Photodiode Mode: The Choice Between Speed and Silence in Barcode Readers |
Executive Summary |
This article provides a comprehensive, accessible exploration of the two primary operating modes for photodiodes in barcode readers: photovoltaic (zero bias) and photoconductive (reverse bias). We examine why this seemingly simple choice has profound implications for reader performance, affecting everything from signal-to-noise ratio and response time to circuit complexity and power consumption. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Texas Instruments, Hamamatsu Photonics, and ams OSRAM. We explore the physics behind each mode, the practical trade-offs in real-world applications, and the specific requirements of barcode reading that drive the design decision. The article covers both simple discrete designs and sophisticated integrated solutions, with special attention to the practical implications for circuit design, component selection, and system performance. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting photodetector circuits for barcode reader systems. |
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Chapter 1: The Photodiode at the Crossroads |
Every photodiode, regardless of its manufacturer or specifications, must be operated in one of two fundamental modes. These modes determine the speed, noise, linearity, and sensitivity of the device. The choice between photovoltaic mode and photoconductive mode is one of the most consequential decisions in barcode reader design. |
The photodiode is a semiconductor device that converts light into an electrical current through the photovoltaic effect . When photons strike the PN junction, they generate electron-hole pairs, producing a current that is directly proportional to the incident light intensity. This linear response makes photodiodes ideal for precision optical measurements such as barcode scanning . |
The two operating modes differ in how the photodiode is biased. In photovoltaic mode, the photodiode operates with zero bias, generating a voltage from the light it receives. In photoconductive mode, the photodiode is reverse-biased, using an external voltage to modify its electrical characteristics . |
The choice between these modes is not merely academic. It affects the reader's ability to capture fast-moving barcodes, to detect weak signals in low-light conditions, and to reject noise from ambient light. Understanding this choice is essential for anyone designing or selecting photodetector circuits for barcode reading applications. |
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Chapter 2: The Physics Behind the Modes |
To understand the difference between photovoltaic and photoconductive modes, it helps to understand the physics of the photodiode itself. |
A photodiode is a PN junction, the interface between P-type and N-type semiconductor materials . When light strikes the junction, photons with sufficient energy excite electrons, creating electron-hole pairs. The built-in electric field of the depletion region separates these carriers, producing a photocurrent . |
In photovoltaic mode, the photodiode is operated with zero external bias. The built-in electric field is solely responsible for separating the photocurrent carriers. This mode exploits the photovoltaic effect, which is the same principle that powers solar cells . The output is a voltage that builds up as the photocurrent flows out of the device. |
In photoconductive mode, the photodiode is operated with a reverse bias . This external voltage increases the width of the depletion layer, reducing the junction capacitance . The reduced capacitance dramatically decreases the response time, allowing the photodiode to detect fast-changing light signals. However, the reverse bias also induces a small current known as dark current or saturation current, which adds noise to the system . |
The reverse bias in photoconductive mode has another effect: the photocurrent remains virtually the same as in photovoltaic mode, but the response is faster. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance in both modes . |
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Chapter 3: Photovoltaic Mode --- The Quiet Operator |
Photovoltaic mode is the mode of choice when low noise is the paramount concern. By operating the photodiode with zero bias, the designer eliminates dark current as a source of noise . |
Dark current is the leakage current that flows through a photodiode even when no light is present. In photoconductive mode, the reverse bias increases the dark current, adding a DC offset and increasing the noise floor. In photovoltaic mode, there is no dark current because there is no bias . |
The elimination of dark current makes photovoltaic mode ideal for low-signal detection. The noise equivalent power (NEP) is lower, allowing greater sensitivity to weak light signals . This is particularly valuable for applications where the reflected light from the barcode is very weak, such as long-distance reading or low-contrast barcodes. |
However, photovoltaic mode has a significant disadvantage: it is slower than photoconductive mode. The junction capacitance is higher because the depletion region is narrower. This higher capacitance limits the response speed, making photovoltaic mode unsuitable for high-frequency applications . |
The response time in photovoltaic mode depends on the load resistance. With a 50-ohm load, the rise time for a typical photodiode may be around 2 microseconds . This is adequate for relatively slow scanning, but it is too slow for high-speed barcode reading where the bars pass the detector in microseconds. |
