PCB design: PCB wiring of the eight classic question and answer

One, ask: in small signal circuit in a short section of copper wire has the resistance must not important?

A: printed circuit board PCB conductive belt is relatively wide, the gain error will be reduced. It is generally used in analog circuits as well, but a lot of designers (and printed circuit board design procedures) prefer to use the minimum width of the conductive tape to facilitate the layout of the signal lines. In a word, it is very important to calculate the resistance of the conductive strip and analyze its effect in all possible problems.

Two, asked: before the introduction of the problem of simple resistance, there must be some resistance, its performance is in line with our expectations. What will be the resistance of a section of the wire?

Answer: the situation is not the same. You are referring to a conductor or conductor of a PCB circuit board. Due to room temperature superconductor has been published, so any a metal wire plays low resistance resistor role, it also has the role of capacitance and inductance), so you must consider its impact on the circuit.

Three, ask: the width of the conductive belt with the printed circuit board and the back of the metal layer of the metal layer will have a problem?

Answer: the problem is very small. Although it is important to be a capacitor that is made up of a printed wiring board, it should be estimated first. If there is no such case, even if a wide band formed a large capacitor will not bring the problem. If the problem can be removed, the area of a small ground plane can be removed to reduce the capacitance to the ground.

Four, ask: what is the ground plane?

Answer: if a piece of printed circuit board of the side (or a multilayer printed circuit board of the sandwich) copper foil to earth, then this is what we mean when we say connected plane. The arrangement of any ground to make it as small as possible resistance and inductance. If a system uses a ground plane, it is less likely to be affected by the ground noise. And the ground plane has the function of shielding and heat dissipation.

Five, asked: the ground plane said here is very difficult for manufacturers, right?

A: there are some problems in this area 20 years ago. Today, with the improvement of the adhesive, solder resist and wave soldering technology in the printed circuit, the manufacture of the ground plane has become the common operation of the printed circuit board.

Six, ask: you say a system to use a ground plane to make it suffer from the possibility of noise is very small, left to the ground noise problem still can't solve?

Answer: Although there is a ground plane, but its resistance and inductance is not zero, if the external current source is strong enough, it will affect the precision of the signal. By rational arrangement of printed circuit board, the high current can not flow to the precision of signal generated voltage region. This problem can be reduced to a minimum. Sometimes in the next plane disconnected slots, or can make the grounding current from the sensitive area to change the flow, but forced to change the ground plane can also enable signal a detour into the sensitive area, so this technology must be used carefully.

Seven, ask: how can we know the voltage drop generated in 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 conduction band, but the calculation may be very complex. Voltage in the DC to low frequency (50kHz) range can be used as an instrumentation amplifier. If the amplifier is separated from its power supply, the oscilloscope must be connected to the power supply circuit of the power supply circuit. LED lighting

The resistance between any two points on the ground plane can be added to these two points to be measured. Amplifier gain and the scope of the sensitivity of the oscilloscope can be integrated to measure the sensitivity of 5 V/div. Amplifier noise will increase the width of the oscilloscope waveform curve, is about 3 mu V, but there are still may make the resolution of the measurements to achieve the level of about 1 mu V, which is enough to distinguish most ground noise, and confidence is up to 80%.

Eight, ask: how to measure the high frequency ground noise?

A: it is difficult to measure the high frequency ground noise using a suitable broadband instrument amplifier, so it is more suitable to use high frequency and very high frequency passive probe. It consists of ferrite ring (diameter 6 ~ 8mm), there are two coil magnetic ring, each coil is 6 ~ 10 turns. In order to form a high frequency isolation transformer, a coil is connected to the input end of the spectrum analyzer, and the other coil is connected to the probe. The test method is similar to the low frequency, but the frequency spectrum analyzer is used to represent the noise. This is different from the time domain characteristics, the noise source can be easily distinguished according to their frequency characteristics. In addition the use of spectrum analyzer sensitivity at least than the use of broadband oscilloscope high 60dB.


 

(Source: Inside information)

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