Grounding: The Unseen Architecture of Noise-Free Barcode Reading |
Executive Summary |
This article provides a comprehensive exploration of grounding techniques for barcode readers, with a special focus on the star ground strategy that is essential for maintaining signal integrity in mixed-signal designs. We examine how proper grounding separates sensitive analog circuits from noisy digital components, preventing corruption of the tiny photodetector signals that carry barcode information. Rather than focusing on abstract theory, we ground every concept in practical design examples and recommendations from industry leaders including Texas Instruments, Analog Devices, and leading PCB design software providers. We explore the fundamental separation of analog and digital grounds, the role of the star point in preventing ground loops, the practical layout considerations for implementation, and the key exceptions that require careful attention. The article covers both traditional split-plane approaches and the modern trend toward unified ground planes with careful component placement. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing mixed-signal PCBs for barcode reading applications. |
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Chapter 1: The Problem of Ground Noise |
In any electronic system, the ground is supposed to be a stable reference point---a solid foundation against which all signals are measured. But in reality, the ground is never perfect. Currents flowing through the ground plane create voltage drops, and these voltage drops appear as noise to sensitive circuits. |
This problem is particularly acute in barcode readers, which are classic mixed-signal systems. The analog front end---photodetector, transimpedance amplifier, filters, and comparator---handles signals that can be as small as microvolts. The digital backend---microcontroller, memory, and communication interfaces---switches currents that can be hundreds of milliamps, with edge rates measured in nanoseconds. If these digital currents flow through the same ground path as the analog signals, they create voltage drops that corrupt the analog measurements. |
A Texas Instruments application note explains the objective: 'The objective of any good grounding scheme is to provide a stable reference, without noise and other oscillations, for the IC and its surrounding circuits' . This is the foundation upon which reliable barcode reading is built. |
The Cadence design resource elaborates on why this matters: 'When you have analog and digital components on a PCB, you'll have issues like EMI when the grounding is not executed properly' . A star ground provides the clear separation needed to prevent noise from coupling between different modules. |
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Chapter 2: The Star Ground Concept |
The star ground is a grounding topology where all ground connections from different modules or sections of a circuit are connected to a single central point . The name comes from the visual appearance---like a multi-pointed star, with the ground connections radiating outward from the center. |
The key principle is that the ground paths for different functional sections are kept separate from each other and only meet once at a single point . Despite this separation, all modules share the same ground reference as the power supply, ensuring consistent voltage levels across the system. |
A Texas Instruments application note defines the distinction clearly: 'In star-ground distribution, all reference points are located centrally; however, the source may not be centralized. This method balances the common impedance across all resources' . This is different from single-point distribution, where all reference points originate from the source and each resource has its own uninterrupted ground path. |
The star ground prevents a common problem: ground loops. A ground loop occurs when there is more than one ground return path in the circuit. A large ground loop that goes around the PCB can turn the board into an EMI-radiating antenna . By forcing all grounds to meet at a single point, the star ground eliminates these multiple paths. |
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Chapter 3: Analog and Digital Ground Separation |
In a barcode reader, the analog and digital sections must be separated to prevent digital noise from corrupting the analog signals. The JLCPCB design guide explains the distinction: 'Analog ground is dedicated to analog components and circuits, which handle continuous, varying and low speed signals. Analog ground is essential for maintaining signal fidelity, reducing noise, and preserving the accuracy of analog measurements' . |
Digital ground, by contrast, is dedicated to digital components and circuits, which handle binary, on-off signals. Digital ground provides a reference point for digital signals and manages switching noise . The key point is that analog ground and digital ground are typically kept separate to prevent digital (high speed) noise from contaminating analog signals . |
The separation extends beyond the ground plane itself. Analog and digital components should be physically separated on the PCB to prevent coupling between analog and digital signals. This separation helps reduce crosstalk and interference, ensuring the integrity of both analog and digital signals . |
