Differential Signals EECS 713 Project by Jay Fuller :) What are they? When to use them Traces, connectors, terminations, etc.

Differential Signals Single-Ended Signal Differential Signal

Differential Signals You’ve seen this before!

Differential Signals Noise Noisy signal

Differential Signals Very noisy single-ended signal

Differential Signals Noisy diff Signal Extracted Signal N - N = 0 NOT N - (-N) = 2N (V - 0) - (0 - V) = 2V

Differential Signals Traces proximity of traces Consistent as possible Noise cancellation Trace impedance Nearby planes Induced current loops can likely be eliminated Return current on opposite trace Immune to plane interruptions

Differential Signals Traces Z Diff,Microstrip = 2Z 0 ( e -0.96S/h ) Z Diff,Stripline = 2Z 0 ( e -2.9S/h )

Differential Signals Impedances Trace impedance affected by coupling between traces Coupling factor K, same for both traces Trace 1 V 1 = Z 1 * I 1 V 1 = Z 1 * I 1 + Z 1 * K * I 2 Trace 2 V 2 = Z 2 * I 2 V 2 = Z 2 * I 2 + Z 2 * K * I 1 Z 1 = Z 2 = Z 0,single I 1 = -I 2 Z 0,Pair = Z 0,Single * (1 - K) V 1 = Z 0S * I 1 + Z 0S * K * -I 1 V 1 = Z 0S * I 1 * (1 - K) V 1 = Z 0P * I 1 V 2 = Z 0S * I 2 + Z 0S * K * - I 2 V 2 = Z 0S * I 2 * (1 - K) V 2 = Z 0P * I 2

Differential Signals Connectors Select appropriate connector Use adjacent pins for signal pairs Provide enough routing space, including well-spaced vias, and possibly multiple layers Match characteristic impedance of connector

Differential Signals Terminations Unbiased end termination Unbiased source termination Biased end termination Biased source termination Special “bridge” termination

Differential Signals Basic terminations Almost same as single-ended, Except with two traces instead of one Biased end terminationUnbiased end termination Unbiased source terminationBiased source termination

Differential Signals Bridge termination Still only 1 resistor: R P = 2Z 0P = 2Z 0S (1 - K) Assuming equal but opposite current in each transmission line, there is no current flow to ground. If this is the case, the connection from the middle resistor junction to ground can be eliminated, leaving 2Z 0 between the ends of the transmission lines.

Differential Signals References

Differential Signals Questions?