Hysteresis in the Comparator: The Art of Clean Transitions |
Executive Summary |
This article provides a comprehensive, accessible exploration of hysteresis in comparator circuits for barcode readers. We examine how this essential feedback technique prevents the 'chatter' that would otherwise occur when a noisy analog signal crosses the decision threshold, ensuring clean, reliable digitization of barcode patterns. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real patent disclosures from industry leaders including Symbol Technologies, Texas Instruments, and Johnson & Johnson Clinical Diagnostics. We explore the fundamental principle of positive feedback, the use of resistor networks to set hysteresis levels, the challenges of symmetrical versus asymmetrical hysteresis, and advanced techniques like adaptive hysteresis that tracks signal amplitude. The article also covers practical implementations such as the false transition gating circuit and the margin threshold circuit for enhanced noise immunity. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing comparator circuits for barcode reading applications. |
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Chapter 1: The Chatter Problem |
Imagine a person trying to decide whether it is dark or light outside at twilight. The light level is changing slowly, and every small fluctuation makes the person change their mind back and forth. This is exactly what happens when a comparator without hysteresis tries to digitize a noisy barcode signal near the threshold. |
The comparator is the decision-maker in a barcode reader. It compares the analog signal from the photodetector to a threshold voltage. When the signal is above the threshold, the output is high (representing a white space). When the signal is below the threshold, the output is low (representing a black bar). |
The problem is that the analog signal is never perfectly clean. It contains noise from the photodetector, the amplifiers, and the environment. When the signal is near the threshold, this noise can cause the comparator output to rapidly switch back and forth between high and low states. This is called 'chatter' or 'oscillation,' and it produces a digitized signal that is full of false edges. |
A patent from Johnson & Johnson Clinical Diagnostics explains the challenge: 'The use of hysteresis produced by positive feedback in comparator circuits may also limit the dynamic range of bar code speeds. Large amounts of positive feedback will tend to limit the ability of a system to detect low amplitude signals' . |
Hysteresis is the solution to this problem. It is a form of positive feedback that shifts the threshold slightly after the comparator switches, creating a 'dead zone' that rejects noise. |
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Chapter 2: The Principle of Hysteresis |
Hysteresis is a simple but powerful concept. When a comparator with hysteresis switches from low to high, the threshold is shifted slightly higher. When it switches from high to low, the threshold is shifted slightly lower. This creates a gap between the two switching points---a dead zone where the comparator will not change state. |
The dead zone prevents chatter. If the signal is rising, the comparator switches high when the signal exceeds the upper threshold. If the signal then falls slightly (due to noise), it must fall all the way to the lower threshold before the comparator switches back. This ensures that the comparator does not switch back and forth due to small noise fluctuations. |
The amount of hysteresis is typically set by a resistor network in the comparator's feedback path. A larger hysteresis provides more noise immunity but also limits the sensitivity of the comparator. A smaller hysteresis provides more sensitivity but less noise immunity. The choice depends on the application. |
A patent from Symbol Technologies describes the application of this principle in a barcode reader digitizer: 'The level at which the gating comparator trips is determined by the amount of hysteresis so as to ensure maximum digitizing accuracy' . |
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Chapter 3: Implementing Hysteresis with Resistors |
The simplest way to implement hysteresis is with two resistors connected between the comparator output and the non-inverting input. This creates a positive feedback loop that shifts the threshold. |
The resistor network works as follows: when the comparator output is high, the feedback resistor pulls the non-inverting input slightly higher. This raises the upper threshold. When the comparator output is low, the feedback resistor pulls the non-inverting input slightly lower. This lowers the lower threshold. |
The ratio of the two resistors determines the amount of hysteresis. A higher ratio gives more hysteresis. The switching points are equally placed above and below the reference voltage . |
A patent from Johnson & Johnson Clinical Diagnostics explains the trade-off: 'The selection of the ratio is a trade-off between increased sensitivity to bar and space peaks, and immunity from unwanted transitions caused by noise. As the ratio increases, the comparator input must be larger and larger in order to change the comparator output' . |
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Chapter 4: Texas Instruments' TLC352C --- A Practical Example |
Texas Instruments' TLC352C comparator is a practical example of a device that can be used with an adaptive hysteresis circuit. This comparator can operate with a single-ended supply voltage as low as 1.4 volts and has an open-drain CMOS transistor output that can switch down to its negative supply rail . |