However, the separation is not absolute. Analog and digital ground planes must still be connected at a single point---the star ground---to ensure a common reference voltage and prevent ground loops . |
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Chapter 4: Why Star Grounding Matters for Barcode Readers |
Barcode readers are particularly sensitive to ground noise because of the tiny signals involved. The photodetector current can be in the nanoamp range, and the transimpedance amplifier must amplify this current with high gain. Any noise on the ground reference can be amplified along with the signal, corrupting the barcode data. |
The Analog Devices guide on grounding mixed-signal devices explains the challenge: 'Most ADC, DAC, and other mixed-signal device data sheets discuss grounding relative to a single PCB... This has been a source of confusion in trying to apply the principles described here to multicard or multi-ADC/DAC systems' . The recommendation is usually to split the PCB ground plane into an analog plane and a digital plane, and then connect them at a single point---the star ground. |
In a barcode reader, all noisy digital currents flow through the digital power supply to the digital ground plane and back to the digital supply. They are isolated from the sensitive analog portion of the board. The system star ground occurs where the analog and digital ground planes are joined together at the mixed-signal device . |
This approach works well for a single PCB system, but it requires careful implementation. As the Analog Devices guide notes, 'In systems having several ADCs or DACs on different PCBs (or on the same PCB, for that matter), the analog and digital ground planes become connected at several points, creating the possibility of ground loops and making a single-point 'star' ground system impossible' . |
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Chapter 5: The Single Connection Point |
The most critical rule of star grounding is that the analog and digital ground planes must be connected at only one point. If there are multiple connections, ground loops can form, allowing noise currents to flow between the planes. |
The ScienceDirect reference on analog ground planes explains: 'Separate grounds does not mean that the grounds are electrically separate in the system. They have to be common at some point, preferably a single, low impedance point' . This single point is the star ground. |
The star ground is often located at the mixed-signal device---the component where analog and digital signals meet, such as an ADC or DAC. In a barcode reader, this might be the microcontroller with its integrated ADC, or a separate ADC chip. |
The Altium design resource notes that 'the idea is that each connection is made at a single central point' . In star grounding, all analog and digital ground connections should terminate at the star ground. This topology reduces the possibility of ground loops and EMI radiation, but it requires careful tracking of the return path . |
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Chapter 6: Practical Implementation |
Implementing a star ground on a PCB requires careful planning. The first step is to decide how many branches of the star are needed in the design. This means separating the components according to their functional modules . |
The Cadence design resource recommends: 'You need to arrange the components by module, but keep space in the center for the common ground connection. The connecting point is often shaped as a polygon and it has to be able to handle the total amount of current flowing through the different modules' . |
Components that belong to the same circuit generally have their ground connected together before routing it to the center of the star. However, there are exceptions. For example, an audio IC may have both analog and digital pins on the same component; in such cases, the ground must be connected only at the center of the star . |
The JLCPCB guide adds another layer of consideration: 'When placing components, attention should be paid to the orientation of traces and ground planes to minimize loop areas and reduce electromagnetic interference (EMI). Keeping traces short and direct can help minimize signal distortion and improve signal integrity' . |
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Chapter 7: The Role of the Ground Plane |
In modern PCB design, a solid ground plane is typically used instead of individual ground traces. The ground plane provides a low-impedance path for return currents and serves as a reference plane for signal propagation . A large, continuous ground plane acts like a very big star ground when there is only one power supply point on the board . |
The Cadence resource explains: 'The ground planes provide a low-impedance path for return currents and serve as reference planes for signal propagation' . This is essential for maintaining signal integrity, especially at high frequencies where parasitic inductance becomes significant. |
However, the ground plane is not a magic solution. The return current from the digital section can still induce noise in the analog section if the planes are not properly separated. The Analog Devices guide notes that when the digital and analog signals operate at high speeds (less than nanosecond rise times) or high frequencies (above megahertz), the return path follows closer to the path of least reactance, which tends to be closer to the components and traces . |
