There is one class of signals we have not yet covered, and that is those signals which pass within the unit from one board to another. Typically these are digital control signals or analog levels which have already been processed, so are not low-level enough to be susceptible to ground noise and are not high current enough to generate significant quantities of it. To be thorough in your consideration of ground return paths, these signals should not be left out: the question is, what to do about them?
Often the answer is nothing. If no ground return is included specifically for inter-board signals then signal return current must flow around the power supply connections and therefore the interface will suffer all the ground-injected noise Vn that is present along these lines.
But, if your grounding scheme is well thought out, this may well not be enough to affect the operation of the interface. For instance, 100 mv of noise injected in series with a CMOS logic interface which has a noise margin of 1 V will have no direct effect. Or, AC noise injection onto a DC analog signal which is well-filtered at the interface input will be tolerable.
There will be occasions when taking the long-distance ground return route is not good enough for your interface. Typically these are:
where high-speed digital signals are communicated, and the ground return path has too much inductance, resulting in ringing on the signal transitions;
when interfacing precision analog signals which cannot stand the injected noise or low-voltage DC differentials.
If you solve these headaches by taking a local inter-board ground connection for the signal of interest, you run the risk of providing an alternative path for power supply return currents, which nullifies the purpose of the local ground connection. A fraction of the power return current will flow in the local link (Figure 1.13), the proportion depending on the relative impedances, and you will be back where you started.
If you really need the local signal return, but are in trouble with ground return currents, there are two options to pursue:
Separate the ground return (Figure 1.14) for the input side of the interface from the rest of the ground on that PCB. This has the effect of moving the ground noise injection point inboard, after the input buffer, which may be all that you need. A development of this scheme is to include a “stopper” resistor of a few ohms in the gap X–X. This prevents DC ground current flow because its impedance is high relative to that of the correct ground path, but it effectively ties the input buffer to its parent ground at high frequencies and prevents it from floating if the inter-board link is disconnected.
Use differential connections at the interface. The signal currents are now balanced and do not require a ground return; any ground noise is injected in common mode and is cancelled out by the input buffer. This technique is common where high-speed or low-level signals have to be communicated some distance, but it is applicable at the inter-board level as well. It is of course more expensive than typical single-ended interfaces since it needs dedicated buffer drivers and receivers.
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