Understanding grounding structures are important to enable your circuits to work well and sometimes even to work at all. Proper grounding of circuits is very important to achieving the desired results. As a young engineer, I remember a saying by one of the senior engineers that has stuck with me my whole career. He said, “Ground is a cruel joke.” This one phrase captures the intent of this blog. I would like to discuss grounding and its effects on circuits. Ground is not always ground. Ground can be at zero volts, or it may be an analog ground where the ground potential is volts above earth ground. Such rises in potential can be intentional or unwanted.
These changes in ground are often the cause of reduced performance and sometimes can cause your circuits to not work. So the question is how we avoid grounding issues. I will provide some of my thoughts on the subject, but realize that, just as with my last blog, Capacitors: Bigger Is Not Always Better, to effectively make your grounding system work, you must understand where sensitivities occur in your circuits. With that being said, let's discuss some possibilities to avoid grounding issues.
The first thing to remember is that ground is a relative term, and under specific conditions, ground may not be what you anticipated. Under specific unwanted conditions during operation of your circuit or another circuit on your board, your circuit ground may not provide the needed current path. This condition can be the result of ground rise from currents combined into a single point ground, or it can result from the fact that the grounds used for your circuits are frequency dependent. Therefore, it is best to think of ground as a complex impedance and model it as such when assessing ground effects in your circuits.
As I said, grounding is frequency dependent, and it is important to remember this when setting up grounds for return currents. In addition, it is important to understand the frequencies presented in the circuit you are developing and how the ground for this circuit will react with the other grounds on the board and/or integrated circuit. Furthermore, it is important to understand the various ground paths that exist inside your system. The system can be an integrated IC or a development on a printed circuit board. Ground paths determine where currents flow and influence the interference you might see in your circuits or how your circuits may influence other circuits in your system.
Remember that the current must return to its source, and the current will take the lowest impedance ground path you provide, so provide one that does not influence circuits in its path. This means that the return current ground paths you provide must be over the frequency bandwidth where your circuit shows sensitivity, as well as over the bandwidth of interest to your system performance criteria.
Shown in Figures 1 and 2 are two commonly implemented grounding systems. Figure 1 shows a ground where a single point is the common ground point for many circuit blocks. In this scenario, multiple currents share a single common ground and can result in unwanted ground rise that can affect your circuit blocks. As I said, ground is frequency dependent. Therefore, a signal with high frequency transients can not only cause your circuits that share the ground to experience these transients but possibly result in unwanted mixing of these frequency components into your circuit's bandwidth of interest. Therefore, in cases where you are aware of sensitive circuits to mixing, it would be best to implement your ground system as shown in Figure 2, where each ground is separated and connected to a common ground plane.
As shown in Figure 2, the best ground is a ground plane, provided the plane is truly the common ground for your system. The use of such a plane provides an infinite source of recombination for holes and electrons. When you do not provide such a situation, currents can travel in unwanted directions. With these simple ideas in mind, I have outlined a small checklist to keep in mind when setting up your grounding system:
- Keep ground and signal leads as short as possible.
- Clock signal loop areas should be kept as close to zero as possible.
- Return current path must be kept local and short. Follow the ideas outlined in my last blog on capacitors, and use local decoupling.
- Long static lines can pick up noise and need to be kept as short as possible.
- Do not share grounds that present high frequency transients with other sensitive circuit grounds.
- If possible, use a package that provides low inductance to ground and be aware of the pins that present the most inductance.
- For choice of package pins, choose pins for grounds that provide the best and lowest impedance for the circuits that are sensitive.
- Understand frequencies in your system, and avoid ground loops that can be a source of coupling into your ground or transmission of your signals to other circuits.
One last item: Whether it is designing a board for implementing circuits or floor planning an integrated circuit, the path of current is important. The idea of separate ground planes in board design does not always provide the best outcome. Sometimes it won't work and will seriously degrade performance. As I stated in my last blog, if you can keep the circulating currents local to the block, this is the best situation for grounding, and here separate grounds may be more appropriate.
Do you have other items that I have missed for providing appropriate grounds in your circuits?
Have you had any experiences where the grounding system put in place caused unwanted distortion or interference into your circuits?