Chapter 8: Modern Approaches: The Unified Ground Plane |
While split ground planes with a star connection are still common, many modern designers recommend using a single, unified ground plane with careful component placement. The Altium design resource argues: 'In a multilayer PCB, or on the back side of a single-layer PCB, large copper regions can be placed on a layer and grounded. The goal is to provide a large region with consistent ground potential' . |
The argument is that physically separated ground planes can create problems. If signals must cross between the analog and digital sections, routing over a gap between ground planes creates an undefined return path with large parasitic loop inductance, which can become a strong emitter and receiver of EMI . |
The unified ground plane approach uses a single ground plane for both analog and digital sections, but the sections are arranged so that return currents from the digital section do not flow through the analog section. This requires careful component placement and routing. As the Altium resource notes, 'the typical strategy is not to separate the ground plane into two physically separated planes. Instead, you should try to arrange the digital ground in one area, and the analog ground in a different area, both of which occupy the same plane' . |
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Chapter 9: The Exception: Mixed-Signal ICs |
A notable exception to the rule of connecting digital ground to the digital plane and analog ground to the analog plane concerns mixed-signal ICs such as ADCs and DACs. The ScienceDirect reference explains: 'The pin names analog ground and digital ground refer to internal connections in the IC, not the plane to which they should be connected. Both should connect to the analog ground plane' . |
The connection would have been made inside the IC, but it is impossible to get low enough impedance at the typical geometries inside ICs. The IC designer actually counts on the end user to supply a low impedance connection outside the IC . |
One might suspect that the digital portions of the converter would make circuit performance worse by coupling digital switching noise onto the analog ground and power plane. However, converter designers realize this and design digital portions without a lot of output power to minimize switching transients . |
The Embedded article on grounding mixed-signal devices adds nuance: 'In systems having several ADCs or DACs on different PCBs (or on the same PCB, for that matter), the analog and digital ground planes become connected at several points, creating the possibility of ground loops and making a single-point 'star' ground system impossible' . |
Chapter 10: Texas Instruments' Star Ground Example |
Texas Instruments provides a practical example of star ground implementation in their TPA32xx amplifier application note. The note describes a star ground connection scheme where input stage ground nodes are connected with separate traces directly to the amplifier's analog ground reference . |
The TI document explains: 'With this star ground connection scheme, G1 G2 G3 and G4 are well coupled and share the same ground potential. Therefore, no noise voltage can be developed between the input stage grounds and the TPA32xx analog ground' . This is the essence of the star ground: ensuring that all grounds share the same potential, preventing noise voltages from developing between them. |
The note also mentions that the supply grounds of the operational amplifiers are not shown to connect to a specific point on the grounding scheme. The high PSRR offered by the op-amp means that ground noise on the supply pins will have little impact on the performance . |
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Chapter 11: Avoiding Overlap Between Planes |
A critical rule in mixed-signal PCB design is that analog and digital ground planes should not overlap. If they overlap, the distributed capacitance between the overlapping portions couples high-speed digital noise with the analog circuitry, defeating the purpose of isolated planes . |
The ScienceDirect reference states: 'Do not overlap digital and analog planes. Place analog power coincident with analog ground and digital power coincident with digital ground. If any portion of analog and digital planes overlap, the distributed capacitance between the overlapping portions couples high speed digital noise with the analog circuitry. This defeats the purpose of isolated planes' . |
This rule applies not only to ground planes but also to power planes. The power and ground planes for each section should be coincident to minimize noise coupling. The reference also notes that 'it is important to isolate the power planes as well' . |
Chapter 12: The Broadcasting Antenna Problem |
One of the most common mistakes in mixed-signal PCB design is routing digital signals over analog ground planes or vice versa. The ScienceDirect reference warns: 'It is important to keep digital signals away from analog portions of the circuit. It makes little sense to isolate planes, keep analog traces short, and place passive components carefully if high speed digital traces are running right next to the sensitive analog traces' . |
The reference includes a vivid description: 'If not, the design will include a new schematic symbol... the broadcasting antenna! Most digital clocks are high enough in frequency that even small capacitances between traces and planes can couple significant noise' . |
