PCB Static effect and interference in PCB
Printed circuit boards (PCB) comprise of single or multi insulating layers and one or more copper layers which contain the signal traces and powers lines. The design of printed circuit boards is important as the design of the electrical circuit. Modern systems consist of up to 8 layers. No matter how carefully the board was designed there are few practical difficulties that keep on showing. Let’s see PCB static effect and mutual interference in detail.
- PCB Static effect
Current leaking through PCB to sensitive trace lines by means of leakage resistance is generally referred to as PCB Static effect. PCBs that are contaminated with chemical residues and waste debris particles from manufacturing process can offer a conductive path between the traces and which in turn disrupts the signal. Since digital systems are mostly immune to this effect most of the sensitive analog systems are vulnerable. Usually a few nano amperes of current can be found near the supply line above 12V. Which is not much but this can cause significant error in the inputs to devices such as Op-Amp.
To illustrate the effect let us consider the following example. If a leakage current of 5nA is dumped into the sense rail of op amp with impedance of 10MΩ then the Op-Amp reds a jump in 50mV so it may result in error up to few percentages considering the input range is between few volts.
1.1 Solution to PCB static effect
A simple solution to this problem is to rinse the board with Isopropyl alcohol to get rid of the unwanted wastes. Though this does not guarantee 100% success in order to eliminate this problem the problem should be rectified in the design stage by introducing guard trace
This method introduces a guard line between the sensitive leads to ground the leaking currents. The guard trace must cover the most of the input leads to eliminate the possibility of leaking. The following example shows a comparator configured to set a primary first stage FET high oh appropriate input signal. The figure also illustrates the guard trace surrounding the input terminals of the Op-Amp.
Fig 1.1.1 Illustrating Guarding
- Mutual induction
Even though inductance is a major component in electronic circuits unwanted inducted inductance can lead to inefficient system. Mutual inductance occurs when two closed circuit loops of which one carrying alternative current is placed sufficiently closer the other loop an EMF is induced in the second one.
In order to reduce the inducted current few practical methods can be used
2.1 Reducing effective closed loop area
By reducing the area of the loop as illustrated by the figure induced current can be significantly reduced.
Fig 2.1 Reducing total loop area