Grounding of ADC grounds

Question

Fast, high resolution ADCs, especially ones that have parallel output, usually have a separate supply pin (DRVDD, (drive vdd) or OVDD (output vdd)) presumably because they don't want to couple noise to the sensitive analog supply while all the digital output signals toggle.

Most ADC datasheets recommend a single unbroken ground plane right under the device and connect the OGND and GND to this plane with the least possible inductance.

We have a situation where we have several of these ADCs on a single board. I'm wondering whether the "single unbroken ground plane" recommendation still holds even when there are multiple ADCs on the PCB.

In our design we went with two separate ground planes, one for GND (gnd of VDD), the other one for OGND (gnd of OVDD), and we connected these two planes near the edge of the PCB, where power enters through an adapter jack.

Any ideas, real world examples or links to reference documents will be appreciated.

Answer 1

Consider this a theoretically based answer - I've worked with ADCs, but not dealt with multiple ADCs and a separate ground plane. This will (hopefully) not be your star answer but may raise some issues worth noting. Also - if any of this sounds like hogwash or ill advised (variations on the same theme :-) ) please say so (preferably gently) - leaving uncommented advice which you consider misleading reduces the worth of the material as a resource for others. .

• What you have done sounds close to ideal. A second ground plane is a luxury not always available in "lesser" systems.

• One may be tempted to partition the ground plane into N segments radially expanding from the single common ground point, but that has good and bad points.

• Considering where and how you return the grounds of the signal sources can be an interesting exercise.

If possible you return the sources' grounds to the analog ground plane, but that then raises issues re sources which are powered but which do not themselves have separate power and analog grounds. How do you return the source power ground to the power ground plane and the source analog ground to the analog ground plane?

In the case of eg instrumentation amplifiers this may be easy as the analog ground is conceptually separate from the power ground.

In the case of single ended sources you may need to look closely at what happens to ground currents between power and analog. If the local power ground has a potential dc offset relative to analog ground you may wish to isolate this component from analog ground. To do this you may even go as far as providing an AC filtered DC feed to power ground for the sources analog portion and an AC ground path to the analog ground plane. This effectively creates a local analog ground for the source's circuitry - eg perhaps an inductor from power ground plane to local analog ground with a capacitor from local analog ground to analog ground plane.This sort of magic is liable to be needed only in extreme cases - it is to be hoped that in cases where DC components are large enough to matter that the device designers have accommodated it (as they have done with your dual gnd ADC's.

An example where this may not be the case is eg a microcontroller with internal DAC being used as a signal source for an ADC. For this arrangement to make sense (DAC-ADC) there will probably be some other analog function or convolved signal as well as the DAC output. In this case, how do you treat the microcontroller ground and what differences do the choices make.

• Both ground planes will probably be interrupted by vias interconnecting other planes. In extremely demanding cases, which yours sounds like, care needs to be taken re unbalancing of go and return signal paths for critical analog signals. An analog signal track which crosses a break in it's analog ground plane creates a slot antenna which may be both a radiator and a receiver. In many cases the effect may be small enough to be neglected but you need to know that this is so by design and not by good (or bad) luck. Ground plane breaks also provide increased loop area which can be important in critical cases. (Loop area between go and return can occur in fully balanced cases when tracks are used for both paths - usually eliminated by proper groundplane use.)

Answer 2

The answer depends on the resolution your are looking for in your ADC. For low resolution there is probably no need to isolate your digital circuitry from the analog ones(except the link itselft) and yes, connect all the grounds from the ADCs together and independent from digital logic. With more more resolution say 16 bit you must not connect the two ground planes together.

Quoting:

In higher resolution systems requiring more noise isolation, you might worry about stray digital currents flowing across the analog-ground region of your pc board. These currents can interfere with some extremely sensitive analog circuitry.

In general, when you encounter undesirable currents flowing in a particular pathway, you can address the problem through one of three approaches:

Reduce the level of the aggressive signal. Disrupt the stray current by putting a high impedance in series with it. Introduce a low-impedance element to shunt the stray current somewhere else.

There are problems which arise There is a website explaining the various approaches(google was your friend in this one)

Link to a set of problems and solutions regarding ADC bonding