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Chapter 4: Photoconductive Mode --- The Speed Demon |
Photoconductive mode is the mode of choice when speed is the paramount concern. By reverse-biasing the photodiode, the designer widens the depletion region, reducing the junction capacitance and enabling fast response times . |
The reduced capacitance is the key advantage of photoconductive mode. Capacitance limits the bandwidth of the photodiode circuit; a lower capacitance allows higher frequencies to pass. For barcode reading, where the barcode pattern produces rapidly changing light signals, the speed advantage of photoconductive mode is essential. |
The reverse bias also improves linearity over a wider range of light intensities. Photoconductive mode provides a linear current response to light intensity, making it suitable for precise light measurement . The current is directly proportional to the incident light, preserving the barcode's contrast information. |
However, photoconductive mode has a significant disadvantage: increased noise. The reverse bias introduces dark current, which contributes to the noise floor . The dark current is typically in the nanoamp range for silicon PIN photodiodes , but it doubles with every 10C increase in temperature. |
Despite the increased noise, photoconductive mode is the preferred choice for most barcode readers. The speed advantage is simply too important to sacrifice, especially for readers that must capture moving barcodes. |
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Chapter 5: The PIN Photodiode --- Optimized for Speed |
The PIN photodiode is the most common type of photodiode used in barcode readers. The 'PIN' stands for P-type, Intrinsic, N-type, referring to the structure of the device. The intrinsic layer between the P and N regions increases the width of the depletion region, reducing the junction capacitance and improving the response speed. |
The PIN structure is inherently optimized for photoconductive mode. The wide depletion region, which makes the PIN photodiode fast, is further widened by reverse bias. This is why PIN photodiodes are almost always operated in photoconductive mode in barcode readers. |
The BPW34 family from ams OSRAM is a classic example of PIN photodiodes designed for barcode scanning. The BPW34FAS, for instance, has a response time of just 20 nanoseconds, a low dark current of 2 nanoamps, and a low capacitance of 7 picofarads . The low capacitance, combined with reverse bias, provides the speed needed for high-frequency optical detection. |
The Hamamatsu S8385 is another example of a PIN photodiode optimized for photoconductive mode. This device has a capacitance of 12 picofarads and is specifically designed for applications such as barcode scanners, laser radar, and optical switches . The S8385 is specified for operation in photoconductive mode, reflecting the manufacturer's expectation that it will be used with reverse bias. |
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Chapter 6: Hamamatsu's Photoconductive Photodiode --- The S8385 |
Hamamatsu Photonics is a leading manufacturer of photodiodes for barcode readers. The company's S8385 is a silicon PIN photodiode that is explicitly designed for photoconductive operation . |
The S8385 has a spectral range of 320 to 1100 nanometers, covering both visible and near-infrared light. Its responsivity ranges from 0.4 to 0.56 amps per watt, meaning it converts 40 to 56 percent of the incident light power into electrical current . This high efficiency is essential for detecting weak reflected light from barcodes. |
The dark current of the S8385 is specified as 0.1 to 1 nanoamp, which is exceptionally low for a photoconductive photodiode . This low dark current helps to minimize noise, even with reverse bias. The capacitance is 12 picofarads, which is low enough for high-speed detection. |
The S8385 is designed for applications including barcode scanners, free-space optical communications, optical switches, and laser radar . The versatility of this photodiode demonstrates how photoconductive mode can be applied to a wide range of optical sensing applications. Hamamatsu offers a wide range of optoelectronic devices for industrial applications, with photodetectors featuring high sensitivity, low noise, and high reliability . |
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Chapter 7: ams OSRAM's BPW34 Series --- A Workhorse in Photoconductive Mode |
The BPW34 series from ams OSRAM is among the most popular photodiode families for barcode scanners. These PIN photodiodes are designed for fast response and high sensitivity, making them ideal for photoconductive operation . |
The BPW34FAS variant has a spectral range of 730 to 1100 nanometers, a peak sensitivity at 940 nanometers, and a response time of just 20 nanoseconds . The dark current is 2 nanoamps, and the capacitance is 7 picofarads. These specifications are typical of a photodiode designed for photoconductive mode in high-speed applications. |
The BPW34 family is used in a wide range of applications, including barcode scanners, optical communications, industrial automation, and consumer electronics . The fast response time and high sensitivity make the BPW34 particularly suitable for detecting rapidly changing light signals, such as those produced by a moving barcode. |
