Differential Pair
Understanding Differential Pairs
Differential pairs are the standard interconnect for high-speed signals because of their noise immunity. Any noise that couples equally to both lines (common mode) is rejected by the differential receiver, leaving only the intended differential signal.
Differential Pair Properties
- Differential impedance: Z_diff = 2 x Z_odd. Typically 100 ohms (two 50-ohm lines, loosely coupled).
- Common-mode impedance: Z_cm = Z_even / 2. Typically 25-30 ohms.
- Coupling: Tighter coupling reduces differential impedance and increases common-mode rejection.
Design Rules
- Maintain consistent spacing between the pair.
- Route both traces on the same layer with the same length.
- Avoid splitting the reference ground plane.
- Length match within 5 mils for multi-GHz signals.
Frequently Asked Questions
What is a differential pair?
A differential pair is two transmission lines carrying equal and opposite signals. Differential signaling rejects common-mode noise, reduces EMI, and is immune to ground bounce. Standard for high-speed digital (USB, PCIe) and some RF applications.
Why is differential better than single-ended?
Differential rejects noise that couples equally to both lines (ground bounce, supply noise, external EMI). It also radiates less EMI because the equal and opposite currents create canceling fields. The trade-off is two traces instead of one.
What is differential impedance?
Differential impedance is the impedance seen by the differential signal: Z_diff = 2 x Z_odd. For loosely coupled 50-ohm lines: Z_diff = 100 ohms. As coupling increases, Z_odd decreases and Z_diff decreases below 100 ohms.