The Complete Analogue Schematic - A Design Example: Building the Front-End of a Barcode Scanner from the Ground Up |
Subtitle: A Deep Dive into the Component Selection, Circuit Topologies, and Practical Considerations for a Real-World Barcode Reader - with Examples from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Analog Devices |
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Opening Summary |
Throughout this series, we have explored each stage of the barcode scanner's signal chain in isolation: the photodetector, the transimpedance amplifier, the AC coupling, the gain stage, the adaptive threshold, and the comparator. Now, it is time to put it all together. This chapter presents a complete analogue schematic - a detailed design example that integrates all the stages into a working front-end. This is not a theoretical exercise but a practical design that could be used in a real product, based on the best practices of the industry's leading companies. |
This article is dedicated to the complete analogue schematic. We will walk through each stage, explaining the component choices, the design trade-offs, and the practical layout considerations. We will base our design on the TIDA-00857 reference design from Texas Instruments, which is a proven, well-documented example of a barcode scanner front-end. We will also draw on design principles from Symbol's LS2208, Honeywell's 1900 imager, and Datalogic's PowerScan series. We will discuss the selection of the photodiode, the op-amps, the resistors, and the capacitors. We will examine the power supply and the PCB layout. We will also discuss testing and debugging. |
By the end of this journey, you will have a complete understanding of how a barcode scanner's analogue front-end is designed and built. You will see how the individual stages work together to transform a faint light signal into a clean, digital pulse. |
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Full Article |
Section 1: The Design Goals - A Practical, Robust, and Cost-Effective Front-End |
Before we start selecting components, we must define our design goals. Our target is a general-purpose barcode scanner front-end that can be used in a handheld device. The design should be robust, reliable, and cost-effective. It should be able to read standard 1D barcodes (Code 39, UPC, Code 128) at typical hand-scanning speeds. It should have a good depth of field and be able to handle moderate variations in label contrast and ambient light. |
Our design will be based on the TIDA-00857 reference design from Texas Instruments. This design is a proven, well-documented example of a barcode scanner front-end. It uses readily available components and has a clear, straightforward topology. |
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Section 2: The Block Diagram - An Overview of the Signal Chain |
The complete analogue front-end consists of the following stages: |
1. Photodetector: A PIN photodiode (OSRAM SFH 203 P) that converts the reflected light into a current. |
2. Transimpedance Amplifier (TIA): A TLV272 op-amp with a 1-megaohm feedback resistor and a 2.2-picofarad feedback capacitor. This converts the photodiode's current into a voltage. |
3. AC Coupling (High-Pass Filter): A 2.2-microfarad capacitor and a 10-kilohm resistor. This removes the DC offset from the TIA's output. |
4. Gain Stage: A TLV272 op-amp (the second half of the dual op-amp) configured as a non-inverting amplifier with a gain of 68. This amplifies the AC signal. |
5. Adaptive Threshold: A peak detector circuit using a BAT54S Schottky diode, a 1-microfarad capacitor, and a 1-megaohm resistor. This generates a threshold that tracks the signal's average level. |
6. Comparator: A TLV3501 comparator with a built-in hysteresis of 15 millivolts. This converts the analogue signal into a digital square wave. |
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Section 3: The Photodetector - Selecting the PIN Photodiode |
We will use the OSRAM SFH 203 P PIN photodiode. This is a classic choice for barcode scanners. It has a fast response time (5 nanoseconds), a low capacitance (2 picofarads at 5 volts), and a good responsivity (0.6 A/W at 650 nm). It is also inexpensive and widely available. |
The photodiode will be reverse-biased at 5 volts. This reduces the capacitance and increases the speed. The reverse bias is provided by the TIA's virtual ground. |
Section 4: The Transimpedance Amplifier - The First Hero |
The TIA is built around the TLV272 op-amp. The TLV272 is a CMOS, rail-to-rail op-amp with a low input bias current (1 picoampere), a moderate voltage noise (19 nV/(v)Hz), and a gain-bandwidth product of 3 MHz. It is a cost-effective, reliable choice. |
The feedback resistor is 1 megaohm. This gives a gain of 1 volt per microampere. The feedback capacitor is 2.2 picofarads. This stabilizes the amplifier and sets the bandwidth to about 72 kHz. |
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Section 5: The AC Coupling - Removing the DC Offset |
The AC coupling network consists of a 2.2-microfarad capacitor and a 10-kilohm resistor. The corner frequency is about 7 Hz. This is low enough to pass the barcode signal (which has frequencies down to 50 Hz) and high enough to reject the DC offset and low-frequency drift. |
The capacitor must be a low-leakage type. A ceramic X7R capacitor is a good choice. |
Section 6: The Gain Stage - The Second Hero |
The gain stage is a non-inverting amplifier built around the second half of the TLV272. The gain is 68, set by a 100-kilohm feedback resistor and a 1.5-kilohm ground resistor. The gain stage amplifies the AC signal from a few millivolts to about 1.36 volts peak-to-peak. |
The gain stage's output is a clean, amplified version of the barcode signal. |