The BPW34FAS is available in a surface-mount package, making it suitable for compact handheld readers . The device operates over a wide temperature range of -40 to 100C, ensuring reliability in various environments . |
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Chapter 8: Texas Instruments' TIPD176 --- A Practical Photoconductive Circuit |
Texas Instruments provides a practical reference design for a photodiode amplifier that operates in photoconductive mode. The TIPD176 is a single-supply transimpedance amplifier with a bandwidth greater than 1 MHz . |
The TIPD176 is designed to amplify the light-dependent current of a photodiode, producing output voltages from 0.1 to 4.9 volts for photodiode currents of 0 to 90 microamps . The circuit includes a small bias voltage derived from the positive supply and applied to the op-amp's non-inverting input. This prevents the output from saturating at the negative supply rail in the absence of input current . |
The TIPD176 is explicitly designed for applications such as barcode scanners, optical networking, and spectrometers . The reference design includes theory, component selection, simulation, PCB schematic and layout, bill of materials, and measured results . This comprehensive documentation makes it easier for designers to implement photoconductive mode photodiode circuits in their barcode readers. |
The TIPD176's 1 MHz bandwidth is typical for barcode scanner applications. This bandwidth is sufficient to capture the barcode pattern as it moves past the reading window, preserving the high-frequency components of the signal. |
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Chapter 9: The Bias Voltage --- How Reverse Bias Works |
The reverse bias voltage is the key to photoconductive mode. By applying a negative voltage to the photodiode's cathode relative to its anode, the depletion region is widened, and the junction capacitance is reduced. |
The reverse bias is typically in the range of 5 to 20 volts for silicon PIN photodiodes. The Hamamatsu S8385, for example, has a maximum reverse voltage of 20 volts . The BPW34FAS has a maximum reverse voltage of 16 volts . |
The reverse bias voltage must be chosen carefully. Too high a voltage can damage the photodiode, while too low a voltage may not provide sufficient speed. The optimal voltage is a balance between speed and noise, as higher voltages increase the dark current. |
The bias voltage is usually derived from the power supply through a resistor network. The TIPD176 reference design, for example, uses a bias voltage derived from the positive supply and applied to the op-amp's non-inverting input . This approach is simple and effective, requiring no separate bias supply. |
In some designs, the bias voltage is generated by a dedicated voltage regulator or charge pump. This is particularly important when the photodiode requires a bias voltage higher than the main power supply. |
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Chapter 10: Photovoltaic Mode Applications --- Where Silence Matters |
While photoconductive mode is the preferred choice for most barcode readers, photovoltaic mode has its place in specific applications where low noise is paramount . |
Photovoltaic mode is ideal for low-light detection because the absence of dark current reduces the noise floor. This allows the photodiode to detect very weak signals that would be swamped by noise in photoconductive mode . |
The photovoltaic mode is also advantageous in applications where power consumption is critical. Because there is no bias current, the photodiode consumes no power. This is a consideration for battery-powered readers, although the power consumption of the bias circuit is usually minimal. |
However, the speed limitation of photovoltaic mode is a significant drawback for barcode reading. The response time of a photodiode in photovoltaic mode can be orders of magnitude slower than in photoconductive mode . This makes photovoltaic mode unsuitable for high-speed scanning, where the barcode pattern must be captured in microseconds. |
For these reasons, photovoltaic mode is rarely used in barcode readers. It may be used in specialized applications where the barcode is stationary and the light level is very low, but these are niche applications. |
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Chapter 11: The Capacitance-Speed Trade-off --- A Deeper Dive |
The relationship between capacitance and speed is central to the choice between photovoltaic and photoconductive modes. The junction capacitance of the photodiode forms a low-pass filter with the load resistance, limiting the bandwidth of the circuit. |
In photovoltaic mode, the depletion region is narrow, and the junction capacitance is relatively high. This high capacitance limits the response speed, making the photodiode unable to track rapidly changing light signals . |
In photoconductive mode, the reverse bias widens the depletion region, reducing the junction capacitance. This lower capacitance allows higher frequencies to pass, enabling faster response times . |
The capacitance of a photodiode is specified in the datasheet. The BPW34FAS has a capacitance of 7 picofarads, which is typical for a fast PIN photodiode . The Hamamatsu S8385 has a capacitance of 12 picofarads . These low capacitances are essential for achieving the 20 nanosecond response times specified for these devices. |
The load resistance also affects the bandwidth. A lower load resistance reduces the time constant and improves the response speed, but it also reduces the signal amplitude. The designer must balance these trade-offs to achieve the desired performance. |
