Why the Analog & Digital Ground to be connected separate in the design?

In this tutorial we are going to learn about Why the Analog & Digital Ground to be connected separate in the 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/more layers on multilayer PCBs usually are devoted to Ground planes. When the PCB designer is not careful, the analog circuitry will be connected directly to these “Ground” planes. The analog circuitry return, after all, is the same net in the net list as digital return. Auto-routers respond accordingly, connecting all of the grounds together—creating a disaster.
  • Ground & Power planes are at the same AC potential, due to decoupling caps & distributed capacitance.
  • Very important to isolate the power planes as well. Don’t overlap digital & analog planes. Place analog power coincident with analog ground & digital power coincident with digital gnd. When any situation some part of analog & digital planes overlaps, the distributed capacitance between the overlapping portions will couple high- speed digital noise into the analog circuitry. This defeats the purpose of isolated planes.
  • Separate ground doesn’t mean that the ground is electrically separate in the system. They have to be common at some point preferably a single, low-impedance point. In the system there is only one ground— the electrical safety ground in an ac-powered system or the battery ground in a dc-powered system. Everything else “returns” to that gnd. Refer to everything that is not a ground as a “return.” All returns should be connected together at a single point, which is system “gnd.” At some point, this will be the chassis. It’s important to avoid ground loops by multiple connections to the chassis Insuring only one chassis ground 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 cause 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 several connector pins almost always is. If this cannot be done, then it will be necessary to make the two returns a single net on the PCB—with very special routing precautions.
  • Most important to keep digital signals away from analog portions of the circuit. It makes little sense to isolate planes, keep analog traces short, and 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 analog ground & power planes.
  • Most digital clocks are high enough in frequency that even small capacitances between traces & planes can couple significant noise. Remember that its not only the fundamental frequency of the clock that can cause a potential problem but also the HF harmonics.
  • Best guideline for designer to locate analog circuitry as close as possible to the I/O connections of the board.
  • If design the high-current ICs then it will be tempted to make a 50-mil trace run several inches to the analog circuitry—thinking that reducing the resistance in the trace will help get rid of noise. What will actually result is a long, skinny capacitor that couples noise from digital ground & power planes into the op amp, making the problem worse!
  • Generally to keep digital runs away from analog runs. The fast rise times of digital signals cause them to more easily couple into adjacent runs. With 3.3V or 5V logic, a 0.2V spike from an adjacent digital signal won’t cause a problem, but that same 0.2V spike will wreak havoc in an analog circuit. The longer the distance that two runs are run in parallel or the closer they are to each other, the more coupling will occur. Such runs can even couple through adjacent layers of a circuit board. It is best to try to keep runs on adjacent layers at roughly right angles to each other. This technique usually facilitates easier layout & reduces crosstalk. Be aware that sometimes radiation from adjacent circuit boards can couple into some analog circuits & may require shielding or more careful layout. It has been shown that using copper fill between areas of signal runs reduces crosstalk & radiation if the copper fill is grounded (never leave it ungrounded). Also note that some digital runs (especially clocks) can couple into the VREF pins of some A/D converters, causing “strange lumps” in the spectrum of the noise floor of the A/D. Keep all connections to VREF pins very short & isolated from digital runs. In some designs, it may be necessary to run parallel sets of runs for long distances.
  • Be sure to design the configuration of these conductors carefully to reduce the effects of crosstalk between the conductors. In some cases, it may be necessary to intersperse some ground runs between other runs to act as shielding. Also remember to terminate &, in some cases, back-terminate digital runs to control overshoot & undershoot if the runs have significant length (more than a few centimeters).

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