how much the effect to add a test point in high-speed differential signal

Hi Mengyao,

There are many reasons why we try to avoid adding any test points or test pads to high-speed signal traces. One reason is due to the unwanted parasitic capacitance that is introduced to the signal trace with the addition of a pad or test point. The addition of a test pad or test point also changes the characteristic impedance of the signal trace in that section of the trace, which could result in reflections of the signal. Specifically, to differential signal traces, their performance is dependent on the traces being tightly coupled and any mismatch between the signal traces will affect their ability to minimize the noise injection and radiated electromagnetic field from other signals. Having test points or test pads can cause mismatch and result in a many of the problems highlighted above. The rules we highlight in our documents are good rules of thumb to follow when designing PCBs and good references are the board designs of TI evaluation modules (EVMs).

Best Regards,

Ignacio