The recommendation is that digital signals must be routed around analog circuitry and not overlap analog ground and power planes. It is important to remember that it is not only the fundamental frequency of the clock that can cause a potential problem but also the higher frequency harmonics . |
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Chapter 13: Component Placement Strategy |
The placement of components is critical for effective grounding. Analog components should be located as close as possible to the input/output connections of the board. Digital components should be placed closer to the power supply. The ScienceDirect reference notes: 'It is a good idea to locate analog circuitry as close as possible to the input/output connections of the board' . |
The reason is that long traces from the analog section to the connector can act as antennas, picking up noise from the rest of the board. The reference warns that digital designers, used to high current ICs, may be tempted to make a 50 mil trace run several inches to the analog circuitry, thinking that reducing the resistance in the trace will help get rid of noise. 'What they actually do is create a long, skinny capacitor that couples noise from digital ground and power planes into the op amp, making the problem worse' . |
The JLCPCB guide reinforces this: 'Analog and digital components should be physically separated on the PCB to prevent coupling between analog and digital signals. This separation helps reduce crosstalk and interference, ensuring the integrity of both analog and digital signals' . |
Chapter 14: Connector Grounding |
In systems with multiple PCBs or cables, connector grounding is critical. The connector is one of the few places in the system where all signal conductors must run in parallel; it is therefore imperative to separate them with ground pins to reduce coupling between them . |
The Analog Devices guide explains: 'Multiple ground pins are important for another reason: They keep down the ground impedance at the junction between the board and the backplane. The contact resistance of a single pin of a PCB connector is quite low (on the order of 10 m) when the board is new; as the board gets older, the contact resistance is likely to rise and the board's performance may be compromised' . |
The recommendation is to allocate extra PCB connector pins so that there are many ground connections---perhaps 30 to 40 percent of all the pins on the PCB connector should be ground pins. For similar reasons there should be several pins for each power connection, although there is no need to have as many as there are ground pins . |
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Chapter 15: Ferrite Beads and Schottky Diodes for Ground Isolation |
In some designs, ferrite beads or back-to-back Schottky diodes are used to connect analog and digital ground planes. The Analog Devices guide explains: 'The Schottky diodes prevent large DC voltages or low-frequency voltage spikes from developing across the two planes. These voltages can potentially damage the mixed-signal IC if they exceed 300 mV because they appear directly between the AGND and DGND pins' . |
As an alternative to the back-to-back Schottky diodes, a ferrite bead provides a DC connection between the two planes but isolates them at frequencies above a few MHz where the ferrite bead becomes resistive . |
The guide recommends that the initial board layout should provide for non-overlapping analog and digital ground planes, but pads and vias should be provided at several locations for the installation of back-to-back Schottky diodes or ferrite beads, if required. Pads and vias should also be provided so that the analog and digital ground planes can be connected together with jumpers if required . |
Chapter 16: The Role of Decoupling Capacitors |
Decoupling capacitors play a critical role in maintaining ground integrity. They provide a local energy reservoir for high-frequency currents, preventing these currents from flowing through the ground plane and creating noise. |
The Analog Devices guide explains the mechanism: 'The method... depends on the decoupling capacitor between VD and DGND to keep the digital transient currents isolated in a small loop. However, if the digital currents are significant enough and have components at DC or low frequencies, the decoupling capacitor may have to be so large that it is impractical' . |
In practice, a combination of decoupling capacitors with different values is used to provide effective decoupling across a wide frequency range. Small capacitors (0.1 uF) are placed close to each IC to handle high-frequency transients, while larger capacitors (10 uF or more) provide bulk decoupling for lower frequencies. |
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Chapter 17: The LM25119 Example |
Texas Instruments provides another practical example of star ground implementation in the LM25119 datasheet. The datasheet illustrates the recommended PCB layout with a clear star ground technique . |
The datasheet shows solid ground plane connections with bold lines, high current paths with thick lines, and small signal paths with narrow lines. The small signal paths should be kept away from radiated noise and away from traces that may couple noise capacitively. The points requiring maximum bypassing of high-frequency switching noise should be isolated from the high-frequency switching noise of other channels . |
