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Guide to grounding, powering complex circuits

Posted: 27 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:EMI? noise interference? linear regulator? USB connector? GND?

The final system demands and complexities of mobile and stationary devices are becoming evermore sophisticated as electronics applications continue to become increasingly compact, powerful and versatile. This complexitywhich demands wireless and wired interconnectivity of analogue and digital circuitsrequires system engineers to use multiple power rails and mixed disciplines of circuit design. Circuits with analogue and digital signals tend to cause declaration of several ground references, often leading to a spaghetti-like result, where ideas are distorted and what appear to be solid solutions turn out to be chaotic failures.

In order to put engineering foundations back into complex systems, it is imperative that power and grounding solutions are proactively engineered in a manner that optimises performance and heat dissipation while reducing EMI radiation and signal to noise interference. This article demonstrates how to optimise complex circuits from the point of view of power delivery, improved signal integrity and properly grounded functional blocks to implement the final system. The focus is on understanding circuit needs and pre-planning for the final system, because the result of those two steps is a project that effectively moves from the schematic to the final printed circuit board. By taking the time during the design stage to consider each block of a complex system from the current path and noise susceptibility point of view, then placing blocks and powering circuits based on the simple axiom that current always flows in a loop, the complexity faced by today's system engineers can be broken down into manageable pieces, and implemented into a final, robust design.

To demonstrate the theory, let's examine a simple circuit and consider the shown connections. This basic circuit consists of three elements, a low-drop out (LDO) linear regulator, a micro-processing USB data-to-audio driver, and a speaker. All are powered by a USB plug connected to some computing host. In this example, the USB-to-audio driver must be powered by 3.3V. Since the speaker is powered by the audio driver output, and the audio driver input needs the +3.3V LDO which is powered by the USB connector (+5V), it seems an obvious conclusion to place them on the board just like the schematic in figure 1(a) shows.

However, with this configuration, the current making the speaker play will create a voltage bounce while it returns to the driver that sourced the current. That voltage bounce will, in turn, flow back to the LDO and finally to the USB connector. In this example, the reference voltage converting the USB data to music will bounce at the rate the music plays. The phase shift due to the speaker inductance will increase the error and this will be compounded by higher volumes due to increased current levels. The bounce also will cause ripple that will degrade the sound quality from the speaker.

There are a couple of ways to minimise the impact of the ripple current. One is to reduce the ripple by adding a capacitor (C1) very close to the USB to Audio IC from the VLDO node to the GND pin such that the capacitor is centred between these nodes. The ripple reduction should be aimed at the frequencies of interest, in this case the audible range

This will reduce the ripple to DC. Then the current only causes a voltage drop, and it will not vary with time as much (?t from above should be considered as an average of the audible frequency, 12-14kHz). Error can then be controlled by using wide power and GND connections between each IC to limit the voltage drop (product of current times resistance) governed by Ohms' Law.

Figure 1: A simple circuit showing power circuit causes bounce and must return to source.

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