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Section 7: The Adaptive Threshold - The Tracking Reference |
The adaptive threshold is generated by a peak detector circuit. The peak detector uses a BAT54S Schottky diode, a 1-microfarad capacitor, and a 1-megaohm resistor. The BAT54S is a dual Schottky diode in a small SOT-23 package. It has a low forward voltage drop (about 0.3 volts), which minimizes the error in the peak detection. |
The positive peak detector captures the signal's peak (white level). The negative peak detector captures the signal's valley (black level). The outputs are combined through a resistor divider to produce the midpoint voltage. |
Section 8: The Comparator - The Decision Maker |
The comparator is the TLV3501. This is a high-speed comparator with a propagation delay of 5 nanoseconds and a built-in hysteresis of 15 millivolts. The built-in hysteresis simplifies the circuit and ensures clean switching. |
The comparator's output is a digital square wave. The high level represents a white space. The low level represents a black bar. |
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Section 9: The Power Supply - The Quiet Heart |
The entire analogue front-end is powered by a 5-volt supply. The supply must be clean. A linear regulator (LDO) is used to provide a stable, low-noise 5-volt rail. |
The LDO is decoupled with a 10-microfarad tantalum capacitor and a 100-nanofarad ceramic capacitor. Each op-amp has its own 100-nanofarad decoupling capacitor placed as close as possible to the power pin. |
Section 10: The PCB Layout - The Art of the Design |
The PCB layout is critical for the analogue front-end. The layout must minimize parasitic capacitance, reduce noise pickup, and ensure stability. |
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. A ground plane is used under the analogue section. The analogue and digital grounds are separated and connected at a single point (star ground). A shield can is placed over the photodiode and the TIA. |
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Section 11: The Component Selection - A Detailed BOM |
The bill of materials (BOM) for the analogue front-end is as follows: |
- U1: TLV272 (dual op-amp) |
- U2: TLV3501 (comparator) |
- D1: BAT54S (dual Schottky diode) |
- PD1: OSRAM SFH 203 P (PIN photodiode) |
- R1: 1 megaohm, 1%, 0603 (TIA feedback resistor) |
- R2: 10 kilohms, 1%, 0603 (AC coupling resistor) |
- R3: 100 kilohms, 1%, 0603 (gain stage feedback resistor) |
- R4: 1.5 kilohms, 1%, 0603 (gain stage ground resistor) |
- R5, R6: 1 megaohm, 1%, 0603 (peak detector resistors) |
- C1: 2.2 picofarads, COG, 0603 (TIA feedback capacitor) |
- C2: 2.2 microfarads, X7R, 0805 (AC coupling capacitor) |
- C3, C4: 1 microfarad, X7R, 0603 (peak detector capacitors) |
- C5, C6, C7: 100 nanofarads, X7R, 0603 (decoupling capacitors) |
- C8: 10 microfarads, tantalum, 1206 (bulk decoupling capacitor) |
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Section 12: The TIDA-00857 Reference Design - A Proven Example |
The design we have just described is based on the TIDA-00857 reference design from Texas Instruments. The TIDA-00857 is a complete, documented example of a barcode scanner front-end. It includes a detailed schematic, a PCB layout, and a firmware example. |
The TIDA-00857 is an excellent starting point for any engineer designing a barcode scanner. |
Section 13: Symbol's LS2208 - A Comparison |
Symbol's LS2208 uses a similar topology but with some differences. The LS2208 uses the TLV2371 op-amp (a single version of the TLV272). The feedback resistor is 470 kilohms, and the gain is 68. The LS2208 uses an LM393 comparator with external hysteresis. |
The LS2208's design is simpler and less expensive. It is a testament to the classic design's robustness. |
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Section 14: Honeywell's 1900 - A More Integrated Approach |
Honeywell's 1900 imager uses a more integrated approach. The analogue front-end is integrated into the CMOS sensor chip. The sensor includes a programmable gain amplifier (PGA) and an ADC. The adaptive threshold is calculated in the digital domain. |
The integrated approach is more complex but offers better performance and flexibility. |
Section 15: Datalogic's PowerScan - A High-Performance Design |
Datalogic's PowerScan series uses a high-performance analogue front-end with a fully differential TIA and an automatic gain control (AGC). The differential topology provides better common-mode rejection and a higher signal-to-noise ratio. |
The PowerScan's design is more expensive but is optimized for industrial applications. |
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Section 16: The TIA - A Deeper Look |
The TIA is the most critical stage. The choice of the op-amp, the feedback resistor, and the feedback capacitor determines the TIA's gain, bandwidth, and noise performance. |
The TLV272 is a good, cost-effective choice. For higher performance, the AD8615 from Analog Devices could be used. |
Section 17: The Gain Stage - A Deeper Look |
The gain stage is a non-inverting amplifier. The gain is set by the ratio of the feedback resistor to the ground resistor. The gain must be high enough to bring the signal to the comparator's input range but not so high that the signal clips. |
The gain of 68 is a good compromise. |
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Section 18: The Adaptive Threshold - A Deeper Look |
The adaptive threshold is generated by the peak detectors. The peak detectors capture the signal's peak and valley. The threshold is the midpoint between the peak and valley. |
The time constant of the peak detectors must be carefully chosen. A time constant of 1 second is typical. |
Section 19: The Comparator - A Deeper Look |
