Is the digital circuit isolated from the analog circuit with 0 ohms or magnetic beads?

Q: Does the circuit design use a 0 ohm resistor or a magnetic bead to isolate the digital ground and the analog ground?

I made a test board. I don't know if I should use a 0 ohm resistor or a magnetic bead to isolate the digital ground from the analog ground.

The crystal oscillator on the board is: 24MHz, 50MHz, 27MHz, etc., the board inlet voltage is 5V, the chip demand voltage rail: 3.3V, 2.5V, 1.5V, 1.2V.

Please expert guidance!

A: 0 ohm resistor

Ground ground in analog ground and digital ground. As long as it is ground, it will eventually be connected together and then into the earth. If they are not connected together, they are "floating", there is a pressure difference, and it is easy to accumulate electric charges and cause static electricity. The ground is referenced to 0 potential, all voltages are derived from reference ground, the ground standards are the same, so the various grounds should be shorted together.

It is believed that the earth can absorb all the electric charges and always maintain stability, which is the ultimate ground reference point. Although some boards are not connected to the earth, the power plant is connected to the earth, and the power supply on the board will eventually return to the power plant.

If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. There are four ways to solve this problem: 1. Connect with magnetic beads; 2. Connect with capacitors; 3. Connect with inductors; 4. Connect with 0 ohm resistors.

The equivalent circuit of the magnetic bead is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For frequencies that are uncertain or unpredictable, the beads do not match. The capacitor is connected straight to the ground, causing floating. The inductor is bulky, has many stray parameters, and is unstable.

The 0 ohm resistor is equivalent to a very narrow current path, which effectively limits the loop current and suppresses noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads.

When the jumper is used for the current loop, when the electric ground plane is divided, the shortest return path of the signal is broken. At this time, the signal loop has to bypass, forming a large loop area, and the influence of the electric field and the magnetic field becomes stronger, and it is easy to interfere. / Being disturbed.

By connecting a 0 ohm resistor across the partition, a shorter return path can be provided to reduce interference. Configuration circuit In general, do not have jumpers and dip switches on the product. Sometimes the user will tamper with the settings, which may cause misunderstanding. In order to reduce the maintenance cost, the 0 ohm resistor is used instead of the jumper to solder on the board.

The vacant jumper is equivalent to the antenna at high frequencies, and the effect of the chip resistor is good. Inter-route debugging/testing for other purposes wiring temporarily replaces other SMD devices as temperature compensation devices

More often than not, it is due to EMC countermeasures. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the via, and the via also affects the ground plane (because the hole is to be dug). Large size 0 ohm resistors can also be used as jumpers. The middle can be traced and there are different sizes of 0 ohm resistors that allow different currents to pass, generally 0603 for 1A, 0805 for 2A.

Therefore, different currents will be selected in different sizes. When the position is reserved for the magnetic beads and inductors, the package must be packaged according to the size of the magnetic beads and the inductor. 0503, 0805 and other different sizes have 0 ohm resistors. In a circuit for mixing signals, in order to reduce mutual interference between the digital portion and the analog portion.

Their power ground wires are separated, but they need to be connected together at the entrance point of the power supply. They are usually connected by a 0 ohm resistor, which achieves no voltage difference between the digital ground and the analog ground, and uses 0. The parasitic inductance of the ohmic resistor filters out the interference of the digital portion to the analog portion.