude (mV)400
Noise Injection
CM Noise (mV)50
Noise Freq (MHz)200
Pair Config
Skew (ps)0
Signal Quality
CMRR --
Mode Conv 0%
Eye Open --
Oscilloscope: D+ and D- (with noise)
Oscilloscope: Differential & Common-Mode
D+
D-
Vdiff
VCM

Impedance Relationships

Differential impedance depends on the coupling between traces. Tighter coupling (smaller spacing, same layer) reduces Zdiff below 2×Z0. The coupling coefficient k ranges from 0 (no coupling) to ~0.3 (tight coupling).

Zdiff = 2 × Z0 × (1 - k)
ZCM = Z0 × (1 + k) / 2

Common target impedances: USB 3.x = 90Ω diff, PCIe = 85Ω diff, HDMI = 100Ω diff, Ethernet = 100Ω diff.

Coupling Factors

Spacing (S)
Smaller S → higher k → lower Z_diff. Typical: S = W (1:1 rule).
Dielectric Height (H)
Lower H → stronger ground coupling → less mutual coupling → k drops.
Trace Width (W)
Wider W → lower Z0, but coupling depends on S/H ratio.
Edge-Coupled vs Broadside
Broadside (vertically stacked) gives tighter coupling but harder to manufacture.

Interactive: Differential Pair Impedance Calculator

LIVE CALCULATION
Geometry
Width W (mil)5.0
Spacing S (mil)5.0
Stackup
Height H (mil)4.0
Er (dielectric)4.0
Computed Impedance
Z0 50 Ω
k (coupling) 0.15
Zdiff 85 Ω
ZCM 29 Ω
Standards Match
Cross-Section: Differential Pair Geometry
Oscilloscope: Impedance vs Spacing Sweep

Intra-Pair Skew

Skew is the time difference between D+ and D- arriving at the receiver. It converts differential energy into common-mode, degrading CMRR and creating EMI. Sources: unequal trace lengths, asymmetric via transitions, connector pin differences, glass weave effects.

Maximum Skew Budgets
USB 3.2: < 15ps | PCIe 5.0: < 5ps | 28G SerDes: < 3ps

Mode Conversion Impact

Differential → Common Mode
Skew converts diff signal to CM noise. CM current radiates EMI since fields don't cancel.
Eye Diagram Degradation
Mode conversion reduces eye height and increases jitter at the receiver.
Length Matching Fixes
Serpentine tuning within 5mil of each other keeps skew within budget.

Do's

✓ Maintain constant spacing throughout the route
✓ Route on single layer when possible (no layer transitions)
✓ Length-match D+ and D- to <5 mil (use serpentine)
✓ Keep 3×S clearance from other traces
✓ Place ground vias at layer transitions

Don'ts

✗ Route D+ and D- on different layers
✗ Break the ground plane beneath the pair
✗ Place components or vias between D+ and D-
✗ Route near switching power or clock traces
✗ Change spacing at bends or breakouts
Question 1 of 6 Score: 0/6

Scenario

Route a USB 3.2 Gen 2 (10 Gbps) differential pair from a Type-C connector to a USB controller IC. Distance: 3 inches. Stackup: 4-layer, 62-mil, FR-4 (Er=4.2). Required: Zdiff = 90Ω ±10%.

Recommended Geometry

W = 4.5 mil, S = 5.5 mil, H = 4.0 mil
Z0 ≈ 52Ω, k ≈ 0.13
Zdiff = 2×52×(1-0.13) ≈ 90.5Ω ✓

Skew Budget

USB 3.2: max intra-pair skew = 15 ps
At vp ≈ 6 in/ns → 15ps = 0.09 mil length diff
Length-match to < 5 mil (3ps budget)

Ready to complete this module?

Mark this module as complete once you have reviewed all sections and passed the quiz.