E
Eeyore
- Jan 1, 1970
- 0
Jim said:Crap. Graham can't even give good advice on how to wipe ;-)
Let me explain for you then ! After wiping you 'wash'. A moist tissue is fine.
Graham
Jim said:Crap. Graham can't even give good advice on how to wipe ;-)
John said:Just recently, I put a power supply (linear and switching regs), a
temperature controller (thermocouple-input, 150 watt PWM heater
output), a PPM-noise-level, 3-amp NMR gradient coil driver, and a uP
on a single small board, with a single ground plane. Works fine and
passed FCC and CE first time.
And another board has a VME interface, switching regs, uP, and twelve
12-bit ADCs clocked at 40 MHz, with sub-LSB RMS noise levels. Single
plane.
Sounds like you're working at somewhat higher freqiencies than me ! ;~)
Hello Graham,
Plus professors often teach that stuff. That, and those reference
designs, have kept my business humming. When I started out I really
didn't want to go into EMI but split grounds have kind of sucked me into
that funnel.
It can also mean that the chip designers didn't have much of a clue
about the system side of things ;-)
The worst case was a slew of field failures. Some pulses had hit into
the substrate diodes and much of the stuff across the AGND-DGND devide
fried. Nobody knew where the pulses came from but after a re-layout with
a common plane there were no more field failures.
In EMI we have an old rule: The EMI performance of a given design is
inversely related to the number of toroids it requires to make it behave.
Jim said:Yep. You don't want a common wirebond to cross-couple noise.
Sounds like you're working at somewhat higher freqiencies than me ! ;~)
Eeyore said:Many reference designs use 'split' grounds.
There's a bit of a clue when the chip itself has dedicated AGND and DGND pins !
Graham
Aug 2006 said:Just recently, I put a power supply (linear and switching regs), a
temperature controller (thermocouple-input, 150 watt PWM heater
output), a PPM-noise-level, 3-amp NMR gradient coil driver, and a uP on
a single small board, with a single ground plane. Works fine and passed
FCC and CE first time.
And another board has a VME interface, switching regs, uP, and twelve
12-bit ADCs clocked at 40 MHz, with sub-LSB RMS noise levels. Single
plane.
I also did the NIF timing system: about 2000 client points triggered to
within a few picoseconds accuracy, spread over the world's biggest
laser, the size of a football stadium. No split planes there, either.
Many reference designs use 'split' grounds.
There's a bit of a clue when the chip itself has dedicated AGND and DGND pins !
Not in the case of audio. You have to be careful where those ground currents
flow.
12 bits at over 100MSPS. Plus a time-gain control amp in front to cover
the required range.
Well, of course you have to be careful. But audio isn't very
demanding.
John
John said:Don't do it. Use a single solid ground plane and handle any low-level
problems locally.
Splitting planes implies an impedance between them, hence high
relative AC potentials between "grounds". That will generally cause a
lot more trouble than it cures.
John
Joerg said:Hello Graham,
Very low frequencies is where star-grounds and stuff like that can
indeed work. Ultrasound guys consider audio to be DC ;-)
Problems can still creep up when you install such split ground gear in
the vicinity of a strong RF emitter. Radar bases, coast guard stations,
AM talk radio towers, 13.56MHz "blasters", you name it, I have seen them
pretty much all. Mostly I have to come out there to find out what's
causing EMI and then sit down with the client to gently break the news
that they need a few re-layouts and chassis mods. Usually the engineers
take that in strides but often the enclosure designers don't. Frequently
they are artists rather than engineers and they can become quite livid
when I suggest to move away from an injection molding scheme.
Jim said:Come on, Joerg, It's to avoid the common inductance to ground if you
use a single wirebond.
They are meant to connect together to a single ground plane outside of
the package.
John said:Well, of course you have to be careful. But audio isn't very
demanding.
Jim said:As long as you use Monster Cable ?
Joerg said:Hello Graham,
Very low frequencies is where star-grounds and stuff like that can
indeed work. Ultrasound guys consider audio to be DC ;-)
not really, the purpose of the pins AGND and DGND is not to suggest
that these pins be connected to the system analog and digital ground,
respectively, but to identify which pin is connected to the analog
gound of the IC and which pin is connected to the digital ground of the
IC.
It's up to the designer to decide which gets connected to which ground,
for low power digital mixed signal systems it's best to connect AGND
and DGND to the analog ground (i.e., the mixed signal device is
considered an analog part). For high powered mixed signal systems
(i.e., a fast DSP with built in A/D), then you split them.
See figure 9 for an example of the former and figure 10 for the latter,
http://www.analog.com/en/content/0,2886,760%5F%5F97529,00.html