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Chapter 12: Dark Current and Noise --- The Price of Speed |
The dark current is the primary disadvantage of photoconductive mode. This current flows through the photodiode even when no light is present, adding a DC offset and contributing to the noise floor . |
The dark current increases with temperature. For silicon photodiodes, the dark current approximately doubles with every 10C increase. This temperature sensitivity must be considered when designing barcode readers for use in different environments. |
The dark current also increases with the reverse bias voltage. A higher bias voltage provides faster response but also higher dark current. The designer must balance these factors, choosing a bias voltage that provides sufficient speed without excessive noise. |
The dark current can be compensated by circuit design. The TIPD176 reference design, for example, uses a bias voltage that prevents the output from saturating at the negative rail in the absence of light . This ensures that the dark current does not swamp the signal. |
The noise introduced by the dark current is typically thermal noise (Johnson-Nyquist noise) from the load resistance, rather than shot noise from the photodiode itself. For a good PIN photodiode, the dark current is so low that the thermal noise of the load resistor dominates . |
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Chapter 13: Responsivity --- The Conversion Efficiency |
Responsivity is the measure of how effectively the photodiode converts light into electrical current. It is expressed in amps per watt (A/W) and depends on the wavelength of the light . |
Silicon photodiodes have peak responsivity in the near-infrared range, typically around 850 to 900 nanometers. At these wavelengths, the responsivity is typically 0.4 to 0.6 A/W, meaning that 40 to 60 percent of the incident light power is converted into electrical current . |
The responsivity is generally the same in photovoltaic and photoconductive modes. The reverse bias does not significantly change the quantum efficiency of the photodiode; it only affects the speed and noise characteristics. |
The Hamamatsu S8385 has a responsivity of 0.4 to 0.56 A/W over its spectral range of 320 to 1100 nanometers . The BPW34FAS has a peak responsivity at 940 nanometers, which matches the wavelength of many infrared LEDs used in barcode readers . |
The responsivity is important for barcode reading because it determines the signal strength for a given light level. A higher responsivity produces a stronger signal, making it easier to detect weak reflections from distant or low-contrast barcodes. |
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Chapter 14: Spectral Range --- Matching the Light Source |
The spectral range of the photodiode must match the wavelength of the illumination source. Silicon photodiodes have a spectral range of approximately 190 to 1100 nanometers, covering both visible and near-infrared light . |
The BPW34FAS has a spectral range of 730 to 1100 nanometers, which matches the wavelengths of red and infrared LEDs . This makes it suitable for barcode readers that use these illumination sources. |
The Hamamatsu S8385 has a broader spectral range of 320 to 1100 nanometers, covering ultraviolet, visible, and near-infrared light . This makes it suitable for a wider range of applications, including those that use UV or visible illumination. |
The spectral range is important because the photodiode must be sensitive to the wavelength of light that is reflected from the barcode. If the photodiode is not sensitive to the illumination wavelength, the signal will be weak, and the reader will not perform well. |
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Chapter 15: Photovoltaic Mode in Solar Cells --- A Comparison |
The photovoltaic mode is the basis for solar cells, which are essentially large-area photodiodes . In a solar cell, the photodiode is operated with zero bias, and the photocurrent is used to generate electrical power. |
The comparison with solar cells helps to illustrate the characteristics of photovoltaic mode. In a solar cell, the speed is not critical; the goal is to maximize the conversion efficiency and generate as much power as possible. The photovoltaic mode is ideal for this because it eliminates dark current and provides maximum output voltage. |
In barcode reading, the goal is different. The goal is to detect rapidly changing light signals with high sensitivity and low noise. The photovoltaic mode's speed limitation makes it unsuitable for this application. |
This comparison highlights the fundamental trade-off between photovoltaic and photoconductive modes. The choice depends on the application requirements: speed for barcode reading, efficiency for power generation. |
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Chapter 16: Photoconductive Mode in Optical Communication |
Optical communication is another application where photoconductive mode is essential. In fiber-optic receivers, the photodiode must detect high-speed optical signals, often at rates of gigabits per second. |
The photoconductive mode's fast response time makes it ideal for optical communication. The reverse bias reduces the junction capacitance, allowing the photodiode to respond to the rapidly changing optical pulses. |
The PIN photodiode is the workhorse of optical communication, and it is always operated in photoconductive mode. The low capacitance and fast response time of the PIN structure, combined with reverse bias, provide the speed needed for high-bandwidth communication. |
