Resistors
Pull-up and Pull-down Resistors
1) Clamping: hold a node in a defined stable state.
2) Drive current: address insufficient bus driving capability. A pull-up resistor can source current; a pull-down resistor can sink current.
3) Improve noise immunity: in CMOS chips unused inputs are often not left floating; connecting them to a pull-up or pull-down provides a discharge path for charge to prevent static damage.
4) Impedance matching: on long transmission lines impedance mismatch causes reflections. Pull-up or pull-down resistors can be used for impedance termination to suppress reflected interference.
Lower resistor values increase the pull-up/pull-down strength but also increase power consumption. Excessively large pull-up resistance may cause slow rising edges; excessively small pull-up resistance may raise the low-level voltage (risking low being detected as high and causing data errors).
Functions of a 0 ohm Resistor
1) Functionally it is a jumper on the printed circuit board used for debugging convenience or compatibility in design.
2) It can be used as a solder jumper; if a circuit branch is not needed, omit the resistor to open the line without affecting board appearance.
3) When circuit parameters are uncertain, use a 0 ohm part as a placeholder; replace it with the actual value after tuning.
4) To measure current in a circuit section, remove the 0 ohm resistor and insert an ammeter for easy measurement.
5) If routing is difficult, a 0 ohm resistor can act as a convenient link.
6) At high frequency a 0 ohm resistor may present inductive or capacitive characteristics (depending on external circuit), useful for addressing EMC issues.
7) Single-point grounding: protect, functional, and DC grounds can be kept separate on equipment to form independent systems.
8) When ground planes are split and the shortest return path is broken, signals may follow longer routes creating large loop areas that increase susceptibility to interference. A 0 ohm jumper across the split can provide a shorter return path and reduce interference.
9) Configuration on products: avoid user-accessible jumpers or DIP switches where possible. To reduce maintenance, use 0 ohm resistors instead of jumpers; an unused jumper can act as an antenna at high frequency, so a chip resistor is preferable.
10) Reserved footprint: place 0 ohm resistors in the same package to allow later replacement with other components such as capacitors, inductors, ferrite beads, or fuses.
1 Ohm Resistor Usage
A 1 ohm resistor is commonly used for current measurement: place a 1 ohm resistor in series with the circuit and measure the voltage across it. Since R = 1, the measured voltage in volts equals the current in amperes (I = V/R).
Capacitors
At very low frequencies the capacitor dominates behavior; at very high frequencies parasitic inductance dominates and the capacitor loses filtering effectiveness.
High-speed coupling capacitors: PCIe designs typically place the coupling capacitor near the transmitter, while SATA often requires the capacitor near the connector. Besides providing DC bias, these capacitors also offer over-voltage protection.
Inductors and Ferrite Beads
Inductors
An inductor is a property of a closed loop. When current flows through a coil it generates a magnetic field; the changing magnetic field induces a current that opposes the change. This opposition is inductive reactance, measured in henries (H).
Ferrite Beads
Ferrite beads are used to suppress high-frequency noise and spikes on signal and power lines and can absorb electrostatic pulses.
Ferrite beads have high resistivity and permeability and are effectively a series combination of resistance and inductance, both varying with frequency. Compared to ordinary inductors they provide better high-frequency attenuation and appear resistive at high frequency, maintaining high impedance over a broad frequency range to improve RF filtering.
For power filtering an inductor can be used. The schematic symbol for a ferrite bead is usually an inductor symbol, but the part is specified as a ferrite bead and the frequency characteristics differ from an inductor.
Note: ferrite bead specifications are given in ohms because they are specified by impedance at a given frequency.
Functions of Ferrite Beads
Ferrite beads remove RF noise present on transmission lines. RF energy superimposed on DC levels is unwanted EMI that travels and radiates along traces. A chip ferrite bead acts as a high-frequency resistor: it allows DC to pass while attenuating AC. Typically this applies to frequencies above about 30 MHz, although lower frequencies can also be affected depending on the bead.
Differences Between Inductors and Ferrite Beads
1. An inductor is an energy storage element; a ferrite bead is an energy-dissipating component.
2. Inductors are often used in power filtering circuits; ferrite beads are typically used on signal lines for EMC mitigation.
3. EMI propagates by radiation and conduction: ferrite beads mainly suppress radiated interference, while inductors focus on conducted interference suppression.
4. Ferrite beads absorb very high-frequency signals and are used in RF circuits, PLLs, oscillators, and high-frequency memory supply inputs. Inductors are energy storage elements used in LC oscillators and low- to mid-frequency filters; inductors rarely are used above about 50 MHz.
5. Inductors are generally used for matching and signal quality control. Ferrite beads are often placed where analog and digital grounds join and on signal lines.
The size and impedance-frequency curve of a ferrite bead depend on the interference frequency to be absorbed. A ferrite bead specified as 1000 ohm @ 100 MHz means it presents about 1000 ohm at 100 MHz. Datasheets typically include impedance vs frequency curves. For example, a part labeled 2012B601 may indicate an impedance of 600 ohm at 100 MHz.
Single-point Grounding: 0 ohm Resistor, Inductor, Ferrite Bead
All grounds eventually connect together and return to earth. If grounds are not connected they can float, develop potential differences, accumulate charge, and cause static issues. Ground is the reference 0 potential for voltages, so ground standards must be consistent and various grounds should be tied together.
Although some boards are not connected directly to earth, supply returns ultimately reference earth. Directly connecting large analog and digital ground areas can cause mutual interference.
Four Ways to Connect Analog and Digital Grounds
1. Use a ferrite bead;
2. Use a capacitor;
3. Use an inductor;
4. Use a 0 ohm resistor.
Comparison
1. A ferrite bead behaves like a frequency-selective attenuator and is effective at specific noise frequencies; select a bead based on the expected noise frequency. For unknown or broadband noise, a bead may be unsuitable.
2. A capacitor blocks DC and passes AC, which can create a floating ground.
3. Inductors are bulky and have many stray parameters, making them less stable.
4. A 0 ohm resistor provides a narrow current path that limits loop current and can effectively reduce noise. Every resistor has some impedance, so a 0 ohm resistor also offers attenuation across frequency bands, which can be advantageous compared to a ferrite bead.
