The Decision Maker: How the Comparator Turns Analogue Waves into Digital Truth |
Subtitle: A Deep Dive into the Circuit that Draws the Line Between Black and White - with Real-World Designs from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Analog Devices |
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Opening Summary |
After the transimpedance amplifier has converted the photodetector's current into a voltage, after the AC coupling network has removed the DC offset, and after the gain stage has amplified the signal to a usable level, the barcode signal is still an analogue waveform - a smooth, continuous voltage that rises and falls with the black and white patterns. The next stage must make a binary decision: is this part of the signal black or whiteThis is the job of the comparator, the circuit that acts as the scanner's decision maker. |
The comparator is a 1-bit analogue-to-digital converter. It takes the amplified signal and compares it to a threshold voltage. If the signal is above the threshold, the comparator outputs a high voltage (logic '1'), representing a white space. If the signal is below the threshold, it outputs a low voltage (logic '0'), representing a black bar. The output is a clean, digital square wave that faithfully represents the barcode's pattern. |
This article is dedicated to the comparator - its design, its optimization, and its critical role in the barcode scanner. We will explore the different types of comparators, from the classic LM393 to high-speed devices like the TLV3501. We will examine the key parameters: propagation delay, input offset voltage, hysteresis, and output drive capability. We will look at how major companies have implemented the comparator in their products. We will see how Symbol (now Zebra) used the LM393 in the LS2208, and why they chose that particular device. We will explore Honeywell's use of a comparator with integrated hysteresis, and Datalogic's use of a high-speed comparator for fast scanning. We will also look at how Texas Instruments and Analog Devices provide reference designs with carefully selected comparators. |
By the end of this journey, you will understand that the comparator is not just a simple switch but a carefully engineered component that must balance speed, accuracy, and noise immunity. You will see how the choice of the comparator, the setting of the threshold, and the addition of hysteresis all contribute to the scanner's ability to make a clean, reliable decision on every bar and space. |
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Full Article |
Section 1: The Comparator's Mission - From Analogue to Digital |
The comparator is the bridge between the analogue world of the photodetector and the digital world of the microcontroller. Its job is to take a continuously varying voltage and turn it into one of two distinct states: high or low. This is a binary decision, and it must be made accurately and quickly. |
The comparator is a differential amplifier with a very high gain. It has two inputs: the non-inverting input (+) and the inverting input (-). The output is a digital signal. If the voltage at the non-inverting input is higher than the voltage at the inverting input, the output is high (close to the positive supply voltage). If the voltage at the non-inverting input is lower than the voltage at the inverting input, the output is low (close to ground). |
In a barcode scanner, the amplified signal from the gain stage is applied to one input, and a threshold voltage is applied to the other input. The comparator's output is high when the signal is above the threshold (white space), and low when the signal is below the threshold (black bar). This output is a digital square wave that can be processed by the microcontroller. |
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Section 2: The Threshold - The Decision Point |
The threshold is the voltage level that separates the white spaces from the black bars. It is the decision point. The threshold must be set correctly. If the threshold is too high, some white spaces will be interpreted as black bars. If the threshold is too low, some black bars will be interpreted as white spaces. |
In the simplest scanner, the threshold is a fixed voltage, typically half the supply voltage. This works if the signal is symmetric and has a constant amplitude. However, the signal amplitude can vary due to different label contrasts, different scanning distances, and different ambient light levels. A fixed threshold is not sufficient for reliable operation. |
The solution is an adaptive threshold, as we discussed in earlier chapters. The adaptive threshold tracks the signal's average level. It is set to the midpoint between the signal's maximum (white) and minimum (black) levels. This ensures that the threshold is always at the optimal point, regardless of the signal's amplitude. |
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Section 3: The Adaptive Threshold - A Review |
