How to properly connect the ground. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network
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Leads
Since pickups are a source of unwanted signals, they can be classified as noise. Protection against interference can be achieved through proper grounding, careful mounting location, and shielding. The task of proper shielding can be quite complex, but can be reduced to three simple principles (which are by no means easy to follow): 1) The shield conductor should be connected to the signal reference zero (signal ground) only once (When shielding low-frequency noise in the near field When shielding radio interference, it is better to choose grounding experimentally); 2) shield and signal ground must be grounded at the power supply at the same physical point; 3) all conductors carrying the signal must be placed in the screen. Proper wiring should avoid excessive crowding, long signal paths with or without shielding, and unnecessary wire crossings. In other words, follow good design practice.
Grounding
Poor grounding tends to cause the circuitry to pick up unwanted signals. It is not difficult to formulate the principle of proper grounding, but sometimes it is difficult to follow this principle. This principle can be stated as follows: the ground, through which the load current flows to the power supply, should be carried out by a separate wire, and not combined with the signal ground wire.
On fig. 1 shows an example of the implementation of this principle. The point of this grounding is that often the load current is many times greater than the signal current. The load current, flowing even through wires of a sufficiently large cross section, can cause a voltage drop (IR) on the ground line. This will cause a voltage change on the voltage reference lines of each op amp connected to this ground line. In extreme cases, this change in potential can amount to several millivolts and can be a source of significant error.
Rice. 1. Correct grounding scheme (click to enlarge)
A and B (in circles) - ground wires through which a small current flows; C and D (in circles) - ground wires through which a large current flows; 3 - grounded output.
More about grounding
There are two techniques that are not taken seriously even by those who seem to know - this is grounding and shielding. The main reason for the complications that can arise in this area lies in the formation of so-called feedback loops through the ground.
Rice. 1. A situation leading to severe consequences - ubiquitous ground currents (a); correct grounding (b).
Such a circuit is created by currents flowing in the ground conductor. This current creates a voltage drop that appears to the system as a signal. This situation is shown in Fig. 1a, where the signal ground, the common conductor to the power supply from the amplifier, and the ground conductor of the power supply itself are connected to different currents at the ground surface.
Rice. 2. The method of connecting the ground of analog and digital circuits on a printed circuit board.
The ground surface may be a massive thick piece of copper, or the ground itself, or a large sheet of metal, but even then there is no certainty that the problem has been solved. If the "ground" is a thin wire, then you will almost certainly encounter the problem of feedback circuits through the ground.
In the case shown in Fig. 2, a large current (a "big" current of only a few milliamps) causes a voltage drop between points B and C. This can be interpreted as a DC signal at the input of the amplifier. Although we may think of the input signal as being between point A and the input line of the amplifier, the actual input to the amplifier is voltage relative to point C.
We often talk about the need for grounding, but, apparently, there are also cases when there are too many grounding points. The best situation is a single ground point, as shown in fig. 1b. In this case, the formation of feedback loops through the ground is excluded or at least significantly hampered.
Rice. 2. shows how this can be done when designing a pattern of printed conductors on a printed circuit board. Ground conductors from various circuits converge to one point on the end connector of the board. For higher currents, two parallel leads are used.
Literature
- L. Folkenberry "Application of operational amplifiers and linear ICs"
- J. Kar "Design and manufacture of electronic equipment" M., "Mir" 1980
Publication: cxem.net
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