Example - Comparator with Hysteresis: The LMV7219 in Barcode Scanning Applications |
Executive Summary |
This article provides a practical exploration of comparator circuits with hysteresis, focusing on the Texas Instruments LMV7219 as a concrete design example for barcode scanners and other portable systems. We examine why hysteresis is essential for clean digitization of noisy barcode signals, how the LMV7219 implements internal hysteresis to simplify design, and how designers can add external hysteresis when additional noise margin is required. Rather than focusing on abstract theory, we ground every concept in real component datasheets and application notes from Texas Instruments and National Semiconductor. We explore the key parameters of the LMV7219---its 7-nanosecond propagation delay, 1.1 mA supply current, rail-to-rail output, and internal hysteresis---and explain why these features make it ideal for battery-powered barcode readers. The article also covers the practical design procedure for adding external hysteresis, including resistor selection and the trade-offs between response time and 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: Why Hysteresis Matters in Barcode Readers |
In a barcode reader, the comparator is the final decision-maker in the analog signal chain. It takes the conditioned, amplified signal from the photodetector and decides, for every instant in time, whether the scanner is looking at a black bar or a white space. This decision is made by comparing the signal to a threshold voltage. If the signal is above the threshold, the output is high (white space). If it is below, the output is low (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. |
Hysteresis is the solution to this problem. Hysteresis is a form of positive feedback that shifts the threshold slightly after the comparator switches, creating a 'dead zone' that rejects noise. 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. |
In the context of a barcode scanner, hysteresis is essential for reliable digitization. The Texas Instruments datasheet for the LMV7219 explains: 'The internal hysteresis ensures clean output transitions even with slow-moving inputs signals.' This is particularly important in barcode readers because the signal from a hand-scanned barcode changes relatively slowly compared to the comparator's speed, and noise can easily cause false triggering without hysteresis. |
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Chapter 2: Introducing the LMV7219 Comparator |
The LMV7219 is a low-power, high-speed comparator with internal hysteresis, originally introduced by National Semiconductor and now part of the Texas Instruments portfolio. It is specifically designed for applications such as barcode scanners, portable and battery-powered systems, and high-speed signal conditioning. |
When it was introduced in 1999, the LMV7219 was notable for combining high speed with low power consumption and a compact package. As National Semiconductor's technical marketing manager explained at the time: 'Digital data signals are transmitted at very high speeds over long distances, so these signals need to be conditioned by high-speed comparators... this new chip has a symmetrical output response time that is extremely fast, ensuring that the system can fully utilize its signal efficiency'. |
The LMV7219 is available in space-saving SC-70-5 and SOT-23-5 packages, which are ideal for systems where small size and low power are critical. Its key characteristics for barcode scanner applications include: |
Propagation delay: 7 nanoseconds typical at 5 V |
Supply current: 1.1 mA at 5 V |
Input common mode range: Extends 200 mV below ground |
Internal hysteresis: Ensures clean switching |
Rail-to-rail output: Push-pull configuration |
Operating voltage: 2.7 V to 5 V single supply |
Fast rise and fall times: 1.3 ns |
These specifications make the LMV7219 well-suited for barcode scanners, where both speed and power efficiency are essential. |
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Chapter 3: The LMV7219's Internal Hysteresis |
The LMV7219 includes 7 mV of internal hysteresis, which significantly simplifies circuit design. The Texas Instruments datasheet explains the benefit: 'Standard comparators require hysteresis to be added with external resistors. The fixed internal hysteresis eliminates these resistors'. |
The internal hysteresis works by creating two trip points: one for rising input voltages and one for falling input voltages. The difference between these trip points is the hysteresis. With internal hysteresis, 'when the comparator's input voltages are equal, the hysteresis effectively causes one comparator-input voltage to move quickly past the other, thus taking the input out of the region where oscillation occurs'. |
This is particularly valuable in barcode readers because the input signal from the photodetector can change slowly and may linger near the threshold. Without hysteresis, the comparator output would chatter, creating false edges in the digitized signal. With hysteresis, the comparator switches cleanly and stays switched until the input moves significantly away from the threshold. |
The LMV7219's internal hysteresis is stable across supply voltage and common-mode input variations, as reflected in the datasheet specifications. This stability ensures consistent performance across a wide range of operating conditions, which is important for barcode readers used in diverse environments. |
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Chapter 4: The LMV7219's Push-Pull Output |
The LMV7219 features a push-pull, rail-to-rail output stage. This means the output can swing close to both the positive supply rail (VCC) and ground, providing a clean digital signal that can directly drive logic circuits without external pull-up resistors. |
The push-pull output is an important advantage over open-drain or open-collector comparators, which require external pull-up resistors. In a barcode scanner, eliminating these external resistors reduces component count and simplifies the PCB layout. The rail-to-rail capability ensures that the output signal is compatible with the full logic voltage range, providing maximum noise margin. |