The comparator is the final stage. The comparator converts the analogue signal into a digital square wave. The hysteresis prevents chattering. |
The TLV3501's built-in hysteresis simplifies the design. |
Section 20: The Power Supply - A Deeper Look |
The power supply must be clean. The LDO provides a stable, low-noise 5-volt rail. The decoupling capacitors filter out the high-frequency noise. |
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Section 21: The PCB Layout - A Deeper Look |
The PCB layout is critical. The photodiode and the TIA must be placed close together. The trace from the photodiode to the TIA input must be short. A ground plane is used. The analogue and digital grounds are separated. |
Section 22: The Testing - Validating the Design |
The analogue front-end must be tested. The tests include: |
Gain: Measure the TIA's gain and the gain stage's gain. |
Bandwidth: Measure the frequency response. |
Noise: Measure the output noise. |
Pulse Response: Apply a test pulse and measure the response. |
Stability: Check for oscillation. |
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Section 23: The Decoder Interface - Connecting to the Digital World |
The comparator's output is connected to a digital input pin of the microcontroller. The microcontroller's timer/capture module measures the pulse widths. |
The firmware decodes the pulse widths using the algorithm described in earlier chapters. |
Section 24: The Optical Design - The Lens and the Aperture |
The analogue front-end must be paired with an optical system. The optical system includes a lens and an aperture. The lens focuses the image of the barcode onto the photodiode. The aperture controls the amount of light and the depth of field. |
Section 25: The Illumination - The Light Source |
The scanner also includes an illumination source. The illumination source is usually an LED or a laser. The illumination source is controlled by the microcontroller. |
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Section 26: The Enclosure - The Mechanical Design |
The enclosure protects the electronics. The enclosure includes a window for the light to pass through. The window may include an optical filter. |
Section 27: The ESD Protection - Protecting the Input |
The photodiode input is susceptible to ESD. A clamping diode network is used to protect the input. |
Section 28: The EMI Filtering - Reducing Interference |
The power supply and the signal lines may be affected by EMI. EMI filters are used to reduce the interference. |
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Section 29: The Cost Optimization - A Cost-Effective Design |
The design must be cost-effective. The component selection must balance performance and cost. |
Section 30: The Manufacturing - Producing the Scanner |
The scanner must be manufactured efficiently. The PCB is assembled by a pick-and-place machine. The enclosure is molded. The scanner is calibrated and tested. |
Section 31: The Calibration - Adjusting the Scanner |
The scanner is calibrated at the factory. The calibration adjusts the gain, the offset, and the adaptive threshold. |
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Section 32: The Debugging - Finding the Problems |
If the scanner does not work, the engineer must debug the circuit. The debugging involves measuring the signals at each stage with an oscilloscope. |
Section 33: The Variations - Customizing the Design |
The design can be customized for different applications. The gain, the bandwidth, and the threshold can be adjusted. |
Section 34: The Future - Integration and Digitization |
The future of barcode scanning is integration and digitization. The analogue front-end is being integrated into the sensor chip. The signal processing is being moved to the digital domain. |
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Section 35: The Complete Analogue Schematic - A Final Look |
We have now completed our design example. We have selected the components, designed the circuit, and discussed the layout and testing. This design is a practical, robust, and cost-effective solution for a barcode scanner front-end. |
Section 36: The Complete Analogue Schematic - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for designing a complete analogue front-end for a barcode scanner: |
1. Use a PIN Photodiode: The PIN photodiode is the standard choice. |
2. Use a TIA with a Low-Noise Op-Amp: The TIA is the most critical stage. |
3. Use AC Coupling: The AC coupling removes the DC offset. |
4. Use a Gain Stage: The gain stage amplifies the signal. |
5. Use an Adaptive Threshold: The adaptive threshold is essential for reliable decoding. |
6. Use a Comparator with Hysteresis: The comparator converts the signal to a digital pulse. |
7. Use a Clean Power Supply: The power supply must be clean. |
8. Design the PCB Carefully: The PCB layout is critical. |
9. Test the Design: The design must be tested thoroughly. |
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Final Summary |
The complete analogue front-end of a barcode scanner is a carefully engineered system that integrates multiple stages: the photodetector, the transimpedance amplifier, the AC coupling, the gain stage, the adaptive threshold, and the comparator. Each stage has a specific function, and each stage must be designed to work with the others. |
We have based our design on the TIDA-00857 reference design from Texas Instruments, which is a proven, well-documented example. We have also drawn on design principles from Symbol, Honeywell, and Datalogic. We have selected the components, designed the circuit, and discussed the layout and testing. |
The complete analogue schematic is the heart of the barcode scanner. It transforms a faint light signal into a clean, digital pulse that can be decoded by the microcontroller. |
cs@easiersoft.com