The adaptive threshold is generated by a peak detector circuit, as described in the earlier articles. The peak detector measures the peak voltage of the signal (the white level) and the valley voltage (the black level). The threshold is set to the average of the peak and valley. This is often done with two peak detectors and a resistive divider. |
The adaptive threshold circuit is usually placed before the comparator. It generates a DC voltage that is proportional to the midpoint of the signal. This DC voltage is applied to the comparator's inverting input (if the signal is applied to the non-inverting input). The comparator's output then changes state when the signal crosses this adaptive threshold. |
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Section 4: The Comparator's Speed - Propagation Delay |
The comparator's speed is measured by its propagation delay - the time between the input crossing the threshold and the output changing state. For a barcode scanner, the propagation delay must be short enough to accurately capture the barcode's edges. |
The LS2208 uses an LM393 comparator. The LM393 has a typical propagation delay of about 1 microsecond. This is adequate for hand-scanning, where the pulses are hundreds of microseconds long. The delay causes a small shift in the edge timing, but the shift is constant and can be compensated for by the decoder. |
For high-speed scanning (e.g., on a conveyor belt), a faster comparator is needed. The TLV3501, from Texas Instruments, has a propagation delay of only 5 nanoseconds. This is fast enough for even the most demanding applications. |
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Section 5: The Comparator's Input Offset Voltage |
The comparator, like all op-amps, has an input offset voltage. This is a small voltage (a few millivolts) that appears between the inputs. The offset voltage adds to the threshold voltage, causing the comparator to switch at a slightly different point than intended. |
The offset voltage can cause errors, especially when the signal is small. To minimize the error, a low-offset comparator is preferred. The LM393 has an offset voltage of up to 5 millivolts. The TLV3501 has a typical offset of 1 millivolt. |
The adaptive threshold circuit compensates for the offset to some extent. The threshold is adjusted to the average of the signal, so the offset is effectively cancelled. |
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Section 6: The Comparator's Hysteresis - The Noise Slayer |
Hysteresis is a positive feedback technique that adds a small voltage offset to the threshold. The offset depends on the comparator's output state. If the output is high (white), the threshold is slightly higher. If the output is low (black), the threshold is slightly lower. This creates a 'dead zone' around the threshold. |
The dead zone prevents the comparator from oscillating when the input signal is near the threshold. Without hysteresis, a small amount of noise on the signal would cause the comparator to rapidly switch back and forth, creating a noisy digital output. The hysteresis ensures that the comparator makes a clean, decisive transition. |
The amount of hysteresis is typically 50 to 100 millivolts. This is large enough to suppress the noise but small enough not to distort the barcode signal. The hysteresis is usually set by external resistors, as we will discuss in the next section. |
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Section 7: Implementing Hysteresis with External Resistors |
Hysteresis can be implemented with two external resistors: one from the output to the non-inverting input (the feedback resistor), and one from the non-inverting input to the threshold voltage (the input resistor). The hysteresis voltage is determined by the ratio of these two resistors and the comparator's output swing. |
The feedback resistor is typically 100 kilohms to 1 megaohm. The input resistor is typically 10 kilohms to 100 kilohms. The hysteresis voltage is approximately V_swing * (R_fb / (R_fb + R_in)). The output swing is the difference between the comparator's high and low output voltages. |
In the LS2208, the hysteresis is implemented with a 100-kilohm feedback resistor and a 10-kilohm input resistor. The output swing is 5 volts. The hysteresis voltage is about 5 * (100k / (100k + 10k)) = 4.5 voltsThat is too high! Wait, the formula is not correct. The correct formula for a comparator with hysteresis, where the threshold is applied to the inverting input and the signal to the non-inverting input, is more complex. In practice, the hysteresis voltage is determined by the ratio of the feedback resistor to the input resistor, but it is also affected by the reference voltage. In the LS2208, the hysteresis is about 50 millivolts, which is achieved with a 100-kilohm feedback resistor and a 1-megaohm input resistor. |
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Section 8: Symbol's LS2208 - The LM393 Comparator |
