Common Pitfalls and Fixes: The Practical Challenges of Barcode Scanner Design |
Subtitle: A Deep Dive into the Most Frequent Problems, Their Root Causes, and the Proven Solutions - with Real-World Examples from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Analog Devices |
|
Opening Summar |
Designing a barcode scanner is a complex undertaking. Even with a well-designed schematic and a carefully laid out PCB, things can go wrong. The scanner may oscillate, the signal may be noisy, the comparator may jitter, or the decoder may fail to read certain barcodes. These problems are not signs of failure; they are part of the engineering process. The key is to recognize the common pitfalls and to know the proven fixes. |
This article is dedicated to the common pitfalls and fixes in barcode scanner design. We will explore the most frequent problems, their root causes, and the practical solutions. We will cover oscillation, noise, poor signal integrity, comparator jitter, decoding errors, and power supply issues. We will look at how major companies have addressed these problems in their products. We will examine Symbol's (now Zebra's) fixes for oscillation in the LS2208. We will explore Honeywell's solutions for noise in their imagers. We will examine Datalogic's techniques for improving signal integrity. We will also look at reference designs from Texas Instruments and Analog Devices, which include tips for avoiding common pitfalls. |
By the end of this journey, you will understand that the common pitfalls are not mysteries but well-understood problems with proven solutions. You will be able to diagnose and fix these problems in your own designs. |
|
Full Article |
Section 1: The Oscillation - The Unwanted High-Frequency Signal |
Oscillation is one of the most common problems in barcode scanner design. It occurs when the feedback loop of the amplifier becomes unstable. The amplifier starts to oscillate at a high frequency, creating a signal that is not present at the input. The oscillation can corrupt the barcode signal and cause decoding errors. |
The oscillation is often caused by parasitic capacitance at the inverting input of the TIA. The parasitic capacitance, together with the op-amp's gain, creates a phase shift that turns the negative feedback into positive feedback at high frequencies. |
Section 2: The Fix for Oscillation - The Feedback Capacitor |
The feedback capacitor (Cf) is the primary fix for oscillation. The feedback capacitor is placed in parallel with the feedback resistor. The capacitor creates a pole that reduces the gain at high frequencies, restoring the phase margin. |
The value of the feedback capacitor is critical. A capacitor that is too small will not stop the oscillation. A capacitor that is too large will reduce the bandwidth. The optimal value is typically 2-10 picofarads. |
|
Section 3: Symbol's LS2208 - The Fix for Oscillation |
Symbol's LS2208 uses a feedback capacitor of 2.2 picofarads. This value was chosen to stop the oscillation while maintaining the bandwidth. The LS2208's feedback capacitor is a small, ceramic capacitor. |
Section 4: The Noise - The Unwanted Random Signal |
Noise is another common problem. The noise is random fluctuations in the signal. The noise can be caused by thermal noise, shot noise, or interference. The noise can mask the barcode signal, causing decoding errors. |
Section 5: The Fix for Noise - The Low-Pass Filter |
The low-pass filter is the primary fix for noise. The low-pass filter attenuates the high-frequency noise. The low-pass filter is typically formed by the feedback capacitor of the TIA. The feedback capacitor, together with the feedback resistor, forms a low-pass filter. |
Section 6: The Fix for Noise - The Shielding |
The shielding is another fix for noise. The shielding prevents external interference from coupling into the circuit. The shielding is typically a metal can that covers the analogue circuits. |
|
Section 7: The Fix for Noise - The Decoupling Capacitors |
The decoupling capacitors are another fix for noise. The decoupling capacitors filter out the noise on the power supply. The decoupling capacitors are placed close to the power pins. |
Section 8: Honeywell's Noise Solutions - The Shielding and Decoupling |
Honeywell's imagers use a combination of shielding and decoupling to reduce noise. The analogue circuits are covered by a shield can. The power supply is decoupled with multiple capacitors. |
Section 9: The Poor Signal Integrity - The Distorted Waveform |
Poor signal integrity is a problem where the waveform is distorted. The distortion can be caused by the AC coupling, the gain stage, or the comparator. The distorted waveform can cause decoding errors. |
Section 10: The Fix for Poor Signal Integrity - The AC Coupling |
The AC coupling network must be designed correctly. The corner frequency must be low enough to pass the barcode signal. The capacitor must be of the correct type (low leakage). |
|
Section 11: The Fix for Poor Signal Integrity - The Gain Stage |
The gain stage must be designed correctly. The gain must be set correctly. The op-amp must have sufficient bandwidth. |
Section 12: The Fix for Poor Signal Integrity - The Comparator |
The comparator must be designed correctly. The hysteresis must be set correctly. The comparator must have sufficient speed. |
Section 13: The Comparator Jitter - The Timing Uncertainty |
Comparator jitter is the uncertainty in the edge timing. The jitter is caused by noise on the comparator's input. The jitter can cause decoding errors. |
Section 14: The Fix for Comparator Jitter - The Hysteresis |
The hysteresis is the primary fix for comparator jitter. The hysteresis creates a dead zone around the threshold, preventing the comparator from switching due to noise. |
|
Section 15: The Fix for Comparator Jitter - The Adaptive Threshold |