The push-pull output also provides fast rise and fall times. As the datasheet notes, 'when the output switches, there is a direct path between VCC and ground, causing high output sinking or sourcing current during the transition. After the transition, the output current decreases and the supply current settles back to about 1.1 mA at 5 V, thus conserving power consumption'. |
This power-saving characteristic is important for battery-powered barcode scanners, where every milliampere matters. |
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Chapter 5: The LMV7219's Input Characteristics |
The LMV7219's input characteristics are designed for compatibility with single-supply systems and ground-sensing applications. The input common-mode voltage range extends 200 mV below ground, allowing the comparator to sense signals that are very close to the ground reference. |
The input stage uses a pair of PNP transistors, which means the input bias current flows out of the device. The datasheet explains: 'If either of the input signals falls below the negative common mode limit, the parasitic PN junction formed by the substrate and the base of the PNP will turn on, resulting in an increase of input bias current'. |
If one input goes above the positive common mode limit, the output will still maintain the correct logic level as long as the other input stays within the common mode range. However, the propagation delay will increase. When both inputs are outside the common mode voltage range, current saturation occurs in the input stage, and the output becomes unpredictable. |
These characteristics must be considered when designing the threshold and signal conditioning circuits for a barcode reader. The input signals should be kept within the common-mode range for predictable operation. |
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Chapter 6: The LMV7219's 7 ns Propagation Delay |
The LMV7219's propagation delay of 7 nanoseconds is a key specification for barcode scanner applications. This extremely fast response time allows the comparator to accurately capture the edges of the barcode signal, even when scanning high-density barcodes at high speed. |
The datasheet notes that 'the propagation delay does not increase significantly with large differential input voltages'. This means the comparator responds consistently to both weak and strong signals, which is important for reading barcodes at varying distances. |
The 7 ns propagation delay is achieved while consuming only 1.1 mA of supply current at 5 V. This combination of speed and low power is what makes the LMV7219 suitable for portable and battery-powered systems. |
In a barcode scanner, the comparator's propagation delay, combined with the photodetector and amplifier response times, determines the overall bandwidth of the digitizer. A fast comparator ensures that the digitized signal accurately represents the barcode pattern, preserving the edges of the bars and spaces. |
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Chapter 7: A Practical Design Example - The LMV7219 in a Scanner |
The LMV7219 is explicitly listed as suitable for scanner applications in its datasheet. A typical implementation in a barcode scanner would follow the standard comparator configuration: the conditioned analog signal from the transimpedance amplifier and gain stages is applied to the non-inverting input, while a threshold voltage is applied to the inverting input. |
The threshold voltage is typically derived from a resistor divider or from the output of a digital-to-analog converter controlled by the microcontroller. In an adaptive threshold system, the threshold can be adjusted dynamically based on the signal amplitude. |
The output of the LMV7219 is a clean digital signal that can directly interface with the microcontroller's input capture or interrupt pin. The push-pull output eliminates the need for an external pull-up resistor, reducing component count. |
The internal hysteresis of 7 mV is sufficient for most barcode scanner applications. However, if additional hysteresis is required---for example, when the signal is particularly noisy or has a low contrast---the designer can add external hysteresis using the method described in the datasheet. |
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Chapter 8: Adding External Hysteresis to the LMV7219 |
While the LMV7219 includes internal hysteresis, there may be applications where additional hysteresis is desirable. The datasheet provides a detailed design procedure for adding external hysteresis using three resistors. |
The typical method of adding external hysteresis uses positive feedback. The positive feedback shifts the comparator trip point depending on the state of the output. The design procedure involves three steps: |
First, select R3. The current through R3 should be greater than the input bias current to minimize errors. The current through R3 at the trip point is (VREF - VOUT) / R3. Consider the two possible output states when solving for R3, and use the smaller of the two resulting resistor values. |
Second, choose the amount of hysteresis (VHYS). The hysteresis is determined by the resistor network and the output voltage swing. |
Third, calculate the remaining resistor values. The specific equations are provided in the datasheet. |
The datasheet warns that 'the positive feedback method slows the comparator response time'. This is an important trade-off: additional hysteresis provides more noise immunity but reduces the comparator's speed. |
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Chapter 9: Design Considerations for External Hysteresis |
When adding external hysteresis to the LMV7219, several practical considerations must be kept in mind. First, the resistor values should be chosen to minimize errors due to input bias current. The LMV7219 has a typical input bias current of 450 nA, which is relatively low, but can still cause errors if the resistor values are too high. |
Second, the hysteresis voltage should be chosen to provide sufficient noise immunity without overly degrading the comparator's sensitivity. A typical hysteresis voltage for barcode scanner applications might be on the order of 10 to 50 mV, depending on the signal-to-noise ratio. |
