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

All,

My design includes a differential amplifier cascaded pair at 20 kHz to

5 MHz. The amplifer I am using is the AD8331. I have routed the signals differentially through, the differential pairs mostly reside on the top layer and layer 3 (between ground layers)

From what I have heard online, it appears that trace lengths in differential routing is very important - however how much impact does it have under 5 MHz. It appears that my routes are are around 20/30 mils mismatch, though I am trying to reduce it.

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 for differential pairs to be on the top or internal layer? Is there a good book or a resource that can throw more light on this?

Thanks

Ray

Reply to
namdeguerre
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Electromagnetic radiation travels about 1 foot (30cm) per nanosecond in a vacuum, and at about 8 inches (20cm) per nanosecond in FR4. Your

5MHz frequency has a wavelenght of 200 feet in a vaccuum and about 130 feet on you printed circuit board. 20/30 mills of mismatch is unlikely to be all that important.

If you are going to bother regarding your printed circuit board traces as transmission lines, you should aim to lay them out to have a constant impedance.

Buried traces are called "stripline"

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and offer a lower characteristic impedance (for the same trace width) as "microstrip" running over the outer surface of the board. They are also better screened.

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Having a track pop up to the outer layer to connect to an integrated circuit does create a small impedance discontinuity, but a much smaller discontinuity than that created by the capacitance to ground at the integrated circuit input.

One time when I was feeling excessively perfectionist, I narrowed my traces for about 2cm around an integrated circuit input to compensate for the extra capacitance - I was tracking in a 200MHz clock and it was almost certainly a complete waste of time.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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

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/
Reply to
Joerg

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?

Reply to
namdeguerre

I haven't used this chip on a circuit board yet. Too expensive or, as Jim T. would say, I am a cheapskate when it comes to circuit design. But I did use the uA733 which is also differential. The situations where I'd place controlled impedance traces are mostly for really long traces all across a backplane and stuff like that. Especially since it costs nothing and we don't even need to grab the old slide rule any longer:

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--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Small point: "FR4" is pretty generic. The relative dielectric constant can be anything over a pretty large range from a bit less than 4 to maybe 5 or a bit more. I have some in front of me that I measured at about 4.8 yesterday, and I have some boards made with Isola 370HR that's rated at just about 4.0. Anyway, in the 4.8 material, I expect a velocity factor no higher than .57 for a microstrip on the surface, but for a stripline, it will be 1/sqrt(4.8) = .456. So, instead of 20 cm/ns for microstrip, I'd expect more like

17 cm/nsec, and for things buried in the dielectric, it'll be less than 15 cm/ns (13.6 for my 4.8 dielectric constant material). The variability in dielectric constant of FR4 also means that it's tough to get good accuracy on trace impedance, unless you specify a particular substrate material.

Cheers, Tom

Reply to
Tom Bruhns

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