8 Layer Board - Non-standard Stackup

If you really have few traces to route, why so many layers? Multiple ground planes seldom help, but all those layers drive up the cost some.

What is the frequency range, impedance, and nature of the signals? What do you mean by "highly sensitive"? What's the environment?

Is this audio?

John

It depends on what "very sensitive to noise" means. If you're doing low level, high impedance circuitry in an environment where there's going to be capacitive pickup, put ground planes on the outside, at least in the analog front end part. I've had to re-spin boards to do this, after picking up everything in the universe. If the top of the board faces a metal box, you might be all right with the stackup you've got.

Run completely separate power supplies for the analog and digital sections, and don't run the digital supply planes through the analog areas. They usually use different voltages anyway. An analogue supply can be isolated amazingly well using a capacitance multiplier, even if the raw supply is really ugly. Just the capacitive pickup from a single

1 mm square pad sitting over a very mildly ugly PS plane blew one of my low-level circuits completely out of the water. Splitting PS planes makes much more sense than splitting grounds.

Then there's other stuff, e.g. magnetic pickup from the power supply.

Cheers,

Phil Hobbs

In December I did an 900Mhz RF/ 100Mhz Micro board. Four layers:

Top: parts and traces Mid1: Ground plane 100% Mid2: Multiple Power planes Bottom: Signal traces

Passed FCC first time.

(Except for the BNC connector the buyer switched on me, the board had to be re-done)

No one else will ever use your board for anything, so you can stack up traces anyway you want.

donald

Actually, it is ultrasound from around 20 kHz to 20 MHz range with around 100 dB gain maximum with differential pairs. I would admit that I am not very experienced in board design and from internet sources, it seemed that a 8 layer was the recommended stack.

Regarding the term "highly sensitive", basically, the ultrasound board will be used with sensors to get through parts that are traditionally difficult and so you maybe at max gain, you are looking at a signal that is around 100 mV. In this case, you need the noise floor to be at least half of the signal so it can be measured with reasonable SNR.

This board does not need to pass FCC or other certificati> > >Friends,

The computer will probably be your largest source of noise!

--
Ben Jackson AD7GD

http://www.ben.com/

Yes,

Actually, that is why I am worried. > >

Running several regulators etc off a single supply makes the grounding tricky especially if you are limited by the form-factor of the pcb. I used separate small isolated ground areas for each - with a track back to the source power ground. The analogue and digital ground should not overlap (if you have them on separate layers). Adjacent boards can radiate so it might be worth laying out the sensitive input area so that is can be shielded (both sides). I know the outer planes should provide screening - but the components are still unscreened.

Geo

cky

te

er

em on

input

ld

When you mean separate ground areas for each, do you mean separate grounds for each linear regulator output - Currently, my ground plane is the same for all the regulator outputs. I can separate them, however this will mean more complications as each 4 linear regulators link to one switcher which links to the main ground from the PC supply.

I have separate grounds for the digital and analog areas though and all the analog circuitry runs off voltages derived from the +12V line and all digital logic runs off voltages derived from the +5V line.

Also, has anyone use optoisolators to allow signals to jump from the digital domain to analog domain in case of a single supply PCI board?

I said it gets complicated... depends a lot on your signal paths and the type of chips in use. For instance programmable gain amps have digital and analogue signals and supplies (although the digital control lines can be held static during measurement).ADCs have analogue and digital grounds and require the (electronic) design engineer to specify where he wants them joined. This can give problems where there is more than one ADC. In this case I used to provide several solder link pads by the chips to let them experiment to find the optimum for their purposes.

Geo

Hi, can you please send me gerber for 8-layer board with circuit diagram

sp