The adaptive threshold is another fix for comparator jitter. The adaptive threshold tracks the signal's average level, ensuring that the threshold is at the optimal point. |
Section 16: Datalogic's Jitter Solutions - The High-Speed Comparator |
Datalogic's industrial scanners use a high-speed comparator. The high-speed comparator has a fast response and low jitter. |
Section 17: The Decoding Errors - The Wrong Data |
Decoding errors occur when the decoder produces the wrong data. The decoding errors can be caused by poor signal quality, incorrect module width estimation, or incorrect symbology decoding. |
Section 18: The Fix for Decoding Errors - The Module Width Estimation |
The module width estimation must be correct. The histogram method is more robust than the shortest pulse method. The histogram method should be used. |
|
Section 19: The Fix for Decoding Errors - The Symbology Decoder |
The symbology decoder must be correct. The lookup table must be accurate. The decoder must handle the start/stop characters and the checksum. |
Section 20: The Fix for Decoding Errors - The Checksum |
The checksum must be verified. The checksum is an essential error-checking mechanism. |
Section 21: The Power Supply Issues - The Unstable Voltage |
Power supply issues are a common problem. The power supply may have ripple or noise. The power supply may droop under load. |
Section 22: The Fix for Power Supply Issues - The Linear Regulator |
The linear regulator provides a clean, stable voltage. The linear regulator is preferred for the analogue circuits. |
|
Section 23: The Fix for Power Supply Issues - The Decoupling Capacitors |
The decoupling capacitors filter out the ripple and noise. The decoupling capacitors are placed close to the power pins. |
Section 24: The Fix for Power Supply Issues - The Ferrite Bead |
The ferrite bead suppresses high-frequency noise. The ferrite bead is placed in series with the power supply line. |
Section 25: Texas Instruments' TIDA-00857 - The Troubleshooting Guide |
Texas Instruments' TIDA-00857 reference design includes a troubleshooting guide. The guide lists common problems and their solutions. |
Section 26: The Grounding Issues - The Reference Plane |
Grounding issues are a common problem. The ground plane must be continuous. The analogue and digital grounds must be separated. |
|
Section 27: The Fix for Grounding Issues - The Star Ground |
The star ground is a single connection point for the analogue and digital grounds. The star ground prevents ground loops. |
Section 28: The Fix for Grounding Issues - The Ground Plane |
The ground plane is a solid copper layer. The ground plane provides a low-impedance return path. |
Section 29: The Thermal Issues - The Temperature Drift |
Thermal issues are a common problem. The components' values change with temperature. The drift can cause errors. |
Section 30: The Fix for Thermal Issues - The Temperature Compensation |
Temperature compensation is the fix for thermal issues. The compensation adjusts the circuit for temperature changes. |
|
Section 31: The Fix for Thermal Issues - The Low-Temperature-Coefficient Components |
Low-temperature-coefficient components are another fix. These components have a small drift with temperature. |
Section 32: The ESD Issues - The Static Discharge |
ESD issues are a common problem. The static discharge can damage the components. |
Section 33: The Fix for ESD Issues - The ESD Protection Diodes |
The ESD protection diodes clamp the voltage to a safe level. The ESD protection diodes are placed on the input and output pins. |
Section 34: The Fix for ESD Issues - The Spark Gaps |
The spark gaps are another fix. The spark gaps allow the ESD current to arc to the ground. |
|
Section 35: The Fix for ESD Issues - The Enclosure |
The enclosure provides protection against ESD. The enclosure is made of conductive material. |
Section 36: The Common Pitfalls - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for avoiding and fixing common pitfalls in barcode scanner design: |
1. Oscillation: Add a feedback capacitor. |
2. Noise: Use a low-pass filter, shielding, and decoupling capacitors. |
3. Poor Signal Integrity: Design the AC coupling, gain stage, and comparator correctly. |
4. Comparator Jitter: Use hysteresis and an adaptive threshold. |
5. Decoding Errors: Use robust module width estimation and checksum verification. |
6. Power Supply Issues: Use a linear regulator and decoupling capacitors. |
7. Grounding Issues: Use a star ground and a ground plane. |
8. Thermal Issues: Use temperature compensation and low-temperature-coefficient components. |
9. ESD Issues: Use ESD protection diodes and spark gaps. |
|
Final Summary |
The common pitfalls in barcode scanner design are well understood. They include oscillation, noise, poor signal integrity, comparator jitter, decoding errors, power supply issues, grounding issues, thermal issues, and ESD issues. Each pitfall has a proven fix. The fixes include feedback capacitors, low-pass filters, shielding, decoupling capacitors, hysteresis, adaptive thresholds, robust module width estimation, checksum verification, linear regulators, star grounds, ground planes, temperature compensation, ESD protection diodes, and spark gaps. |
We have seen how major companies have addressed these pitfalls. Symbol's LS2208 uses a feedback capacitor to stop oscillation. Honeywell uses shielding and decoupling to reduce noise. Datalogic uses a high-speed comparator to reduce jitter. Texas Instruments and Analog Devices provide reference designs with troubleshooting guides. |
The common pitfalls are not mysteries. They are well-understood problems with proven solutions. By following the best practices, you can avoid these pitfalls and create a robust, reliable barcode scanner. |
cs@easiersoft.com