The datasheet emphasizes: 'Use the star ground PCB layout technique and minimize the length between the grounds' . This is a practical demonstration of the star ground technique in a real power supply design. |
Chapter 18: A Forum Discussion on Ground Plane Splitting |
A practical discussion on the EEWorld forum illustrates the real-world challenges of ground plane design. The forum user asks about the best method for copper layout when analog and digital sections are separated by a 0-ohm resistor . |
One response notes: 'Method 1 focuses more on isolation. Layers (1, 2, 4) are divided by dotted lines to form a completely separated GND, which is suitable for analog-sensitive occasions' . This approach prioritizes isolation between analog and digital sections. |
Another response notes: 'Method 2 has a complete reference plane, so the noise propagation will be less. At the same time, laying digital GND on the 4th layer BOTTOM can enhance the heat dissipation and overcurrent capabilities. There is no need to worry about cross-segmentation problems' . |
A third response adds wisdom: 'Where is the end point of your power supply or signal returnIf it is at the bottom of the digital part, it may be meaningless to divide it like this. Single-point grounding is to minimize the impact on both sides and allow them to return separately. It is not literally just a single point connection. The path and position must be considered' . |
This discussion highlights the importance of considering the return path and the system architecture, not just the layout of the ground planes themselves. |
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Chapter 19: When to Use Star Grounding |
Star grounding is most beneficial in systems where the return currents from low-speed and low-frequency signals follow paths of lower resistance and are harder to track. In these systems, the return currents may pass between the digital and analog sections of a large ground plane, creating interference . |
The Altium resource notes: 'Because the return currents from low-speed/low-frequency signals follow paths of lower resistance and are harder to track, it's possible they will pass between the digital and analog sections of a large ground plane. These signals need to be isolated from each other, thus the star point grounding method' . |
For high-speed and high-frequency designs, a unified ground plane with careful component placement is often recommended. The Altium resource explains: 'When the digital and analog signals operate at respectively high speeds... the return path follows closer to the path of least reactance, which tends to be closer to the components and traces' . This makes the star ground less critical because the return paths are naturally confined. |
Chapter 20: The Return Path and Its Importance |
Understanding the return path is essential for effective grounding. Every signal has a return path---the path that the current takes to return to its source. The return path for a signal is as important as the signal path itself. |
In high-speed designs, the return current follows the path of least inductance, which is typically directly under the signal trace. This is why a solid ground plane is so important: it provides a low-inductance path for the return current, keeping the loop area small and reducing EMI. |
In low-speed designs, the return current follows the path of least resistance, which is the shortest path back to the source. This can cause the return current to flow through any available conductor, potentially crossing between analog and digital sections. |
The Altium resource explains: 'At lower speeds/frequencies, the return path follows closer to a straight line back to the power return point(s). This means that, at lower frequencies, digital return currents from the digital section of the ground plane are at risk of traveling near analog components' . |
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Chapter 21: The Chassis Ground Consideration |
In systems with a metal chassis, the chassis ground adds another layer of complexity. The chassis is often connected to the PCB ground through mounting screws or standoffs, creating additional ground paths. |
The Altium resource warns: 'If you're using the chassis as an additional reference, be careful as capacitive coupling between displacement current in the chassis and other grounds in your board will create common-mode currents that appear as the noise at components' . |
The goal is to ensure that the chassis ground and the PCB ground are at the same potential. When separate grounds have some parasitic capacitance between them, and the potential between these two sections is 0 V, there will be no displacement current induced back into the system from the chassis . |
Chapter 22: The Role of Ground Planes in RFID/Barcode Combo Devices |
A patent for a handheld combination barcode/RFID reader illustrates the practical use of ground planes in real products. The reader includes a printed circuit board with electronic components on one side and a ground plane on the reverse side. The ground plane serves the dual role of providing an electrical ground for the RFID and control circuitry, as well as providing a physical ground connection for the patch antenna assembly . |
