In this tutorial we are going to learn about WHAT IS GROUND PLANE IN PCB DESIGN ?
Good grounding is a system level design consideration. Proper grounding should be planned into the
product from the first conceptual design reviews.
Separate grounding for analog & digital portions of circuitry is one of the simplest & most effective
methods for noise suppression. One or more layers on multi-layer PCB’s are usually devoted to GND
plane. If PCB designer will not take carefull then the analog circuitry will be connected directly to these “GND” planes. Auto-routers respond accordingly–& connect all of the GNDs together, creating a disaster. GND & PWR planes are at the same AC potential, due to decoupling capacitors & distributed capacitance. Therefore, its important to isolate the PWR planes as well. Don’t overlap digital & analog planes. Place analog PWR coincident with analog GND, & digital PWR coincident with DGND. If any portion of analog & digital planes overlap, then generated capacitance between the overlapping portions will couple high- speed digital noise into the analog circuitry. This defeats the purpose of isolated planes.
“Separate GNDs” doesn’t mean that the GNDs are electrically separate in the system. They have to be common at some point, preferably a single, low impedance point. System-wise, there is only one GND – the electrical safety GND in an AC powered system or battery GND in a DC powered system. Everything else “returns” to that GND. All “returns” should be connected together at a main point, which is system “GND”. At some point, this will be the chassis. It is important to avoid GND loops by multiple connections to the chassis. Insuring only one chassis GND point is one of the most difficult aspects of system design. If at all possible, dedicate separate connector pins to separate returns, & combine the returns only at system GND. Aging & repeated mating causes connector pins to increase in contact resistance, so several pins are needed. Many digital boards consist of many layers & 100s or 1000s of nets. The addition of one more net is seldom an issue, but the addition of a several connector pins almost always is. If this can’t be done, then it will be necessary to make the 2 returns a single net on the PCB – with very special routing precautions. In PCB design should be keep analog traces short, Isolate Plane & place passive components carefully if there are high-speed digital traces running right next to the sensitive analog traces. Digital signals must be routed around analog circuitry, & not overlap AGND & power planes. Most digital clocks are high enough in frequency that even small capacitances between traces & planes can couple significant noise. Remember that it is not only the fundamental frequency of the clock that can cause a potential problem, but also the HF harmonics. Genrally locate analog circuitry as close as possible to the I/O connections of the PCB. Digital designers, used to high current IC’s, will be tempted to make a 50 mil trace run several inches to the analog circuitry, Skinny capacitor that couples noise from digital ground & power planes into the opamp, making the problem worse! one entire side of a PCB (or one entire layer, in the case of a multi-layer PCB) consists of continuous copper which is used as GND this is known as a “GND plane.” It will have the least possible resistance & inductance of and GND configuration. If a system uses a GND plane, it is less likely to suffer GND noise problems