Harmonizing Circuit Design and EMC DesignPart 8 Evaluation Circuits and Boards (2) Handling of Ground Wires (GND)

Hello! I’m Inagaki, at ROHM.

This 8th article is the second on evaluation circuits and boards. Here I would like to explain methods for connecting the ground wires of evaluation boards for semiconductor integrated circuits (LSIs, ICs) and layout of ground wiring on an evaluation board, and related topics.

I’ll begin with connection of the ground wire of an evaluation board. Essentially, the ground pin of a semiconductor integrated circuit should be thought of as a reference or base point. Depending on the product, there may be multiple ground pins; many of these are shorted together directly below the LSI or IC. On an evaluation board, a ground wire is placed so as to lead from there toward the outside of the board.

The ground voltage that serves as a reference for an evaluation board is the value at the ground pin or pins of the semiconductor integrated circuit (LSI or IC). Remember this much, and it will be useful when performing measurements where accuracy is demanded. In measurements of noise levels, crosstalk and the like (extremely small signals) in particular, attention must be paid to the position at which the ground wire is connected. When using a noise meter or spectrum analyzer, if the ground-side probe is connected at this position, that is, as close as possible to the ground pin of the semiconductor integrated circuit, the measurement will return a value close to the true value.

Next let’s consider a case in which there are multiple evaluation boards and multiple semiconductor integrated circuits (LSIs, ICs). One very common question is,

“Is it alright to keep the ground wires separate?”

The reason for such a question is the opinion that

“there are evaluation boards for an analog system and for a power system, and during testing, not connecting the ground wires, keeping them separate, means noise characteristics will be better.”

But is this really the case?

The answer is “no”. As a basic rule when there are multiple evaluation boards and components to be evaluated, it is extremely important that the ground pins always be shorted together. If they are not connected to each other, the potential difference between the grounds will be indeterminate, and the circuits cannot operate normally.

In the example of this question, short-circuiting is not performed intentionally, but it is surmised that short-circuiting occurs indirectly via some path in the circuit. Even if testing and experiments go well, if mass production is begun in this state, the probability of some problem caused by abnormal operation is extremely high. Hence the way ground wires are handled is extremely important.

Let’s extend things a bit here, and consider a case in which a user wants to create the artwork (layout diagram) for an evaluation board himself.

How should he go about drawing the ground wires for semiconductor integrated circuits (LSIs, ICs) in analog, digital, and power systems?

To sum up, there are two possible solution types.

The first is a solution for a case in which the overall electrical characteristics are given priority. The ground wires for each of the above systems (analog, digital, power) are of course shorted together, but a slit (notch) is inserted between the ground wires of the different systems. Where to short-circuit the ground wires to ground also greatly affects the electrical characteristics, but they can never be kept separate.

The second solution is for a case in which electromagnetic compatibility (EMC) characteristics are prioritized. In this case, instead of putting a slit (notch) between ground wires, the ground pins of the systems are shorted together using a ground plane. Here the paths in which the power supply currents flow and the paths in which ground currents flow are in the same positions, above and below the evaluation board (in the case of a multilayer PCB). In the world of EMC (electromagnetic compatibility), such ground currents are called return currents, and it is important that the layout allows the return currents to flow freely. This is because if there are slits (notches) or the like, currents flow in circumventing paths, the magnetic fields due to power supply currents and ground currents are not cancelled, and electromagnetic interference (unwanted radiation, emission) occurs, causing degradation of characteristics.

It can be exceedingly troublesome to decide whether to prioritize electrical characteristics or EMC (electromagnetic compatibility) characteristics; but EMC (electromagnetic compatibility) is related to unanticipated malfunctions and abnormal operation, and so is important for enhancing product reliability. Thus, there is a need to pay close attention to how ground wires (GND) are handled.

Thank you very much for your attention.