Re: Differential pairs at 5 MHz - how important is trace length?

On Jan 30, 8:57 pm, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
namdegue...@xxxxxxxxx wrote:
All,

My design includes adifferentialamplifier cascaded pair at 20 kHz to
5MHz. The amplifer I am using is the AD8331. I have routed the
signals differentially through, thedifferentialpairs mostly  reside
on the top layer and layer 3 (between ground layers)

From what I have heard online, it appears that trace lengths in
differentialrouting is very important  - however how much impact does
it have under5MHz. It appears that my routes are are around 20/30
mils mismatch, though I am trying to reduce it.

As Bill pointed out it's not going to make much of a difference.

Also, my signals route from the top layer (where the ICs are) to layer
3 through vias and mostly travel through layer 3. but occassionaly
come up to the top layer to the ICs. I am not sure if this is good
practice or if there can be problems associated. Now, is there a
guideline fordifferentialpairs to be on the top or internal layer?
Is there a good book or a resource that can throw more light on this?

The AD8331 is a hotrod, it can do >40dB at 100MHz. So you need to make
sure that the input traces don't "see" too much of the output or it
could oscillate. Other than that routing on top would only be an EMI
issue if that's of any concern here.

--
Regards, Joerg

http://www.analogconsultants.com/

Joerg,

Thanks for the advice. It appears you are familiar with the 8331. At 5
Mhz or less, I get the feeling that controlled impedance lines are not
going to matter much. Have you used this in a differential format and
done any contolled impedance or without controlled impedance boards?
.