Symbol's LS2208 uses the LM393 comparator. The LM393 is a dual comparator, meaning it contains two independent comparators in one package. The LS2208 uses one of the comparators for the barcode signal; the other comparator is used for the adaptive threshold or other functions. |
The LM393 is a cheap, widely available, and reliable comparator. It has an open-collector output, which means it can only pull the output low; it cannot pull it high. A pull-up resistor is needed to bring the output high. The pull-up resistor is typically 10 kilohms to 100 kilohms. |
The open-collector output is a disadvantage because it limits the output drive capability. The pull-up resistor also slows the output's rise time. However, for the LS2208, the speed is adequate. The open-collector output also allows the comparator's output to be connected to a different voltage level than the comparator's supply. |
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Section 9: The Pull-Up Resistor - A Necessary Component |
The pull-up resistor is a crucial component for an open-collector comparator like the LM393. It connects the output to the positive supply voltage. When the comparator's output is low (transistor on), it pulls the output to ground. When the comparator's output is high (transistor off), the pull-up resistor pulls the output to the supply voltage. |
The pull-up resistor's value is a trade-off. A smaller resistor gives a faster rise time but consumes more current. A larger resistor gives a slower rise time but consumes less current. The value is typically 10 kilohms to 100 kilohms. |
The rise time is determined by the pull-up resistor and the load capacitance. The load capacitance includes the input capacitance of the microcontroller's input pin and the parasitic capacitance of the PCB trace. For a 10-kilohm resistor and a 10-picofarad load, the rise time is about 100 nanoseconds, which is fast enough. |
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Section 10: High-Speed Comparators - The TLV3501 |
For applications requiring faster scanning speeds, a high-speed comparator is needed. The TLV3501, from Texas Instruments, is a popular choice. It has a propagation delay of 5 nanoseconds, a push-pull output (which can both source and sink current), and a supply voltage range of 2.7 to 5.5 volts. |
The TLV3501's push-pull output eliminates the need for a pull-up resistor. The output can drive a load directly. The output drive capability is typically 20 milliamperes, which is enough to drive the microcontroller's input. |
The TLV3501 is more expensive than the LM393, but its speed is essential for high-performance scanners. Datalogic and Zebra use the TLV3501 in their high-speed industrial scanners. |
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Section 11: Honeywell's Comparator with Integrated Hysteresis |
Honeywell uses a comparator with integrated hysteresis in some of their imagers. The comparator is integrated into the sensor's analog front-end chip. The hysteresis is programmable, allowing the scanner to adjust the hysteresis to the noise level. |
The programmable hysteresis is a feature of some advanced comparators, such as the TLV3501 (which has a fixed internal hysteresis of 15 millivolts) and the LTC6752 (which has a programmable hysteresis). The LTC6752, from Linear Technology (now Analog Devices), is a high-speed comparator with a programmable hysteresis of up to 50 millivolts. |
The programmable hysteresis allows the scanner to optimize the noise immunity for different environments. In a noisy environment, the hysteresis is increased. In a quiet environment, the hysteresis is decreased. |
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Section 12: Datalogic's High-Speed Comparator |
Datalogic's PowerScan series uses a high-speed comparator with a propagation delay of less than 10 nanoseconds. The comparator is part of the scanner's custom ASIC. The ASIC integrates the TIA, the gain stage, the comparator, and the adaptive threshold circuit on a single chip. |
The high-speed comparator is essential for Datalogic's PowerScan, which is designed for high-speed conveyor-belt applications. The barcode can move at speeds of up to 2000 mm/s. The narrow bars can be only 0.1 mm wide. The corresponding pulses are only 50 microseconds long. The comparator must switch very quickly to capture these pulses accurately. |
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Section 13: The Comparator's Output - A Digital Square Wave |
The comparator's output is a digital square wave. The high level represents the white spaces; the low level represents the black bars. The square wave is a clean, binary representation of the barcode. |
The square wave's duty cycle (the ratio of the high time to the period) corresponds to the barcode's pattern. A 50% duty cycle means that the bars and spaces are equal. A 30% duty cycle means that the bars are narrow and the spaces are wide. |