The TLC352C's open-drain output is a key feature for adaptive hysteresis. Because the output can swing to the negative supply rail, the comparator's output swing can be made equal to the full peak-to-peak amplitude of the input signal. |
A patent describes how this comparator is used in an adaptive trigger circuit: 'Since the output of the comparator can switch down to its negative supply rail, its output will drop to -V peak when its output is in the OFF state. The open drain output is connected through a pull-up resistor to +V peak which represents the maximum positive excursion of the differentiated signal amplitude' . |
This arrangement creates a comparator whose trigger thresholds automatically track the signal amplitude, providing optimal hysteresis for any signal strength. |
Chapter 5: The Adaptive Hysteresis Trigger Circuit |
The adaptive hysteresis trigger circuit is a sophisticated implementation that sets the hysteresis level based on the signal amplitude. This ensures that the comparator always operates with the optimal amount of hysteresis, regardless of whether the signal is strong or weak. |
The circuit uses positive and negative peak detectors to capture the maximum and minimum values of the differentiated signal. The positive peak voltage is buffered and used as the positive supply for the comparator. The negative peak voltage is inverted and used as the negative supply . |
This arrangement sets the comparator's upper and lower trigger thresholds at a fixed percentage of the peak-to-peak signal amplitude. The patent explains: 'The comparator will now turn ON when the signal voltage reaches a value (+V peak)(R3/R4) and OFF when the signal voltage falls to (-V peak)(R3/R4)' . |
A ratio of about 0.7 for the resistor network is well suited to avoid noise levels in barcode reading applications. The circuit triggers adaptively at approximately +/-70% of the differentiated signal peaks, regardless of the signal amplitude . |
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Chapter 6: The Symmetrical Hysteresis Challenge |
One challenge in comparator design is achieving symmetrical hysteresis. When the comparator's positive and negative output voltages are not equal, the hysteresis is asymmetric. This can limit the performance of the digitizer. |
A patent from Johnson & Johnson Clinical Diagnostics addresses this problem: 'The asymmetric hysteresis condition mentioned above may limit the operation of the slope detector circuit. For example, if the smaller of the two hysteresis voltages is scaled for the expected input waveform, some small bar or space features may not generate a large enough signal to cause a state change in the comparator' . |
The patent describes a tracking hysteresis circuit that provides a symmetrical hysteresis characteristic. The circuit uses an operational amplifier to track the input signal and regulate the hysteresis symmetrically with respect to it. In this configuration, neither the positive nor the negative hysteresis values are influenced by changes in the comparator output amplitude . |
This symmetrical hysteresis permits a choice of resistor ratio that is large enough to suppress unwanted transitions while still allowing small but valid changes to be sensed . |
Chapter 7: The Tracking Hysteresis Circuit |
The tracking hysteresis circuit is a significant improvement over fixed hysteresis. It adjusts the hysteresis to maintain a constant value regardless of the signal's DC level or the comparator's output amplitude. |
The circuit includes an operational amplifier configured as a buffer, with its output equal to the voltage at its positive input terminal. Diodes clamp the voltage at the input node to a value equal to the input signal plus or minus one diode forward voltage drop . |
The comparator's output voltage is higher than the input signal when high, and lower than the input signal when low. This arrangement sets the hysteresis voltage to a value that is independent of the comparator output amplitude . |
The patent explains: 'The hysteresis voltage amplitude is thus independent of whether the output is high or low. This symmetric hysteresis characteristic permits a choice of the ratio of resistors which, when multiplied by the diode forward voltage drop, is large enough to suppress unwanted transitions. This value is still small enough to allow small but valid changes to be sensed, thereby increasing the performance range' . |
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Chapter 8: The False Transition Gating Circuit |
The false transition gating circuit is a technique that works in conjunction with hysteresis to reject noise. It ensures that the digitizer only changes state in response to valid barcode edges, not noise. |
A patent from Symbol Technologies describes a circuit that includes a peak locating comparator and a false transition gating comparator. The gating comparator 'discriminates against false transitions in the output signal of the peak locating comparator and changes states only upon the detection of transitions in the output signal of the peak locating comparator about a given threshold' . |
The output circuit 'changes the state of its output signal only if the false transition gating circuit means has changed state since the last change of state of the peak locating comparator means to discriminate against false transition signals' . |
This ensures that the digitizer output is clean and free from noise-induced chatter. |