The TIPD176 reference design from Texas Instruments is explicitly designed for optical networking applications . This demonstrates how photoconductive mode is used in both barcode reading and optical communication, two applications with similar requirements for speed and sensitivity. |
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Chapter 17: Hamamatsu's S8385 --- Designed for Photoconductive Mode |
The Hamamatsu S8385 is a compelling example of a photodiode designed for photoconductive mode operation. The datasheet explicitly states that the device is designed for applications such as barcode scanners, FSO, optical switches, and laser radar . |
The S8385's specifications are tailored for photoconductive mode. The dark current is specified as 0.1 to 1 nanoamp, which is low enough for low-noise operation. The capacitance is 12 picofarads, which is low enough for high-speed detection. The maximum reverse voltage is 20 volts, providing a range for biasing . |
The Hamamatsu S8385 is an example of how photodiode manufacturers design their products for specific applications. The device is explicitly intended for photoconductive mode, reflecting the expectation that it will be used with reverse bias in high-speed applications. |
Hamamatsu's commitment to industrial applications is evident in their product portfolio. They offer a wide range of optoelectronic devices, including CMOS linear image sensors for barcode readers, photodetectors for optical encoders, and optical sensors for industrial equipment . |
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Chapter 18: ams OSRAM's BPW34FAS --- Fast and Low-Noise |
The ams OSRAM BPW34FAS is another example of a photodiode designed for photoconductive mode. This device offers a combination of fast response time and low dark current that is ideal for barcode reading . |
The BPW34FAS has a response time of 20 nanoseconds, which is fast enough for high-speed scanning. The dark current is only 2 nanoamps, which is low enough for low-noise operation. The capacitance is 7 picofarads, which is low enough for high bandwidth . |
The BPW34FAS is available in a surface-mount package, which is suitable for compact handheld readers . The device is also available in a TO-5 metal can package, which provides excellent shielding from electromagnetic interference. |
The BPW34FAS is typically operated with a reverse bias to achieve the specified speed. The low dark current ensures that the noise introduced by the reverse bias is minimal, making it an excellent choice for barcode reading applications. |
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Chapter 19: The Comparison Table --- A Summary of Trade-offs |
While this article avoids tables, a conceptual comparison between photovoltaic and photoconductive modes is worth summarizing in narrative form. |
Photovoltaic mode offers the lowest noise because there is no dark current. It is ideal for low-light detection and for applications where power consumption is critical. However, it is relatively slow because the junction capacitance is high. The response time is typically in the microsecond range, which is too slow for high-speed barcode reading. |
Photoconductive mode offers the highest speed because the reverse bias reduces the junction capacitance. The response time is typically in the nanosecond range, which is fast enough for barcode reading. However, the reverse bias introduces dark current, which adds noise to the signal. The dark current must be compensated by circuit design. |
The choice between the two modes is a trade-off between speed and noise. For barcode reading, speed is generally more important, so photoconductive mode is preferred. |
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Chapter 20: The Transimpedance Amplifier --- The Essential Companion |
The photodiode's current output must be converted to a voltage for further processing. This is the job of the transimpedance amplifier (TIA). |
The TIA is a specialized operational amplifier configuration that uses a feedback resistor to convert the input current to an output voltage. The output voltage is equal to the input current multiplied by the feedback resistance. |
The TIA is essential for photoconductive mode because the photodiode's current is small, typically in the microamp range. The TIA amplifies this current to a usable voltage level while preserving the signal's linearity and speed. |
The TIPD176 reference design from Texas Instruments is an example of a TIA designed for photoconductive mode. The circuit has a bandwidth greater than 1 MHz and a gain of 53.6 kilovolts per amp . The TIA is designed to produce output voltages from 0.1 to 4.9 volts for photodiode currents of 0 to 90 microamps . |
The TIA must be carefully designed to avoid instability and oscillation. The photodiode's capacitance can cause the TIA to oscillate, especially at high frequencies. Compensation capacitors are typically added to ensure stability. |
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Chapter 21: The Bias Voltage Source --- Simplifying the Circuit |
The bias voltage for photoconductive mode can be generated in several ways. The simplest method is to derive the bias from the main power supply using a resistor network. |
The TIPD176 reference design uses a small bias voltage derived from the positive supply and applied to the op-amp's non-inverting input. This prevents the output from saturating at the negative supply rail in the absence of input current . This approach is simple and effective, requiring no separate bias supply. |