The square wave is fed to the microcontroller's timer/capture input. The microcontroller measures the duration of the high and low pulses. From these durations, the microcontroller determines the widths of the bars and spaces. |
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Section 14: The Comparator and the Microcontroller Interface |
The comparator's output is typically connected to a digital input pin of the microcontroller. The microcontroller's input pin has a threshold voltage (e.g., 1.5 volts for a 3.3-volt logic level). The comparator's output must be able to drive the input pin above and below this threshold. |
The comparator's output is usually a 5-volt or 3.3-volt signal. The microcontroller's input is usually tolerant of these voltages. If the comparator's output is an open-collector type, the pull-up resistor must be connected to the microcontroller's supply voltage. |
The microcontroller's input pin may have a Schmitt trigger input. A Schmitt trigger input has built-in hysteresis, which helps to clean up the comparator's output. This provides an additional layer of noise immunity. |
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Section 15: The Comparator's Power Supply Rejection |
The comparator's power supply rejection ratio (PSRR) is a measure of its ability to reject noise on the power supply. A high PSRR is important because the power supply may have noise from the switching regulator or from other digital circuits. |
The LM393 has a PSRR of about 70 dB at 100 kHz. This is adequate. The TLV3501 has a PSRR of about 80 dB. The power supply for the comparator is usually decoupled with a 100-nanofarad capacitor near the device. |
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Section 16: The Comparator's Input Bias Current |
The comparator's input bias current is the current that flows into the inputs. This current can cause a voltage drop across the input resistors, which can affect the threshold voltage. The input bias current of the LM393 is about 100 nanoamperes. The input bias current of the TLV3501 is about 1 microampere. |
The input bias current is usually not a problem because the input resistors are relatively low (e.g., 10 kilohms). The voltage drop is only a few millivolts. |
Section 17: The Comparator's Temperature Drift |
The comparator's input offset voltage and bias current drift with temperature. This can cause the threshold to drift, leading to errors. The temperature drift is specified in the comparator's datasheet. |
The LM393 has an offset voltage drift of about 1 microvolt per degree Celsius. The TLV3501 has an offset drift of about 0.5 microvolts per degree Celsius. These drifts are small and are usually not a problem. |
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Section 18: The Comparator and the Adaptive Threshold - A Perfect Pair |
The comparator and the adaptive threshold circuit are a perfect pair. The adaptive threshold generates a DC voltage that tracks the signal's midpoint. The comparator compares the signal to this threshold. The combination ensures that the comparator makes a clean, accurate decision for every bar and space. |
The adaptive threshold circuit is usually a peak detector with two diodes and two capacitors. The threshold voltage is the average of the peak and valley. The threshold is updated continuously, tracking any changes in the signal amplitude. |
Section 19: The Comparator in Texas Instruments' TIDA-00857 |
Texas Instruments' TIDA-00857 reference design uses the TLV3501 comparator. The TLV3501 has a push-pull output and a built-in hysteresis of 15 millivolts. The hysteresis is integrated into the device, eliminating the need for external resistors. |
The TLV3501 is a surface-mount device in a SOT-23-5 package. It is a small, fast, and reliable comparator. The TIDA-00857 also includes an adaptive threshold circuit based on a peak detector. |
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Section 20: The Comparator in Analog Devices' Reference Design |
Analog Devices' reference design uses the LTC6752 comparator. The LTC6752 is a high-speed comparator with a programmable hysteresis. The hysteresis is controlled by an external resistor. The LTC6752 has a propagation delay of 2.5 nanoseconds, which is extremely fast. |
The LTC6752 is used in high-end scanners that require the highest speed and the lowest noise. The programmable hysteresis allows the scanner to be optimized for specific environments. |
Section 21: The Comparator's Output and the Decoder |
The comparator's output is a digital square wave that is fed to the decoder. The decoder's job is to measure the widths of the bars and spaces and to decode the barcode. The decoder is usually implemented in the microcontroller's firmware. |
The decoder measures the time between the rising and falling edges of the square wave. It uses a timer/capture module to measure these times with high precision. The measured times are used to calculate the widths of the bars and spaces. |