The patent notes that 'by utilizing the entire area of the PCB to provide a ground plane for the patch antenna assembly,' the performance of the patch antenna is considerably improved over prior art. The ground plane area is approximately four times greater than the area of the active metal patch . |
This illustrates the importance of a large, continuous ground plane for RF and mixed-signal applications. The ground plane not only provides a stable reference for the electronics but also improves the performance of the antenna. |
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Chapter 23: The Cost of Poor Grounding |
The consequences of poor grounding can be severe. In the Cadence design resource, the author shares a personal experience: 'I've made the mistake of leaving out the star ground in one of my audio projects. The result was a repetitive chirp coupled in one of the audio output channels. Needless to say, the client wasn't pleased and I had to spend hours redesigning the circuit to produce a new set of prototypes' . |
The Cadence resource also notes that 'the worst scenario of having a messy ground connection is that you'll have interference across components and possibly affect other electronic devices' . This can lead to product failures, customer returns, and damage to the company's reputation. |
In a barcode reader, poor grounding can cause intermittent decoding failures, reduced reading range, and susceptibility to EMI. These problems can be difficult to diagnose and fix after the product has been manufactured, which is why proper grounding must be designed in from the start. |
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Chapter 24: Summary --- Grounding in Perspective |
Grounding is the unseen foundation upon which the entire barcode reader is built. A well-designed grounding scheme provides a stable, noise-free reference for the sensitive analog circuits while isolating them from the noisy digital circuits. A poorly designed grounding scheme can corrupt the tiny signals from the photodetector, causing decoding errors and reducing the reader's performance. |
We have examined how different companies and technologies have approached the challenges of grounding in mixed-signal systems: |
Texas Instruments provides practical guidance on star ground implementation in their application notes, emphasizing that the objective is to provide a stable reference, without noise and other oscillations, for the IC and its surrounding circuits. Their TPA32xx application note demonstrates a star ground connection scheme where input stage grounds are connected to the amplifier's analog ground reference . |
Analog Devices offers extensive guidance on grounding mixed-signal devices, including the key exception that the AGND and DGND pins of ADCs and DACs should both connect to the analog ground plane. Their guide addresses the challenges of multicard systems and provides options for ferrite beads and Schottky diodes for ground isolation . |
ScienceDirect provides a comprehensive reference on analog ground planes, emphasizing the importance of not overlapping analog and digital planes and keeping digital signals away from analog sections . |
JLCPCB offers practical guidelines for analog and digital ground separation, component placement, and signal isolation in PCB design . |
Cadence and Altium provide modern perspectives on star grounding, discussing when it is appropriate and the trend toward unified ground planes with careful component placement . |
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The key lessons from our exploration are: |
The star ground connects all grounds at a single point. This prevents ground loops and ensures that all sections of the circuit share the same ground reference. |
Analog and digital grounds must be separated. Digital currents can corrupt analog signals if they share the same ground path. Physical separation of components and ground planes is essential. |
The separation is not absolute. Analog and digital ground planes must still be connected at a single point---the star ground---to ensure a common reference voltage. |
Do not overlap analog and digital planes. The distributed capacitance between overlapping planes couples high-speed digital noise into the analog circuitry. |
Keep digital signals away from analog sections. Digital traces routed over analog ground planes can couple noise through parasitic capacitance. |
Mixed-signal ICs are an exception. The analog and digital ground pins of ADCs and DACs should both be connected to the analog ground plane. |
Consider the return path. The return current for a signal is as important as the signal itself. A solid ground plane provides a low-inductance return path. |
Component placement is critical. Analog components should be placed close to the board's I/O connections, and digital components should be placed close to the power supply. |
Plan for flexibility. Include pads and vias for ferrite beads, Schottky diodes, or jumpers so that the ground planes can be connected or isolated as needed. |
In the end, grounding is a testament to the importance of system-level thinking in barcode reader design. It is a fundamental aspect of the design that touches every other part of the system. The art of grounding lies in the careful balance of isolation and connection, ensuring that the analog circuits have a clean reference while the digital circuits have a low-impedance return path. A well-grounded barcode reader is a reliable barcode reader, and the foundation of that reliability is the star ground. |
cs@easiersoft.com