Chapter 9: The Margin Threshold Circuit |
The margin threshold circuit is another technique for enhancing noise immunity, particularly when the scanner is in the border areas (white margins) of the barcode. |
The circuit receives the differentiated signal and functions as a retriggerable one-shot. It does not time out as long as there is a continuous series of pulses in the signal that exceed the threshold . |
When the pulses stop long enough for the circuit to time out, the digitized bar output is forced back to the white (space) state. When a border area is encountered, the circuit output is high, turning on a transistor that prevents the latch comparator from passing an output . |
This ensures that the digitizer does not produce spurious pulses from noise when no barcode is being scanned. |
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Chapter 10: Hysteresis and the Dynamic Range Trade-off |
The choice of hysteresis involves a fundamental trade-off between noise immunity and sensitivity to low-amplitude signals. This is particularly important for barcode readers, which must handle a wide range of signal amplitudes. |
A patent from Johnson & Johnson Clinical Diagnostics notes: 'Large amounts of positive feedback will tend to limit the ability of a system to detect low amplitude signals, such as those generated by low bandwidth systems when exposed to high speed bar codes. At very low bar code speeds, where an unchanging input signal exists for a significant amount of time, the low hysteresis levels needed for high speed operation may not be adequate in the presence of electronic noise and the input offset voltage inherent in comparators' . |
The adaptive hysteresis trigger circuit addresses this trade-off by making the hysteresis proportional to the signal amplitude. When the signal is strong, the hysteresis is large. When the signal is weak, the hysteresis is small. This provides noise immunity when the signal is strong and sensitivity when the signal is weak . |
Chapter 11: The Waveform Shaping Stage |
The waveform shaping stage is the final step before digitization. It conditions the signal to optimize the performance of the comparator and hysteresis circuit. |
The signal shaping is performed in the analog domain. The output from the differentiator and amplifiers is processed to enhance the transitions and reduce noise. A low-pass filter removes high-frequency noise that could cause false triggering. |
A patent from Symbol Technologies describes an amplifier with low-pass filtering coupled to the output of the differentiator circuit, providing amplification and low-pass filtering for the differentiated signal . |
The waveform shaping stage ensures that the comparator receives a clean signal with well-defined edges, maximizing the effectiveness of the hysteresis. |
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Chapter 12: The Digitizer in the Full Signal Chain |
The digitizer is the final stage in the analog signal chain, following the transimpedance amplifier, the gain stages, the differentiator, and the filter. The signal chain prepares the photodetector signal for digitization. |
The complete signal chain includes a photodetector that converts light to current, a transimpedance amplifier that converts current to voltage, amplifier stages that provide gain, a differentiator that enhances edges, and a filter that removes noise . |
The digitizer then compares the conditioned signal to the threshold, producing a digital pulse stream. The hysteresis in the comparator ensures that this pulse stream is clean and free from chatter. |
The digitized signal is then passed to the decoder, which measures the pulse widths and decodes the barcode. |
Chapter 13: Hysteresis and the Decoding Performance |
The quality of the digitized signal directly affects the decoding performance. If the digitized signal contains false edges or distorted pulse widths, the decoder will be unable to decode the barcode correctly. |
Hysteresis plays a critical role in ensuring the quality of the digitized signal. By preventing chatter, it ensures that the edges are clean and well-defined. By rejecting noise, it ensures that false edges are not introduced. |
The adaptive hysteresis trigger circuit takes this a step further by ensuring that the hysteresis level is always optimal for the signal amplitude. This provides consistent decoding performance across a wide range of signal conditions. |
The patent notes that the adaptive trigger circuit 'generates square wave transitions required by digital signal processing equipment regardless of incoming signal amplitude' . |
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Chapter 14: The Single Low Voltage Power Supply Advantage |
A significant advantage of the adaptive hysteresis trigger circuit is its ability to operate from a single low-voltage power supply. This is important for handheld barcode readers, which are battery-powered. |
The circuit uses the positive and negative peaks of the differentiated signal as power supplies for the comparator. This eliminates the need for a separate negative supply . |
The patent notes that the circuit 'does not use diode based peak detectors which limit how small a signal can be processed, and makes it difficult to use a single, low voltage power supply' . |
This allows the digitizer to operate from a single 5-volt supply, or even lower, reducing power consumption and extending battery life. |
Chapter 15: The Open Drain CMOS Comparator Advantage |