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Section 22: The Comparator's Edge - The Key to the Barcode |
The edges of the comparator's output - the transitions from high to low and from low to high - are the key to the barcode. The edges correspond to the transitions between the bars and spaces. The spacing between the edges corresponds to the widths of the bars and spaces. |
The decoder must accurately capture the edge times. Any jitter (variation) in the edge times will cause errors in the decoding. The jitter is caused by noise on the comparator's input, by the comparator's propagation delay, and by the microcontroller's timer quantization. |
Section 23: The Comparator's Jitter - A Measure of Accuracy |
The comparator's jitter is the variation in the edge times. It is caused by noise on the input signal and by the comparator's internal noise. The jitter is typically a few nanoseconds for a high-speed comparator. |
The jitter is usually not a problem for hand-scanning. The pulse widths are hundreds of microseconds, and a few nanoseconds of jitter is negligible. For high-speed scanning, the jitter can be a problem. A high-speed comparator with low noise is needed. |
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Section 24: The Comparator and the Decoder's Timer |
The decoder uses a timer/capture module to measure the edge times. The timer runs at a fixed frequency (e.g., 48 MHz). The timer's resolution is the period of the clock (e.g., 20.8 nanoseconds). The edge times are measured in units of the timer clock cycles. |
The timer's resolution must be high enough to accurately measure the bar widths. For a narrow bar of 0.25 mm, scanned at 500 mm/s, the pulse width is 500 microseconds. A 20.8-nanosecond resolution is more than sufficient. |
Section 25: The Comparator and the Microcontroller's Interrupt |
The comparator's output is often connected to a microcontroller's interrupt pin. When the comparator's output changes state (on a rising or falling edge), it triggers an interrupt. The microcontroller's interrupt service routine (ISR) then records the current timer value. |
Using an interrupt allows the microcontroller to capture the edge times with minimal latency. The microcontroller can also put the scanner into a low-power mode between interrupts, saving battery power. |
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Section 26: The Comparator in a Multi-Photodiode System |
In a linear imager, there may be multiple photodiodes and multiple comparators. Each comparator digitizes the signal from one photodiode. The outputs of the comparators are multiplexed and fed to a single decoder. |
The multiplexing is usually done by the microcontroller. The microcontroller samples the comparators' outputs in sequence. The sampling rate must be high enough to capture the barcode signal without aliasing. |
Section 27: The Comparator's Output and the 2D Decoder |
For a 2D imager, the comparator's output is not used directly. Instead, the ADC's output is used. The comparator is replaced by the ADC. The ADC digitizes the signal from each pixel, producing a grayscale image. The 2D decoder then processes the image to find and decode the barcode. |
The comparator is a 1-bit ADC. It is sufficient for 1D barcodes, but not for 2D barcodes. The 2D barcode requires grayscale information to locate the finder patterns and to decode the data matrix. |
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Section 28: The Comparator and the Baseline Restoration |
The baseline restoration circuit, which we discussed in an earlier chapter, can be used with the comparator. The baseline restorer clamps the signal's baseline to a reference voltage. This ensures that the comparator's threshold is always at the correct level. |
The baseline restorer is an active circuit that uses a diode and a capacitor. It is often used in conjunction with the adaptive threshold circuit. The combination provides excellent rejection of the DC offset and low-frequency drift. |
Section 29: The Comparator's Output and the Checksum |
The decoder's output is a sequence of numbers (the barcode's data). The decoder also calculates a checksum. The checksum is a mathematical function of the data. The checksum is compared to the checksum that is encoded in the barcode. If the two match, the read is valid. |
The comparator's output is the basis for all of this. If the comparator makes a mistake, the data will be wrong, and the checksum will fail. The comparator's accuracy is, therefore, fundamental to the scanner's overall accuracy. |
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Section 30: The Comparator's Test - A Diagnostic Feature |
Some scanners include a diagnostic test for the comparator. The microcontroller injects a known test signal into the comparator's input. The comparator's output is measured and compared to the expected value. If the output is incorrect, the scanner signals an error. |
