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Developing Prototype Circuits from Home: Part 5, Grounds, References, and Returns

This blog is Part 5 in a series that initiates designing prototype circuits from home. Developing Prototype Circuits from Home: Part 1, Introduction introduced the concept whereas Developing Prototype Circuits from Home: Part 2, Schematic Capture addressed schematic capture (using National Instrument’s Multisim3 and LTSpice 5 ). In Developing Prototype Circuits from Home: Part 3 Board Layout, the emphasis was on board layout using National Instrument’s Ultiboard 4 and Developing Prototype Circuits from Home: Part 4, Suggestions for Effectively Routing a Board

In this fifth segment, the subject of grounding, references and returns will be addressed. Often times a return path is treated like the drains and sewer lines exiting a building or home. Once the electrons (waste) are in the ground (sewage), they are out of sight and out of mind. For circuits, this can cause issues especially in sensitive signals such as control loops. Even at the gate drive level, interactions of currents can cause false signals.

Ground is typically earth ground. For the most part, it’s not meant to carry current (except in fault situations). Instead, ground is for providing a zero voltage reference point. In utility power, ground is earth ground. In power supplies, it’s the third terminal from the AC wall. For the most part, ground is free from any circuit current. When even harmonics exist, currents may traverse ground paths. The advent of power factor correction has controlled harmonics to a point where even harmonics rarely occur.

References are common points in a schematic. In a power supply, references can have three or more signal levels. As shown in the arrows in the figure, these returns can be the high power of the current returning to the input, the medium power to the gate drive, and the sensitive returns for the error amplifier and the comparator. Some amplifiers are so sensitive, even and adjacent, low signal level comparator can affect its performance.

Smart layout involves separating the return references in a manner that keeps signals from interacting with each other. Many integrated circuits now feature power and signal returns. These are tied together on the board in one place. The goal is for the return paths not to share the power and signal return current. Every attempt must be made to keep them separate. A star point grounding system helps to separate signals as shown in the following figure.

Returns can use individual traces or “ground planes”. The term “ground plane” is often used loosely as circuit board planes are more references or returns than actual grounds. Regardless of whether the return uses traces or a plane, the star point ground method works well.

There is a simple method for analyzing the return paths. It can be done using a computer or print outs. Using the printed method involves printing out the schematic and the board layout. Then with colored pencils or crayons, trace the signal path on the schematic and then on the corresponding traces and planes on the board layout. This will not only show the ground paths, it will also show loop areas and potential layout issues involving key components. This method is also possible in many of the more advanced software packages where signals can be highlighted.

When using the printed assessment method, ground plane signals are easy to trace. The signal will enter the ground plane from the component and take a straight line to either the power source or a decoupling capacitor. In the case of a gate drive, it will go from the transistor source to the gate drive’s power return pin. The ground plane acts as a large parking lot with a drain in the middle of it. The water will fall from all of the components and take a direct line to the drain. Drawing these lines along the direct path is the way to see if any signals have overlapping return paths.

Summary

Grounds, returns, and references are key factors in proper circuit operation. Analysis of the routes in relation to critical signals can save a lot of headaches by avoiding signal interference and reducing overall system noise.

References

  1. Developing Prototype Circuits from Home: Part 1, Introduction
  2. Developing Prototype Circuits from Home: Part 2, Schematic Capture
  3. National Instruments Multisim schematic design and evaluation tool
  4. National Instruments Ultiboard board layout tools
  5. LTSpice
  6. Eagle Schematic Capture and Board Layout Software website Eagle Schematic Capture and Board Layout Software website
  7. You Need a Power Supply Designed by When?
  8. Developing Prototype Circuits from Home: Part 3 Board Layout
  9. Digital Isolator Design Guide (Rev. A), Texas Instruments, 2009
  10. Considerations in Designing the Circuit Board of Embedded Power Supplies

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