The use of an open-drain CMOS comparator is a key advantage of the adaptive hysteresis trigger circuit. Unlike open-collector bipolar comparators, the CMOS comparator can switch its output all the way down to its negative supply rail . |
This allows the comparator's output to swing to the negative peak of the differentiated signal when in the OFF state. When triggered ON, the output swings to the positive peak value. |
The patent explains: 'Comparators with open collector bipolar transistor outputs can not do this' . |
This full swing capability is essential for achieving the wide dynamic range and adaptive hysteresis of the circuit. |
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Chapter 16: The Role of the Voltage Divider |
The voltage divider in the comparator's feedback path sets the amount of hysteresis. The ratio of the resistors determines the percentage of the peak-to-peak signal at which the comparator triggers. |
In the adaptive hysteresis trigger circuit, the hysteresis is set by resistors R3 and R4. The upper and lower trigger thresholds are equally placed above and below the reference voltage . |
A ratio of about 0.7 for these resistors is well suited to avoid noise levels in barcode reading applications. The circuit triggers at approximately +/-70% of the differentiated signal peaks . |
This fixed percentage ensures that the hysteresis scales with the signal amplitude, providing optimal noise immunity for any signal strength. |
Chapter 17: The Comparator Output to Logic Level Conversion |
The output of the adaptive hysteresis trigger circuit is not directly compatible with logic-level processing. The output amplitude is equal to the peak-to-peak amplitude of the differentiated signal, which may vary from a few millivolts to a few volts . |
A second comparator is used to convert this signal to a logic-level signal. This comparator compares the output of the first comparator to the reference voltage. The second comparator's output is a clean square wave that swings from the positive supply voltage to the negative peak value . |
A reference circuit clamps the square wave output to the supply ground, producing a signal that switches from ground to about one-third of the supply voltage. A high-speed switching transistor then produces a clean square wave output that goes from ground to the positive supply voltage . |
This final output is suitable for digital signal processing equipment. |
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Chapter 18: The Shaping of Reflected Light and Edge Resolution |
The beam shaping of the reflected light affects the edge resolution of the digitized signal. The adaptive hysteresis trigger circuit complements the beam shaping methods to deliver a high level of performance with respect to depth of range in beam scanning equipment . |
The patent notes that the circuit 'complements the beam shaping methods described earlier to deliver a high level of performance with respect to depth of range in beam scanning equipment' . |
This integration of optical and electronic design is essential for achieving reliable barcode reading over a wide range of distances. |
Chapter 19: The Correlation to Previous Prior Art Circuits |
The adaptive hysteresis trigger circuit represents a significant improvement over prior art circuits. Previous circuits used either the undifferentiated analog signal (for windows or ambient light rejection) or the second derivative signal (for zero crossings). |
The circuit described in the patent 'utilizes neither of these signals in the signal processing except to derive the first differential signal' . |
It is the only known circuit that determines windows with a single comparator. A high-to-low transition of the gating comparator enables the latch comparator to respond to the first transition, and a low-to-high transition enables it to respond to the first transition in the other direction . |
Other prior art circuits create narrow windows around the time period that a real bar-space or space-bar transition is expected, requiring separate windows for each direction . |
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Chapter 20: The Windowless Window Approach |
The 'windowless window' approach of the adaptive hysteresis trigger circuit is a significant innovation. Instead of creating windows around expected transition times, the circuit uses the gating comparator to enable the latch comparator only at the appropriate times. |
The gating comparator 'discriminates against false transitions in the output signal of the peak locating comparator and changes states only upon the detection of transitions about a given threshold' . |
This eliminates the need for separate windows for bar-space and space-bar transitions, simplifying the circuit and improving its performance. |
The patent notes that 'there is always one window for bar-space transitions and another window for space-bar transitions' in prior art circuits, but the new circuit eliminates this complexity . |
Chapter 21: The Use of a Single Comparator for Windows |
The adaptive hysteresis trigger circuit uses a single comparator for window determination. This is a significant simplification compared to prior art circuits, which required multiple comparators. |
The gating comparator trips at a level determined by the amount of hysteresis, ensuring maximum digitizing accuracy . |
The margin threshold circuit receives the differentiated signal and functions as a retriggerable one-shot. It does not time out as long as there is a continuous series of pulses in the signal that exceed the threshold . |
