critical length reflections
The critical length is total distance between the signal source and the line termination - the total length of the trace from end-to-end. The concern is reflection from the termination which affects signal integrity. If you have a single trace with taps at various components, the problem of signal reflections gets a bit more complex.
Each discontinuity presents a point from which energy can be reflected. If the first device on a signal line is at a distance such that reflection occurs in less than the signal risetime (less than critical length), the reflection has little effect. If the second device is further away (greater than critical length from the source), then the reflection will mix with the applied signal, and distortion occurs.
So what do you do? You control the impedance of the track segment that exceeds critical length, and terminate that track with a matched impedance to ensure maximum energy transfer to the load. With matched impedance, the reflected energy is minimized.
If you insert a buffer in series with the signal path, the input of that buffer is the termination on the incoming line, and the output is the source for a new line.
If you find the above hard to follow, try the paper at:
**broken link removed**