PCB Layout - Ground is Not Ground: The Art and Science of Printed Circuit Board Design for Barcode Scanners |
Subtitle: A Deep Dive into Component Placement, Routing, Ground Planes, Shielding, and the Critical Role of the Layout in Achieving High Performance - with Real-World Examples from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Analog Devices |
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Opening Summary |
A brilliant schematic is only half the battle. The other half is the PCB layout - the physical arrangement of components and the routing of traces on the printed circuit board. In the world of high-performance analogue electronics, the layout is just as important as the circuit design. A poorly laid out board can turn a perfect schematic into a noisy, unstable, and unreliable mess. The saying 'ground is not ground' captures the essence of the challenge: in a real PCB, every trace has resistance and inductance, and every ground return path can become a source of noise. |
This article is dedicated to PCB layout - the art and science of arranging components and routing traces to achieve the best possible performance. We will explore the fundamental principles of layout for mixed-signal circuits: the separation of analogue and digital grounds, the use of a ground plane, the minimization of loop areas, the placement of decoupling capacitors, and the shielding of sensitive nodes. We will look at how major companies have implemented PCB layouts in their products. We will examine the layout of Symbol's LS2208, which is a classic example of a well-designed two-layer board. We will explore Honeywell's use of a four-layer board with a dedicated ground plane. We will examine Datalogic's layout techniques for high-speed industrial scanners. We will also look at reference designs from Texas Instruments and Analog Devices, which provide detailed layout guidelines. |
By the end of this journey, you will understand that PCB layout is not a black art but a systematic discipline that can be learned and mastered. You will see how the physical design of the board directly affects the electrical performance of the scanner. |
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Full Article |
Section 1: The Fundamental Challenge - Ground is Not Ground |
In a schematic, ground is a single, perfect node with zero impedance. In a real PCB, ground is a network of traces, vias, and planes that have resistance, inductance, and capacitance. When current flows through these impedances, voltage drops are created. These voltage drops can appear as noise at different points on the ground network. This is why 'ground is not ground.' |
The goal of PCB layout is to minimize these ground voltage drops and to ensure that the sensitive analogue circuits are not affected by the noisy digital circuits. |
Section 2: The Two-Layer Board - A Classic Approach |
Many barcode scanners, including Symbol's LS2208, use a two-layer PCB. A two-layer board has a top layer and a bottom layer. The top layer is used for components and signal routing. The bottom layer is often used as a ground plane. |
A two-layer board is cheaper than a multi-layer board. It is also simpler to design and manufacture. The LS2208's two-layer board is a testament to the fact that a high-performance product can be built on a simple board. |
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Section 3: The Four-Layer Board - A Better Approach |
A four-layer board has four layers: top, ground, power, and bottom. The ground and power layers are solid planes. This provides a low-impedance return path for the current and reduces the noise. |
A four-layer board is more expensive than a two-layer board, but it provides better performance. Honeywell's 1900 imager uses a four-layer board. |
Section 4: The Ground Plane - The Foundation |
The ground plane is a solid copper layer that provides a low-impedance return path for the current. The ground plane also provides shielding, reducing the pickup of electromagnetic interference. |
The ground plane must be as continuous as possible. Any breaks or splits in the ground plane can create noise. |
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Section 5: The Separation of Analogue and Digital Grounds |
The analogue and digital grounds must be separated. The analogue ground is the reference for the sensitive analogue circuits. The digital ground is the reference for the noisy digital circuits. The two grounds are connected at a single point (star ground). |
The separation prevents the digital currents from flowing through the analogue ground, which would create noise. |
Section 6: The Star Ground - A Single Connection Point |
The star ground is a single connection point for the analogue and digital grounds. The star ground is typically located near the power supply. All the ground connections from the analogue and digital circuits are routed to the star ground. |
The star ground ensures that the digital currents do not flow through the analogue ground. |
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Section 7: The Component Placement - The First Step |
