Grounding

12 bit ! ? I should damn well hope there's no noise above an LSB !

20 bits here my lad ! Without split grounds the 'digital noise' was present in the analogue signal at ~ -85dB wrt FSD ( which is +20dB wrt normal operating level so S/N was degraded to 65dB ). Split grounds made it essentially inaudible

- easily better than 100dB down - not practical to measure it in fact, inside the overall noise floor.

Graham

currents'

pins !

CMOS with supplies approaching 75A with a 3GHz clock. ;-) Well, that was last year...

--
  Keith

!

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.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Now whyever might that be I wonder ? Something to do with resistance perhaps ?

Graham

Hello Graham,

!

That's not really the reason. There simply is no way to provide a low impedance ground path on a chip, at least not at a reasonable cost. All you have is a few teeny bond wires and those cannot connect the chip's "plane" to the board plane very well. So you may need two returns there. Very common on ADCs. But that certainly does not mean you have to keep AGND and DGND split outside the chip.

--
Regards, Joerg

http://www.analogconsultants.com

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,

In message , dated Tue, 8 Aug 2006, John Larkin writes

This is NOT the way to gain friends among your colleagues.(;-) The British English expression is 'jammy barsteward'.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

currents'

Most of the eval board have a single chip, which the vendor considers to be the center of the universe; they'll also tell you to bypass every power pin with three capacitors, typically. A single plane-plane connection under one chip gets messy when there are 10 ADCs on the board. A bigger problem is that each of the ADCs (or whatever) has to connect to the outside world, which can cause more ground loops and EMI.

That reduces ground bounce caused by wirebond inductance. Connecting these pins to separate planes (at separate potentials!) can pump serious noise currents through the AGND wirebond, which can be bad news.

John

Well, of course you have to be careful. But audio isn't very demanding.

John

Hello Joerg,

that is of course challenging enough in itself, but is not the type of situation I thought of. In the module I quoted, I do 10 MSPS, but at 14 bits. If you have meant ground splitting will cause more inductive issues than it will solve, I agree with you. But at 14+ bits, with a power supply switcher on the board, the sheer resistance of the ground plane may be not low enough (on a small board)... On the board you have seen on my website, you can probably well guess where I have split the ground. One of the things that helped was the fact there are no fast digital signals entering the analog section (just relay driving lines :-), otherwise the split would have again caused problems (signal "return" path related).

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

As long as you use Monster Cable ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I agree, I have never found an advantage in splitting the grounds. I try to keep the ground impedance as low as possible.

I do carefully manage the current flow from the digital and power sections to minimize noisy current flow in traces used for analog signals. Usually this means shaping the ground planes or cutting it in various places - I keep the connection between these sections as broad as possible though - definitely not a narrow trace or zero ohm resistor.

Other techniques are to put resistors in series with the digital signals driving A/Ds, so reduce the current flow in the input capacitance of the A/D. Often it is useful to buffer the A/D outputs with digital buffers so that the A/D does not drive significant capacitative load and thus put high currents in its ground system.

I like to put lots of copper everywhere to keep as low an impedance gorund as possible - for boards I do myself using Eagle CAD the polygon capability works well for this. Admittedly this can increase capacitance to ground which may be undesirable.

Be careful to avoid the digital signals passing over the ground region for the analog signals. Analog Device has some useful app notes on this subject.

Which approach is used depends upon the signal frequency. At low frequencies the resistance of the ground system is the dominant effect and using star wiring back to the power source may be the most appropriate - at video frequencies and above (includes most digital systems) the inductance is a major problem so putting lots of paths in parallel and avoiding capacitative or inductive coupling between sections will probably work better.

A design I am working on currently uses 17 LTC1403A 14-Bit 2.5MHz A/Ds in an optical measurement system with bandwidth of around 1MHz.. I have surrounded the pre-amp sections with the A/Ds such that the control signals (isolated with resistors at the driving ends) pass around the periphery of the analog section. I have flooded the analog section with copper to reduce the ground impedance.

These A/Ds are pretty nice to work with - they have differential inputs and serial outputs - reduces the number of digital lines causing noise, and a pretty small MSOP-10 package.

In initial measurements the noise level seems to be pretty much as expected.

cheers

kevin

non-split ground

and the

plan A !

Quite so. My own view about most of the stuff I do is that it's 'moving DC' too !

Ah ! I ensure I'm involved in enclosure design right from the start.

Graham

yes, this may helpful to some

110+ dB dynamic range isn't demanding ?

Graham

Monster cable is for the 'aftermarket' clowns. It's not even especially well made I'm told.

Graham

Some of us consider millimeter-waves to be dc too!

Low frequency ground currents aren't nearly as well localized as at high frequency, due to the lack of inductive reactance, so they tend to smear out all over the board unless you do some strategic cutting.

Cheers,

Phil Hobbs

John,

I know you have enough experience. My "advice" related comment was (hopefully obviously) directed at those participant(s) of the thread who were looking for and advice.

As for "not a single split plane", my latest reply to Joerg says what I think of it. It is all a matter of small and precise enough until you run into plane resistance problems. Have a look at

, the split plane sure does not hurt and I am quite sure unsplitting it will yield a measurable (although not dramatic) worsening effect. Again, it is a matter of size/precision/speed.

And many years ago (15....), I was mad enough to do a similar thing with a _flyback_ convertor on board (these here are just step-down). On that design, without the split planes with all the peak currents flyback convertors have, I would just have been lost, it would have not been just about being "measurable".

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

John Lark>

pins !

And for audio you split them too. See page 9 also. Nice PCB film

Graham