Electronic – Connecting Analog Signals Between Boards

Consider this a theoretically biased answer - I've 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.)

I assume "as an analog signal" means on an oscilloscope as opposed to a logic analyser.

For a digital signal, it is important to be able to check the signal integrity and see whether it is subject to problems such as ringing, crosstalk, reflections, jitter, attenuation, etc.

This can only be done with a scope with a bandwidth > the frequencies present in the signal - remember with a digital signal there are frequencies much higher than the fundamental present, how high is dictated by the rise time of the signal. For a 1MHz digital signal you would generally want at least a 5MHz bandwidth, preferably much higher.
For debugging a typical small microcontroller (e.g. PIC, Atmel AVR, Arduino, etc) a scope bandwidth of at least 50MHz is preferable. This should be capable of handling just about all situations you might encounter.

There are many signals above 1MHz that need checking, most microcontroller clock signals are > 1MHz, SPI is often > 1MHz, USB, etc. FPGA designs may run at 100s of MHz, high speed ADCs and DACs, etc.

On a logic analyser all you can see is whether it is above a certain level or below a certain level (like a 1-bit scope) so while useful in other ways they are not suitable for checking signal integrity.

The image below (taken on an MSO - Mixed Signal Oscillscope, a combination of a scope and logic analyser) is a good example of crosstalk causing problems and why a scope is needed to see what's really happening. Notice the waveforms are quite a way from the idea of a "perfect" digital signal:

For the leftmost red arrow the second trace down is the transmitting trace, and the top trace down is the "victim" (receiving trace) and the right hand pulse they are reversed. We can see on the rise of the "transmitting" signal it causes a spike in the receiving trace, resulting in a unwanted glitch on the logic display, which is what the digital receiver would "see".

In this image at the top we can see signal degradation caused by an incorrectly terminated trace, causing reflections. At the bottom we can see the same signal after it has been correctly terminated:

On the logic analyser, both signals may work, but there is no way of knowing how marginal the first signal is without checking with a scope. The incorrectly terminated trace may only cause problems intermittently, so it's important to be able to check it's integrity.

Looking at your link to the DPScope design, I see it's dsPIC based. It won't be comparable to anything you can buy (you can get a 20MHz analogue scope for << £50 nowadays, and a 5-10MHz DSO for similar)
However, it would be a great project for educational purposes, and you will get something perfectly useable for low frequency (e.g. audio, UART, PWM) purposes. Plus you'll have fun building it. If your thinking of doing so, I'd say go for it, just don't expect it to take care of all your debugging needs. If your budget is limited, get a cheap analogue scope - you will generally get the highest bandwidth for your money.
Remember the chicken and egg problem - you need a scope in order to build and test a scope ;-)