Critical Trace Length

[Budda]

critical length rise time

when calculating critical trace length, does this trace length start again if you put a buffer chip in the line?

Eg: if you run a data and address line to RAM and EPROM and then to a buffer or bus transciever to go off your pc board to another device on another pc board.

Would this mean you calculate another critical length from the buffer?

Kevin

 

[House_Cat]

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**

 

[jdhar]

edge rate calculation rise time critical length

just out of curiosity, how fast is your critical net...

 

[Budda]

edge rate trace length

running at 66Mhz, but have to run bus to some plug in cards so not sure about routing.

 

[House_Cat]

affect rise time critical length track pcb

running at 66Mhz, but have to run bus to some plug in cards so not sure about routing.

Critical length is not about the frequency - it is about the signal risetime. You can have a low frequency with the same critical length as a high frequency. The idea behind critical length is to prevent reflections from distorting the signal in the stable interval following the edge. Distortion after the driver settles can result in false triggering or pulse width distortion, among other problems.

It appears that your concerns may be more about coupling and termination than ciritical path length. If you are really concerned with critical path length, what is your edge rate and total rise?