When probing UART Tx/Rx signals with my 3204D-MSO, I sometime have some glitch on the lines.

I did never experienced this kind of problem with other scopes (Lecroy, Rohde & Schwarz).
Changing the input threshold did not solve the problem. Some threshold results to unexpected results because the scope trigs on a glitch.
What is the input impedance of the digital lines ?
Thank you