Eight classic questions and answers about PCB layout

I. Q: In a small signal circuit, the resistance of a short copper wire must not be important?

A: The conductive tape of the printed PCB circuit board is made wider, and the gain error is reduced. It is preferable to use a relatively wide conductive strip in an analog circuit, but many printed circuit board designers (and printed circuit board design programs) prefer to use a conductive strip of a minimum width to facilitate the arrangement of signal lines. In short, it is very important to calculate the resistance of the conductive strip and analyze its effect wherever there is a problem.

Second, Q: In the previous question about the simple resistance, there must be some resistance, and its performance is completely in line with our expectations. What is the resistance of a piece of wire?

A: The situation is different. You are referring to a length of wire or a piece of conductive tape in a PCB board that acts as a wire. Since room temperature superconductors have not yet been introduced, any piece of metal wire acts as a low-resistance resistor (it also has the function of capacitance and inductance), so its influence on the circuit must be considered.

Eight classic questions and answers about PCB layout

3. Q: Is there a problem with the capacitance of the conductive strip with too much width and the metal layer on the back of the printed circuit board?

A: The problem is very small. Although the capacitance made up of the conductive strips of the printed circuit board is important, it should always be estimated first. If the above situation does not exist, even a wide conductive strip forms a large capacitance and does not cause a problem. If there is a problem, remove the area of ​​a small ground plane to reduce the capacitance to ground.

Fourth, ask: What is the ground plane?

A: If the copper foil of the entire side of a printed circuit board (or the entire interlayer of a multilayer printed circuit board) is used for grounding, then this is what we call the ground plane. Any ground wire should be arranged to have as little resistance and inductance as possible. If a system uses a ground plane, it is less likely to be affected by ground noise. And the ground plane has the function of shielding and heat dissipation.

V. Q: The ground plane mentioned here is very difficult for the manufacturer, right?

A: There are some problems in this area 20 years ago. Today, the fabrication of ground planes has become a routine operation of printed wiring boards due to improvements in adhesives, solder resists, and wave soldering techniques in printed circuits.

6. Q: You said that a system uses a ground plane to make it less likely to suffer from ground noise. What remains behind the ground noise problem?

A: Although there is a ground plane, its resistance and inductance are not zero. If the external current source is strong enough, it will affect the precise signal. This problem can be minimized by properly arranging the printed circuit board so that large currents cannot flow to areas that affect the ground voltage of the precision signal. Sometimes breaking or slitting on the ground plane can cause large ground currents to change from sensitive areas, but forcing a change in the ground plane also allows signals to bypass the sensitive area, so such techniques must be used with care.

Q: How can I know the voltage drop generated on a ground plane?

A: Usually the voltage drop can be measured, but sometimes it can be calculated based on the resistance of the ground plane material and the length of the conductive strip through which the current passes, but the calculation can be complicated. An instrumentation amplifier can be used for voltages ranging from DC to low frequency (50 kHz). If the ground of the amplifier is separated from its power ground, the oscilloscope must be connected to the power ground of the power circuit used. Led lighting

The resistance between any two points on the ground plane can be measured by adding the probe to these two points. The combination of amplifier gain and oscilloscope sensitivity results in a measurement sensitivity of 5μV/div. The noise of the amplifier will increase the width of the oscilloscope waveform by approximately 3μV, but it is still possible to achieve a resolution of approximately 1μV, which is sufficient to discriminate most ground noises with a confidence level of 80%.

8. Q: How is the high frequency grounding noise measured?

A: It is difficult to measure high frequency ground noise using a suitable wideband instrumentation amplifier, so it is appropriate to use high frequency and very high frequency passive probes. It consists of a ferrite magnetic ring (outer diameter of 6 to 8 mm) with two coils on the magnetic ring, each of which is 6 to 10 inches. To form a high frequency isolation transformer, one coil is connected to the input of the spectrum analyzer and the other coil is connected to the probe. The test method is similar to the low frequency case, but the spectrum analyzer uses the amplitude-frequency characteristic curve to represent the noise. This is different from the time domain characteristics, and noise sources can be easily distinguished based on their frequency characteristics. In addition, the sensitivity of the spectrum analyzer is at least 60 dB higher than that of a wideband oscilloscope.

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