This single-comparator windowing approach reduces the component count and power consumption of the digitizer, making it suitable for portable barcode readers. |
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Chapter 22: The Zener Diode Reference Voltage |
The adaptive hysteresis trigger circuit uses a Zener diode to provide a stable reference voltage. The Zener diode ensures that the signal levels are centered at the reference voltage, typically 2.5 volts . |
The Zener diode circuit is coupled to the circuit to 'ensure that the signal levels therein are centered at 2.5 V, and which can be considered to be part of the power supply circuit' . |
This stable reference voltage is essential for the accurate operation of the comparator and the hysteresis circuit. |
Chapter 23: The Fixed Gain and Adaptive Gain |
Some barcode readers offer both fixed gain and adaptive gain modes. The adaptive hysteresis trigger circuit can operate in either mode. |
In fixed gain mode, the hysteresis level is set by fixed resistors. In adaptive mode, the hysteresis level tracks the signal amplitude. |
The patent notes that the circuit 'complements the beam shaping methods described earlier to deliver a high level of performance with respect to depth of range in beam scanning equipment' . |
This flexibility allows the reader to be optimized for different barcode reading applications. |
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Chapter 24: The Hysteresis and the Peak Locating Comparator |
The peak locating comparator is a key element of the digitizer. It detects the crossings of the differentiated signal and the delayed differentiated signal, which correspond to the peaks of the edges. |
The patent describes the peak locating comparator as receiving the differentiated signal as a first input and the delayed differentiated signal as a second input, and detecting crossings of the signals when they are equal, which result in transitions in the output signal . |
The hysteresis in the gating comparator ensures that only valid transitions from the peak locating comparator are passed to the output. |
Chapter 25: The Delay Circuit for Peak Location |
The delay circuit is essential for the operation of the peak locating comparator. It delays the differentiated signal so that it can be compared with the original signal. |
The delay circuit generates a delayed first derivative signal from the first derivative signal. The peak locating comparator compares the original differentiated signal with the delayed version . |
When the two signals cross, a peak is detected. The time of the crossing corresponds to the edge of the barcode. |
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Chapter 26: The Amplifier with Low-Pass Filtering |
The amplifier with low-pass filtering is used to condition the differentiated signal before it is passed to the comparator. This removes high-frequency noise that could cause false triggering. |
The patent describes an amplifier with low-pass filtering coupled to the output of the differentiator circuit, providing amplification and low-pass filtering for the differentiated signal . |
The low-pass filter's cutoff frequency is set to pass the barcode signal while rejecting noise. |
Chapter 27: The Hysteresis in the Gating Comparator |
The gating comparator is the key to the windowless window approach. It uses hysteresis to set the level at which it trips, ensuring maximum digitizing accuracy. |
The level at which the gating comparator trips is determined by the amount of hysteresis . |
When the gating comparator trips, it enables the latch comparator to respond to the next transition from the peak locating comparator. The latch comparator then changes state, producing a clean digitized output. |
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Chapter 28: The Digitizer Output to the Bar Pattern Decoder |
The output of the digitizer is a clean square wave that represents the barcode pattern. This output is passed to the bar pattern decoder, which measures the pulse widths and decodes the barcode. |
The output of the latch comparator is directed to a bar pattern decoder circuit, which it can drive directly in some embodiments . |
In other embodiments, an inverting transistor amplifier couples the output to the decoder circuit. |
The decoder measures the widths of the pulses and compares them to the expected patterns for the barcode symbology being read. |
Chapter 29: The Adaptive Hysteresis and Depth of Range |
The adaptive hysteresis trigger circuit delivers a high level of performance with respect to depth of range in beam scanning equipment. This is achieved by ensuring that the hysteresis is always optimal for the signal amplitude. |
The circuit 'complements the beam shaping methods described earlier to deliver a high level of performance with respect to depth of range in beam scanning equipment, such as bar code readers, object and edge detection devices and the like' . |
This makes the circuit suitable for a wide range of barcode reading applications, from handheld scanners to fixed-mount readers. |
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Chapter 30: The Broad Applicability of the Circuit |
The adaptive hysteresis trigger circuit is not limited to barcode readers. It can be used to shape and condition signals in a variety of applications. |
The patent notes that the circuit 'may also be used to shape and condition signals in fiber optic receivers, optical local area network receivers, RF information receiving equipment and other information signal processing equipment' . |
