0 ohm resistor or ferrite bead to isolate digital ground and analog ground?

Jacktang

0 ohm resistor or ferrite bead to isolate digital ground and analog ground? [Copy link]

The reason for dividing into digital ground and analog ground

Since digital signals are generally rectangular waves with a large number of harmonics, if the digital ground and analog ground in the circuit board are not separated from the access point, the harmonics in the digital signal can easily interfere with the waveform of the analog signal. When the analog signal is a high-frequency or strong electric signal, it will also affect the normal operation of the digital circuit.

Analog circuits involve weak signals, but digital circuits have higher threshold levels, so the power supply requirements are lower than those of analog circuits. In a system with both digital and analog circuits, the noise generated by the digital circuit will affect the analog circuit, making the small signal indicators of the analog circuit worse. The solution to this problem is to separate the analog ground and the digital ground.

The root cause of the problem is that it is impossible to ensure that the resistance of the copper foil on the circuit board is zero. Separating the digital ground and the analog ground at the access point is to minimize the common ground resistance between the digital ground and the analog ground.

In circuit design, should 0 ohm resistors or magnetic beads be used to isolate digital ground and analog ground?

The analog ground and digital ground are single-point grounded. As long as they are grounds, they must be connected together and then into the earth. If they are not connected together, it is a "floating ground" with a voltage difference, which is easy to accumulate charge and cause static electricity. The ground is the reference 0 potential, and all voltages are derived from the reference ground. The ground standards must be consistent, so various grounds should be short-circuited together.

People believe that the earth can absorb all charges and always maintain stability, and 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 on the board will eventually return to the power plant and enter the ground.

If the analog ground and digital ground are directly connected over a large area, it will cause mutual interference. It is also not appropriate not to short-circuit them. There are four ways to solve this problem for the following reasons:

1. Connect with magnetic beads

2. Use capacitors to connect, using the principle that capacitors block direct current and allow alternating current

3. Use inductance to connect, usually a few uH to tens of uH

4. Connect with 0 ohm resistor

Capacitors block direct current and allow alternating current, resulting in floating ground. Inductors are large in size, have many stray parameters, and are unstable. The following mainly discusses magnetic beads and 0 ohm resistors.

Magnetic beads

Magnetic beads are made of ferrite materials with good impedance characteristics in the high frequency band. They are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb static pulses. Magnetic beads have high resistivity and permeability, which is equivalent to resistance and inductance in series, but the resistance and inductance values change with frequency.

It has better high-frequency filtering characteristics than ordinary inductors and exhibits resistance at high frequencies, so it can maintain a higher impedance within a fairly wide frequency range, thereby improving the FM filtering effect.

Ferrite beads have a greater blocking effect on high-frequency signals. The general specifications are 100 ohms/100mMHZ. Its resistance is much smaller than its inductance at low frequencies. Ferrite beads are a rapidly developing anti-interference component. They are cheap, easy to use, and have a significant effect in filtering out high-frequency noise.

Ferrite beads can not only be used to filter high-frequency noise in power supply circuits (can be used for DC and AC output), but can also be widely used in other circuits, and their size can be made very small. Especially in digital circuits, since pulse signals contain high-frequency high-order harmonics, which are also the main source of high-frequency radiation in circuits, magnetic beads can play a role in this occasion. In the circuit, as long as the wire passes through it. When the current in the wire passes through, the ferrite has almost no impedance to the low-frequency current, but will have a greater attenuation effect on the higher-frequency current.

The equivalent circuit of the magnetic bead is equivalent to a band-stop limiter, which can only significantly suppress the noise at a certain frequency. When using it, it is necessary to estimate the noise frequency in advance in order to select the appropriate model. For situations where the frequency is uncertain or unpredictable, magnetic beads are not suitable.

0 ohm resistor

In general, using 0 ohm resistor is the best choice:

• Ensures equal DC potential
• Single point grounding to limit noise
• It has an attenuation effect on noise of all frequencies. 0 ohms also have impedance, and the current path is narrow, which can limit the passage of noise current.

0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress the noise. The resistor has an attenuation effect in all frequency bands (0 ohm resistor also has impedance), which is stronger than the ferrite bead.

When used for current loops during bridging, when the ground plane is divided, the shortest return path of the signal is broken. At this time, the signal loop has to take a detour, forming a large loop area. The influence of the electric and magnetic fields becomes stronger, making it easy to interfere/be interfered with.

Connecting a 0 ohm resistor across the partition can provide a shorter return path and reduce interference. Generally, the configuration circuit should not have jumpers and dip switches on the product. Sometimes users will mess with the settings, which may cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should be soldered on the board instead of jumpers.

The vacant jumper is equivalent to an antenna at high frequencies, and the chip resistor has a good effect. Other uses: temporarily replace other chip devices as temperature compensation devices during cross-line debugging/testing during wiring.

Most of the time, it is due to the need for EMC countermeasures. In addition, the parasitic inductance of a 0 ohm resistor is smaller than that of a via, and the via will also affect the ground plane (because the via needs to be dug). Large-sized 0 ohm resistors can also be used as jumpers, and wires can be routed in the middle. Different sizes of 0 ohm resistors allow different currents to pass through, generally 1A for 0603 and 2A for 0805.

Therefore, different sizes will be selected for different currents. When reserving space for ferrite beads, inductors, etc., they must be packaged according to the size of the ferrite beads and inductors. Different sizes such as 0603 and 0805 are available. 0 ohm resistors are generally used in mixed signal circuits to reduce mutual interference between the digital and analog parts of this circuit.

Their power ground wires are laid separately, but they need to be connected together at the power entry point, usually through a 0 ohm resistor. This not only achieves no voltage difference between the digital ground and the analog ground, but also uses the parasitic inductance of the 0 ohm resistor to filter out the interference of the digital part to the analog part.