Description
Overview: This check evaluates the difference in overall trace length between the two sides of a differential pair.
Passing Criteria: This check passes when the absolute length difference between the two sides of the differential pair falls below a specified tolerance.
Reporting Units: Length (Microns)
Examples
Passing Message:
Length mismatch of 4.79µm within acceptable range (0µm to 10000µm)
Failing Message:
Length mismatch of 12043.32µm outside acceptable range (0µm to 10000µm)
Physics Justification
Length matching in differential pair traces is crucial to ensure the signals arrive at the receiver simultaneously, maintaining their phase alignment. In particular:
Noise Cancellation:
Differential pairs rely on the signals being exact opposites. If they’re misaligned, they can't cancel external noise effectively, reducing signal integrity.
Avoiding Mode Conversion:
Length mismatches can convert the differential signal into common-mode noise, leading to interference and degraded performance.
Signal Timing:
In high-speed circuits, even small timing differences caused by mismatched lengths can introduce errors in data transmission.
Helpful Definitions
Phase Alignment
This means two signals in a pair (like a differential pair) stay in sync, reaching their destination at the same time. Misalignment can distort the signal and cause errors.
Mode Conversion
When differential signals (opposites) lose their balance because of mismatched traces, part of the signal changes into common-mode noise, which is unwanted.
Common-Mode Noise
This is noise shared by both signals in a pair, caused by things like imbalances or interference, and it disrupts the signal the circuit is trying to read.