Help: Ground Noise Removal

I think you've said it all when you describe the pcb layout.

I've designed audio systems with multiple ADCs, FPGa running at 25MHz and processor at 48MHz. Multiple ground and power planes - separate analogue and digital ground and power planes etc. Signal to noise ratio in the audio band > 110dB but every signal looks a bit fuzzy with a scope !! (Not anything like 700mV pp)

You don't tell us the signal bandwidth. Often the simplest thing to do is to filter the analogue signal against high frequency noise with a capacitor from ADC input to ADC analogue ground right next to the chip (3mm trace lengths if you can). If your ADC has differential inputs then you need 2 caps to gound and maybe one between the inputs as well.

If you don't get the grounds right on some ADCs they will never work properly.

Read the data for you ADC and ideally re design your board paying attention to all that stuff about the pcb grounds and power distribution that you find in ADC data sheets.

If you can't do that try and work out where your ADC and logic are sharing a ground path but this usually doesn't help much if the layout is really grim.

Why didn't you lay the board out with ground planes ?

Michael Kellett

Your PCB is designed for audio. It is garbage. Do again, please.

Hi Michael, the PCB was design and layed by anther person. I been task to make it functional.

I read for a 4 layer PCB, flood the gnd plane (digital logic only). If there is a analog/digital, section it off and tie the gnds back to the main power supply.

As for the power plane, (single supply) I should flood the plane. For multi voltage design section it off as needed to distribute as power (without any open areas). The spacing between each section should be ~20mil's.

The two signal layers should have a copper pour (ground) after the PCB has been routed and connected to the power supply ground

As far as layup

I'm I missing something?

thanks slxrti

No. It's usually better to have a single, solid ground plane. Put all the digital and power stuff on one end of the board, mixed (adc, dac) chips in the middle, and low-level analog stuff on the other end. Appreciate that there *will* be system ground loops, and manage them.

Fat traces or better yet artistic pours are good for mixed-voltage power distribution.

Doesn't matter.

That's fine. Keep the 2-3 dielectric thin.

John

You might want to overkill the bypass, or at least leave pads for extra bypass. You don't have to populate them. Also, use different regulators for analog and digital, though that really only helps with lower frequency issues.

slxrti asks

From these comments it is evident yo uare aware of exactly what the cause is, and had clearly inherited the situation rather than designed it yourself, which makes it all the more irksome that you got about 3-4 snide comments about the earth paths - but there tends to be quite a lot of noise on this newsgroup, usually from people who don't bother reading the original posting.

If your budget / timescale / manager / politics does not permit a relayout as a multilayer with ground [perhaps the company's already made several thousand and cannot afford to scrap them], you could perhaps try wire links (eurgh!) leading from the board's primary 0V and power supply points to the ADC in question, and decoupling the ADC. This might just possibly isolate it enough from the ground noise to stabilise its supply. You'll have to cut the power / 0V connections to the ADC first - perhaps easiest by bending its legs up. If it works, you can point to the manager and say "we can either do this, which will involve a fight with Production, or re lay out with a ground plane 'cos this proves it's ground noise."

Your question about which plane is which is pretty spot on. Spacings don't really matter unless you're doing RF layout. If you are really really sure your design is final you can put the power ground planes on the outside, which reduces radiated emissions and improves immunity for the signal tracks inside; but then you'll never be able to do any mods to the board if there's a problem, so folk don't generally do that.

One other bit of advice: ensure your manager understands you were NOT responsible for this layout. Managers not directly involved with tech stuff sometimes associate problems with the last person to work on them.