This broad applicability reflects the fundamental nature of the problem: extracting a clean digital signal from a noisy analog signal. |
Chapter 31: The Importance of the Ratio Selection |
The selection of the resistor ratio for hysteresis is a critical design decision. The ratio determines the amount of hysteresis and affects the comparator's sensitivity to noise. |
A patent from Johnson & Johnson Clinical Diagnostics explains: 'The selection of the ratio is a trade-off between increased sensitivity to bar and space peaks, and immunity from unwanted transitions caused by noise' . |
For the adaptive hysteresis trigger circuit, a ratio of about 0.7 is well suited for barcode reading applications . |
This ratio provides a good balance between noise immunity and sensitivity for typical barcode signals. |
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Chapter 32: The Hysteresis and the Peak Detection |
The hysteresis in the comparator is closely related to the peak detection circuit. The peak detectors capture the maximum and minimum values of the signal, and these values are used to set the hysteresis levels. |
The positive peak detector captures the maximum value, and the negative peak detector captures the minimum value. The resistor network between them sets the hysteresis levels . |
The hysteresis is proportional to the peak-to-peak amplitude of the signal, ensuring that it is always optimal for the current signal level. |
Chapter 33: The Hysteresis and the Common-Mode Rejection |
The hysteresis in the comparator is independent of the common-mode voltage of the input signal. This ensures that the digitizer operates correctly regardless of the signal's DC level. |
The adaptive hysteresis trigger circuit uses the positive and negative peaks of the differentiated signal as power supplies for the comparator. This makes the hysteresis independent of the DC level . |
This common-mode rejection is essential for barcode readers, which must handle a wide range of ambient light levels. |
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Chapter 34: The Hysteresis and the Low-Voltage Operation |
The adaptive hysteresis trigger circuit is designed for low-voltage operation. It can operate from a single supply voltage as low as 1.4 volts . |
This low-voltage operation is important for battery-powered handheld barcode readers. The circuit's low power consumption extends the battery life. |
The use of CMOS comparators, which can operate at low voltages, is a key factor in this low-voltage capability. |
Chapter 35: The Hysteresis and the Signal-to-Noise Ratio |
The hysteresis in the comparator improves the signal-to-noise ratio of the digitized signal. By rejecting noise, it ensures that the digitized signal accurately represents the barcode pattern. |
The patent notes that the adaptive hysteresis trigger circuit 'generates square wave transitions required by digital signal processing equipment regardless of incoming signal amplitude' . |
This high signal-to-noise ratio is essential for reliable barcode decoding. |
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Chapter 36: The Hysteresis and the Decoding Speed |
The hysteresis in the comparator can affect the decoding speed. Too much hysteresis can slow the comparator's response, limiting the maximum scanning speed. |
The adaptive hysteresis trigger circuit is designed for high-speed operation. The CMOS comparators used in the circuit have fast response times, making them suitable for high-speed barcode reading . |
The circuit's adaptive nature ensures that the hysteresis is never larger than necessary, maximizing the response speed. |
Chapter 37: The Hysteresis and the Manufacturing Tolerances |
The resistor network that sets the hysteresis must be designed to accommodate manufacturing tolerances. The resistor values can vary by 1% or more, affecting the hysteresis level. |
The adaptive hysteresis trigger circuit is relatively tolerant of component variations. The hysteresis is set by the ratio of the resistors, not their absolute values . |
This tolerance simplifies the manufacturing process and reduces the cost of the reader. |
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Chapter 38: The Hysteresis and the Temperature Drift |
The hysteresis in the comparator can drift with temperature. The resistor values and the comparator's characteristics change with temperature, affecting the hysteresis level. |
The adaptive hysteresis trigger circuit is designed to minimize temperature drift. The use of matched resistors and the adaptive nature of the circuit reduce the temperature sensitivity . |
This temperature stability is important for barcode readers that must operate over a wide temperature range. |
Chapter 39: The Hysteresis and the Long-Term Reliability |
The hysteresis in the comparator must remain stable over the lifetime of the reader. The resistor values and the comparator's characteristics should not drift significantly with age. |
The adaptive hysteresis trigger circuit uses standard components that are known for their long-term stability. The circuit's design is robust and should provide reliable operation for many years . |
This long-term reliability is important for industrial and commercial barcode readers. |
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Chapter 40: The Hysteresis and the Power Consumption |
The hysteresis in the comparator affects the power consumption of the digitizer. A comparator with hysteresis consumes more power than one without, because it must drive the feedback network. |