Third, the propagation delay of the comparator increases with external hysteresis. The datasheet advises that 'the positive feedback method slows the comparator response time'. If the scanner requires the fastest possible response, the internal hysteresis alone may be sufficient. |
Finally, the PCB layout should minimize parasitic capacitance and inductance on the input and feedback paths. The comparator's 7 ns propagation delay is fast enough that stray capacitance can cause ringing or oscillation, particularly when external hysteresis is added. |
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Chapter 10: The LMV7219 in Battery-Powered Scanners |
The LMV7219's low supply current of 1.1 mA at 5 V makes it ideal for battery-powered barcode scanners. In a handheld scanner, the comparator is typically powered continuously, so its quiescent current directly affects battery life. |
The datasheet explains the power-saving characteristics of the push-pull output: 'When the output switches, there is a direct path between VCC and ground, causing high output sinking or sourcing current during the transition. After the transition, the output current decreases and the supply current settles back to about 1.1 mA at 5 V, thus conserving power consumption'. |
This means the comparator only draws significant current during switching transitions, which is typically a small fraction of the operating time in a barcode scanner. The rest of the time, the comparator draws only the 1.1 mA quiescent current. |
The low supply current is achieved while maintaining a fast 7 ns propagation delay. This combination of speed and low power is achieved through the device's BiCMOS process technology. |
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Chapter 11: The LMV7219's Temperature Range |
The LMV7219 is specified for operation over a temperature range of -40C to 85C, making it suitable for industrial and commercial barcode scanner applications. The device also supports a PCB temperature of up to 105C. |
The wide temperature range ensures stable performance in barcode scanners used in warehouses, outdoor applications, and other demanding environments. The internal hysteresis is stable across temperature variations, ensuring reliable digitization over the entire temperature range. |
The package options---SC-70-5 and SOT-23-5---are designed for surface-mount assembly and are suitable for the compact PCBs used in modern handheld barcode readers. |
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Chapter 12: Comparative Design Example |
To illustrate the LMV7219's advantages, consider a typical barcode scanner application where the conditioned analog signal has a peak-to-peak amplitude of 1 V and the threshold is set at 0.5 V. The signal contains noise of about 10 mV peak-to-peak. |
Without hysteresis, the comparator output would chatter when the signal is near the threshold, causing multiple transitions for a single barcode edge. The LMV7219's 7 mV internal hysteresis ensures that once the comparator switches, it stays switched until the input moves at least 7 mV past the threshold. |
If the noise level exceeds 7 mV, additional external hysteresis can be added. Following the datasheet design procedure, the designer can add 20 mV of hysteresis to provide a comfortable margin. This ensures clean digitization even with noisy signals. |
The 7 ns propagation delay ensures that the edges are captured accurately, preserving the widths of the bars and spaces. The push-pull rail-to-rail output ensures compatibility with the microcontroller's digital inputs. |
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Chapter 13: Summary --- The LMV7219 in Perspective |
The LMV7219 is a practical example of a comparator designed specifically for barcode scanners and other portable systems. Its combination of 7 ns propagation delay, 1.1 mA supply current, internal hysteresis, and rail-to-rail output makes it ideal for the demanding requirements of barcode reading applications. |
We have examined the key features of the LMV7219: |
Internal hysteresis eliminates the need for external resistor networks in most applications, simplifying circuit design and reducing component count. The 7 mV hysteresis provides sufficient noise immunity for typical barcode scanner signals. |
Fast 7 ns propagation delay ensures accurate edge detection, preserving the widths of bars and spaces. This is essential for decoding high-density barcodes. |
Low 1.1 mA supply current makes the LMV7219 suitable for battery-powered handheld scanners. The push-pull output conserves power after switching transitions. |
Rail-to-rail output provides a clean digital signal that can directly interface with microcontrollers without external pull-up resistors. |
SC-70-5 and SOT-23-5 packages provide a compact form factor ideal for space-constrained portable devices. |
The ability to add external hysteresis provides flexibility for applications with particularly noisy signals or low contrast. The datasheet provides a detailed design procedure for adding external hysteresis with three resistors. |
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The key lessons from our exploration are: |
Hysteresis is essential for clean digitization. Internal hysteresis eliminates the need for external resistors in most applications. |
Speed and power consumption must be balanced. The LMV7219 achieves 7 ns propagation delay with only 1.1 mA supply current. |
Package size matters for handheld devices. The SC-70 and SOT-23 packages are ideal for portable systems. |
External hysteresis can be added when needed. The datasheet provides a design procedure for applications requiring additional noise immunity. |
The comparator is the final decision-maker. Its performance determines the quality of the digitized signal and the reliability of barcode decoding. |
In the end, the LMV7219 is a testament to the importance of thoughtful component selection in barcode reader design. It is a component that combines speed, low power, and hysteresis in a small package, making it ideal for the demanding requirements of portable barcode scanning. The art of comparator selection lies in balancing speed, power, and noise immunity to create a digitizer that reliably converts noisy analog signals into clean digital data. |
cs@easiersoft.com