The component placement is the first step in the PCB layout. The components must be placed to minimize the length of the sensitive traces and to separate the analogue and digital circuits. |
The photodiode and the TIA must be placed as close as possible. The trace from the photodiode to the TIA input must be very short. |
Section 8: The Routing - The Second Step |
The routing is the second step in the PCB layout. The traces must be routed to minimize the loop areas and to keep the sensitive traces away from the noisy traces. |
The sensitive traces (the photodiode input and the TIA output) must be routed away from the digital traces. The digital traces (the comparator output and the microcontroller signals) must be routed away from the analogue traces. |
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Section 9: The Decoupling Capacitors - The Noise Filters |
The decoupling capacitors must be placed as close as possible to the power pins of the integrated circuits. The decoupling capacitors provide a local reservoir of charge and filter out high-frequency noise. |
The decoupling capacitor must be connected with a short, low-inductance path to the power and ground planes. |
Section 10: The Shield Can - The EMI Shield |
The shield can is a metal enclosure that covers the analogue circuits. The shield can prevents electromagnetic interference (EMI) from reaching the sensitive analogue circuits. The shield can also prevents the analogue circuits from radiating EMI. |
The shield can is connected to the ground plane at multiple points. |
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Section 11: Symbol's LS2208 - The Classic Layout |
Symbol's LS2208 uses a two-layer board. The top layer is used for components and signal routing. The bottom layer is used as a ground plane. The analogue and digital circuits are separated. The photodiode and the TIA are placed close together. |
The LS2208's layout is a classic example of a well-designed two-layer board. |
Section 12: Honeywell's 1900 - The Four-Layer Layout |
Honeywell's 1900 imager uses a four-layer board. The four layers are: top, ground, power, and bottom. The ground and power layers are solid planes. The analogue and digital circuits are separated. |
The four-layer board provides better performance than the two-layer board. |
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Section 13: Datalogic's PowerScan - The High-Speed Layout |
Datalogic's PowerScan series uses a high-speed layout. The layout is designed for high-speed signals. The traces are controlled impedance. The ground and power planes are continuous. |
The high-speed layout is required for the high-speed operation of the PowerScan. |
Section 14: The Trace Impedance - A High-Frequency Consideration |
At high frequencies, the trace impedance becomes important. The trace impedance is determined by the trace width, the trace thickness, and the distance to the ground plane. The impedance must be controlled to prevent reflections. |
Controlled impedance is important for the high-speed signals in the PowerScan. |
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Section 15: The Loop Area - The Antenna Effect |
A loop is formed by a signal trace and its return path. The loop area acts as an antenna, radiating and receiving electromagnetic interference. The loop area must be minimized. |
A continuous ground plane minimizes the loop area. |
Section 16: The Crosstalk - The Unwanted Coupling |
Crosstalk is the unwanted coupling of signals between adjacent traces. The crosstalk is caused by the mutual capacitance and inductance of the traces. The crosstalk can be minimized by increasing the spacing between the traces and by using a ground plane. |
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Section 17: The Via - The Interlayer Connection |
A via is a plated hole that connects different layers of the PCB. The via has a small inductance and resistance. The via can be a source of noise if not used carefully. |
The vias must be placed to minimize the inductance. |
Section 18: The Solder Mask - The Insulating Layer |
The solder mask is an insulating layer that covers the copper traces. The solder mask prevents short circuits and provides protection from the environment. |
Section 19: The Silkscreen - The Label Layer |
The silkscreen is a layer that prints the component labels and other information on the PCB. The silkscreen is used for assembly and debugging. |
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Section 20: The Test Points - The Debugging Aids |
The test points are small pads that allow access to the signals for debugging. The test points are placed on the PCB to allow the engineer to measure the signals with an oscilloscope. |
Section 21: The Mounting Holes - The Mechanical Attachment |
The mounting holes are holes that are used to attach the PCB to the scanner's housing. The mounting holes are typically connected to the ground plane. |
Section 22: The Edge Connector - The Interface |
The edge connector is a connector that is used to connect the PCB to the outside world. The edge connector is used for power, ground, and signals. |