The adaptive hysteresis trigger circuit is designed for low power consumption. The use of CMOS comparators, which consume very little power, minimizes the power consumption . |
The circuit's single-supply operation also reduces power consumption by eliminating the need for a negative supply. |
Chapter 41: The Hysteresis and the Cost |
The hysteresis in the comparator adds cost to the digitizer. The additional resistor and the feedback network increase the component count and the complexity of the circuit. |
The adaptive hysteresis trigger circuit is designed to be cost-effective. The use of standard components and the simple circuit topology minimize the cost . |
The circuit's ability to operate from a single low-voltage supply also reduces the power supply cost. |
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Chapter 42: The Hysteresis and the Application-Specific Design |
The hysteresis in the comparator must be designed for the specific application. Different barcode reading applications have different noise levels, signal amplitudes, and scanning speeds. |
The adaptive hysteresis trigger circuit is designed for barcode reading applications. The ratio of the resistor network is optimized for typical barcode signals . |
The circuit's adaptive nature makes it suitable for a wide range of applications without requiring redesign. |
Chapter 43: The Hysteresis and the Integration with Other Circuits |
The hysteresis in the comparator must be integrated with the other circuits in the barcode reader. The digitizer, the decoder, and the power supply must work together seamlessly. |
The adaptive hysteresis trigger circuit is designed for integration with other circuits. The circuit's output is a clean square wave that is compatible with standard logic levels . |
The circuit's low voltage operation makes it suitable for integration with battery-powered systems. |
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Chapter 44: The Hysteresis and the Future Developments |
The hysteresis in the comparator is likely to become more sophisticated in the future. Digital signal processing may be used to implement adaptive hysteresis in software, providing even more flexibility. |
The trend toward greater integration will likely continue, with the hysteresis circuit being combined with other functions on a single chip. |
The use of artificial intelligence and machine learning may enable the comparator to learn the optimal hysteresis level for each barcode reading application. |
Chapter 45: Summary --- Hysteresis in Perspective |
Hysteresis is an essential feature of comparator circuits in barcode readers. It prevents the chatter that would otherwise occur when a noisy signal crosses the decision threshold, ensuring clean, reliable digitization. |
We have examined how different companies and technologies have approached the challenges of implementing hysteresis: |
Symbol Technologies developed a digitizer circuit with a gating comparator that uses hysteresis to determine the trip level, along with a false transition gating circuit and a margin threshold circuit for enhanced noise immunity. The circuit uses a single comparator for window determination, simplifying the design . |
Texas Instruments provides the TLC352C comparator, which is used in an adaptive hysteresis trigger circuit. The comparator's open-drain CMOS output allows it to switch down to its negative supply rail, enabling the adaptive hysteresis. The circuit uses positive and negative peak detectors to set the hysteresis levels, triggering at approximately +/-70% of the differentiated signal peaks . |
Johnson & Johnson Clinical Diagnostics developed a slope detector with a tracking hysteresis circuit that provides symmetrical hysteresis. The circuit uses an operational amplifier to track the input signal and regulate the hysteresis symmetrically, ensuring that the positive and negative hysteresis values are equal and independent of the comparator output amplitude . |
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The key lessons from our exploration are: |
Hysteresis prevents chatter. By creating a dead zone around the threshold, hysteresis ensures that the comparator does not switch back and forth due to noise. |
Hysteresis is implemented with positive feedback. A resistor network between the comparator output and the non-inverting input shifts the threshold after the comparator switches. |
Adaptive hysteresis tracks the signal amplitude. The hysteresis level is proportional to the peak-to-peak amplitude of the signal, providing noise immunity when the signal is strong and sensitivity when the signal is weak. |
Symmetrical hysteresis is important. When the positive and negative hysteresis values are equal, the comparator's performance is optimized. |
The resistor ratio is a critical design parameter. The ratio determines the amount of hysteresis and affects the comparator's sensitivity and noise immunity. |
Hysteresis is essential for reliable barcode reading. It ensures that the digitized signal accurately represents the barcode pattern, enabling the decoder to reliably extract the data. |
In the end, hysteresis is a testament to the importance of analog design in barcode readers. It is a simple but powerful technique that transforms a noisy, jittery signal into a clean, well-behaved digital pulse stream. The art of hysteresis lies in the careful balance of noise immunity and sensitivity, creating a digitizer that is robust, reliable, and accurate. |
cs@easiersoft.com