Need advice on designing an eight Layer PCB

Apr 07, 2009 1 Replies
A

I=92m designing an eight Layer PCB Board that has LVDS signals in frequency of 700Mbps. I have read lots of Document regarding Impedance Controlled PCB routing. The desired impedance is 100 Ohm as I read the device datasheet. But in some Application note I=92ve seen that the Impedance should be 75 ohm or 85 ohm. For example as I see in (part of



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the impedance is dependent to the number of layers. What is the exact impedance that I should base my design on it? My stackup is as follow(differential impedance is 87 ohm): Top Layer: Differential signals width 9mil and 4mil for their gap Substrate: 7mil GND Plane Substrate: 10mil VCC Plane Substrate: 10mil Midlayer1: Differential signals width 5mil and 5mil for their gap Substrate: 10mil GND Plane Substrate: 10mil Midlayer2: Differential signals width 5mil and 5mil for their gap Substrate: 10mil GND Plane Substrate: 7mil Bottom: Differential signals width 9mil and 4mil for their gap

As you see the track width is differs by 4mil in top layer relative to mid-layers. How can I change the Stackup that the track width be closer to each other in different layers? Is there software that helps me or a site that has some example of stackup?( I know that the HyperLynx can do this but I don=92t have much time to learn it and I=92m looking for other ways in the little time that I have.)


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J

TXLINE

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will let you check your various impedances.

I note that your dielectrics add up to 64 mils. Adding about a mil or so of copper per layer, the overall board will be about 75 mils thick. Is that OK? Check with your board house to see what standard finished- dielectric stackups they prefer to do; that can save a lot of hassle and cost.

Most of my actual trace impedances seem to come out low. Last board I did was fairly typical, 75 ohm calculated traces became 71 in real life. That's not a big deal for slow (like 700 MBPS!) data, but on my board I was looking for beautiful analog step response, so I tweaked terminator resistor values.

Incidentally, 11801 sampling scopes and SD24 TDR heads are dirt cheap on ebay these days. I just got an SD24 for $200, compared to $2000 a year ago and $14,000 new.

John

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