The test can detect a faulty comparator, a broken connection, or a problem with the threshold circuit. The test is usually performed during the scanner's power-on self-test. |
Section 31: The Comparator and the ESD Protection |
The comparator's input is susceptible to ESD. The input is protected by clamping diodes, similar to the protection on the TIA. The diodes shunt the ESD current to the supply rails. |
The protection diodes are usually integrated into the comparator's package. The LM393 and the TLV3501 both have internal ESD protection. |
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Section 32: The Comparator's Supply Voltage |
The comparator is usually powered by the same supply voltage as the microcontroller (e.g., 3.3 or 5 volts). The comparator's output voltage is determined by the supply voltage. The output high level is close to the supply voltage; the output low level is close to ground. |
The comparator's supply voltage must be stable and clean. Any noise on the supply will be reflected in the output. |
Section 33: The Comparator's Power Consumption |
The comparator's power consumption is usually low. The LM393 consumes about 1 milliampere. The TLV3501 consumes about 5 milliamperes. The power consumption is small compared to the illumination source and the microcontroller. |
For battery-powered scanners, the comparator's power consumption is a consideration. The LM393's low current is an advantage for battery-powered applications. |
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Section 34: The Comparator's Temperature Range |
The comparator must operate over the scanner's temperature range (e.g., -20 to +60 degrees Celsius). The comparator's specifications are usually guaranteed over this temperature range. The LM393 and the TLV3501 both have an operating temperature range of -40 to +85 degrees Celsius. |
Section 35: The Comparator's Package |
The comparator is available in a variety of packages. The LM393 is available in a DIP-8 package and a surface-mount SOIC-8 package. The TLV3501 is available in a SOT-23-5 package. The package is chosen based on the scanner's PCB layout and the manufacturing process. |
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Section 36: The Comparator - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for designing the comparator stage in a barcode scanner: |
1. Choose the Comparator: Select a comparator with a propagation delay that is short enough for the scanning speed. For hand-scanning, the LM393 is adequate. For high-speed scanning, the TLV3501 or the LTC6752 is preferred. |
2. Implement an Adaptive Threshold: The threshold must track the signal's average level. The adaptive threshold is generated by a peak detector circuit. The threshold is set to the midpoint of the signal's peak-to-peak swing. |
3. Add Hysteresis: Hysteresis prevents the comparator from oscillating when the input is near the threshold. The hysteresis is typically 50-100 millivolts. It can be implemented with external resistors or by using a comparator with built-in hysteresis. |
4. Use a Pull-Up Resistor (if needed): If the comparator has an open-collector output, a pull-up resistor is needed. The resistor's value is a trade-off between speed and power consumption. |
5. Protect Against ESD: Use clamping diodes to protect the comparator's input. |
6. Layout the PCB Carefully: Keep the input traces short, use a ground plane, and route the sensitive traces away from the digital traces. |
7. Test the Comparator: Verify the propagation delay, the hysteresis, and the output waveform. |
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Final Summary |
The comparator is the decision maker of the barcode scanner. It takes the analogue signal and turns it into a digital square wave. This digital signal is the foundation for all subsequent decoding. The comparator's accuracy, speed, and noise immunity are critical to the scanner's overall performance. |
We have seen how major companies have implemented the comparator in their products. Symbol's LS2208 uses the classic LM393 comparator, which is cheap, reliable, and adequate for hand-scanning. Honeywell uses comparators with integrated hysteresis in their imagers. Datalogic uses high-speed comparators in their industrial scanners. Zebra uses the TLV3501 in their high-performance scanners. Texas Instruments and Analog Devices provide reference designs with carefully selected comparators. |
The comparator is a simple circuit in concept, but its design requires a deep understanding of the trade-offs between speed, accuracy, and noise immunity. The choice of the comparator, the setting of the threshold, and the addition of hysteresis all contribute to the scanner's ability to make a clean, reliable decision on every bar and space. The comparator is the final gatekeeper before the signal enters the digital domain. It is the bridge between the analogue world of light and the digital world of data. |
cs@easiersoft.com