Well Grounded, Digital Is Analog

文章摘要：The Importance of the Ground PlaneSo far we have spoken of ground as if it is a low-impedance sink for everything, and that the interfering signals are single frequencies. Neither is true. Remember that the fast edges of switching power supplies and digital logic circuits create high-frequency harmo

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The Importance of the Ground Plane

So far we have spoken of ground as if it is a low-impedance sink for everything, and that the interfering signals are single frequencies. Neither is true. Remember that the fast edges of switching power supplies and digital logic circuits create high-frequency harmonics which must be suppressed. Ground is also problematic; seldom is it anywhere near perfect.

It is incorrect to envision ground alone without the complementary power planes or buses. The current must make a complete circle to complete the circuit. However in this note, we will consider only the ground plane and ignore the power planes for simplicity.

It sounds uncharacteristic, even funny, but thinking like an electron does help in this discussion (Figure 4). Electrons by nature are lazy. They always take the easiest path. Water is also a useful analogy. When we observe floodwaters, we see the water spread across the flood plain if it is not constrained in some way. Current behaves the same way in a voltage or ground plane.


Figure 4. A caricature of an electron on a ground plane.

Figure 5a imagines the electrons playing on the ground plane. A close look reveals the "good guys" (the returning signal currents) in the upper left and "bad guys" at the lower right. Some electrons know their place and keep to it. Some are confused, and others can be bought (or co-opted) by the highest bidder or most persuasive force.


Figure 5a. Electrons abound without clear guidelines.

These electrons are free to move over the ground plane. As they travel toward the star point, lower left, these good and bad electron currents can, and do, mix together. (Yes, we know electrons flow the opposite way, but we want the analogy to be simple and to match the concept of water flow.) Noise and interference are the unwanted contamination of one current into another. As the noisy digital electrons flow past the analog circuits, they will induce noise into the linear analog circuit, and vice versa. It is thus quite clear that all circuits are degraded by bad power and grounds. Maxim parts are no exception.

The MAX5400/MAX5401 digital potentiometers can have one side connected to the power bus or ground. Noise on the power or ground would be directly added to the signal. Voltage references like the MAX6133 have a line regulation specification, which indicates the amount of input voltage change that will be reflected in the output voltage. The MAX5532–MAX5535 family of 12-bit DACs has a power-supply rejection ratio (PSRR) of more than 20dB at 1kHz. This means that noise on the power bus will be reduced by a ratio of more than ten times. Nonetheless, having a part that reduces power-supply noise is no excuse for poor power design management on the ground. Good design demands good power.

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