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Section 23: The PCB Material - The Substrate |
The PCB material is the substrate on which the copper traces are etched. The most common material is FR-4, a glass-reinforced epoxy laminate. The PCB material affects the dielectric constant and the loss tangent. |
Section 24: The Copper Thickness - The Current Capacity |
The copper thickness determines the current capacity of the traces. The copper thickness is typically 35 micrometers (1 ounce per square foot). For high-current applications, a thicker copper may be needed. |
Section 25: The Trace Width - The Resistance |
The trace width determines the resistance of the trace. A wider trace has a lower resistance. The trace width must be chosen to handle the current. |
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Section 26: The Trace Spacing - The Voltage Rating |
The trace spacing determines the voltage rating of the PCB. The trace spacing must be large enough to prevent arcing. |
Section 27: The Manufacturing Process - The Production |
The PCB is manufactured by a PCB fabrication house. The fabrication house fabricates the PCB according to the design files. The fabrication process includes etching, drilling, and plating. |
Section 28: The Assembly Process - The Populating |
The components are assembled onto the PCB by a PCB assembly house. The assembly process includes solder paste printing, component placement, and reflow soldering. |
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Section 29: The Testing - The Validation |
The assembled PCB is tested for functionality and performance. The testing includes power-on tests, functional tests, and environmental tests. |
Section 30: The ESD Protection - The Layout Consideration |
The ESD protection must be considered in the layout. The ESD protection diodes must be placed as close as possible to the protected pins. The ESD protection must be connected to the ground plane with a low-inductance path. |
Section 31: The EMI Filtering - The Layout Consideration |
The EMI filtering must be considered in the layout. The EMI filters must be placed as close as possible to the input and output connectors. The EMI filters must be connected to the ground plane with a low-inductance path. |
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Section 32: The Thermal Management - The Layout Consideration |
The thermal management must be considered in the layout. The power-dissipating components must be placed to allow heat to be conducted away. Thermal vias can be used to conduct heat to the ground plane. |
Section 33: The Placement of the Photodiode - A Critical Decision |
The placement of the photodiode is a critical decision. The photodiode must be placed as close as possible to the TIA. The trace from the photodiode to the TIA input must be very short. |
Section 34: The Placement of the Microcontroller - A Digital Noise Source |
The microcontroller is a source of digital noise. The microcontroller must be placed away from the analogue circuits. The digital traces must be routed away from the analogue traces. |
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Section 35: The Ground Plane in Texas Instruments' TIDA-00857 |
Texas Instruments' TIDA-00857 reference design uses a two-layer board with a ground plane on the bottom layer. The top layer is used for components and signal routing. The analogue and digital circuits are separated. |
Section 36: The PCB Layout - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for PCB layout for a barcode scanner: |
1. Use a Ground Plane: A continuous ground plane is essential. |
2. Separate Analogue and Digital Grounds: Connect them at a single point (star ground). |
3. Place the Photodiode Close to the TIA: The trace between them must be very short. |
4. Use Decoupling Capacitors: Place them close to the power pins. |
5. Use a Shield Can: Shield the analogue circuits. |
6. Minimize Loop Areas: Keep the signal traces close to the ground plane. |
7. Route Sensitive Traces Away from Noisy Traces: Keep them separate. |
8. Consider the Manufacturing Process: Design for manufacturability. |
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Final Summary |
PCB layout is a critical part of the barcode scanner design. A well-designed layout ensures that the sensitive analogue circuits are not affected by the noisy digital circuits. The layout must minimize ground noise, reduce loop areas, and provide shielding. |
We have seen how major companies have implemented PCB layouts in their products. Symbol's LS2208 uses a classic two-layer board. Honeywell's 1900 uses a four-layer board with a dedicated ground plane. Datalogic's PowerScan uses a high-speed layout. Texas Instruments and Analog Devices provide reference designs with detailed layout guidelines. |
PCB layout is not a black art. It is a systematic discipline that can be learned and mastered. By following the best practices, you can create a layout that ensures the scanner's performance, reliability, and manufacturability. |
cs@easiersoft.com