In some designs, the bias voltage is generated by a dedicated voltage regulator or charge pump. This is necessary when the photodiode requires a bias voltage higher than the main power supply. The regulator must be low-noise to avoid adding noise to the photodiode signal. |
The bias voltage must be stable and well-regulated. Variations in the bias voltage can affect the photodiode's response time and dark current, degrading the reader's performance. |
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Chapter 22: Temperature Effects --- The Dark Current Challenge |
Temperature has a significant effect on the dark current of a photodiode. The dark current approximately doubles with every 10C increase in temperature. |
This temperature sensitivity is a challenge for barcode readers that must operate in a wide range of environments. A reader that works well in a cool retail environment may perform poorly in a warm warehouse. |
The dark current adds a DC offset to the photodiode signal. This offset must be compensated to prevent the signal from saturating the amplifier. The compensation can be done with a feedback loop, as described in earlier lessons. |
The temperature sensitivity also affects the noise performance of the photodiode. The dark current contributes to the noise floor, and the noise increases with temperature. This is why photoconductive mode is noisier than photovoltaic mode, especially at high temperatures. |
For applications where temperature stability is critical, the photodiode may be cooled or the dark current may be measured and compensated. |
Chapter 23: The Shunt Resistor --- Setting the Bandwidth |
The shunt resistor, also known as the load resistor, is a key component in the photodiode circuit. It converts the photodiode's current into a voltage, and it sets the bandwidth of the circuit. |
The shunt resistor is typically placed in parallel with the photodiode. The voltage across the resistor is proportional to the photocurrent, and this voltage is applied to the amplifier. |
The value of the shunt resistor determines the bandwidth of the circuit. A lower resistance provides a higher bandwidth but a lower voltage signal. A higher resistance provides a higher voltage signal but a lower bandwidth. |
The choice of shunt resistor is a trade-off between signal amplitude and bandwidth. For barcode reading, where speed is important, a relatively low resistance is typically used to achieve the required bandwidth. |
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Chapter 24: The PIN Photodiode's Advantages in Photoconductive Mode |
The PIN photodiode is particularly well-suited for photoconductive mode. The intrinsic layer between the P and N regions increases the width of the depletion region, reducing the junction capacitance and improving the response speed. |
The PIN structure's low capacitance is its key advantage. The low capacitance allows the photodiode to operate at higher frequencies, making it ideal for high-speed applications like barcode reading. |
The PIN photodiode also has a linear response over a wide range of light intensities. This linearity preserves the barcode's contrast information, making it easier to decode. |
The PIN photodiode's low dark current is another advantage. The dark current is typically in the nanoamp range, which is low enough for low-noise operation in photoconductive mode. |
The PIN photodiode is the workhorse of barcode reading, and it is always operated in photoconductive mode to achieve the required speed. |
Chapter 25: Photoconductive Mode in Laser Radar |
Laser radar, or LIDAR, is another application where photoconductive mode is essential. In laser radar, a laser pulse is transmitted, and the photodiode detects the reflected pulse. The time of flight of the pulse is measured to determine the distance. |
The photoconductive mode's fast response time is essential for laser radar. The photodiode must detect the reflected pulse quickly and accurately to measure the time of flight. |
The Hamamatsu S8385 is designed for both barcode scanners and laser radar applications . The fast response time and low dark current of the S8385 make it suitable for both applications. |
The requirements for laser radar are similar to those for barcode reading: speed, sensitivity, and low noise. The photoconductive mode provides all three. |
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Chapter 26: Photovoltaic Mode in Precision Measurements |
Photovoltaic mode is used in precision measurements where low noise is more important than speed. Examples include spectrophotometry, where the absorbance of a sample is measured, and photometry, where the intensity of light is measured. |
In these applications, the photodiode is typically operated with zero bias to minimize dark current noise. The photodiode's output is a voltage that is proportional to the light intensity, and this voltage is measured by a high-impedance voltmeter. |
The photovoltaic mode's low noise makes it ideal for these applications. The dark current is eliminated, and the noise floor is determined solely by the thermal noise of the load resistor. |
The speed limitation of photovoltaic mode is not a problem for these applications because the light intensity is typically constant or slowly varying. |
Chapter 27: The Avalanche Photodiode --- A Special Case |
The avalanche photodiode is a special type of photodiode that operates with a high reverse bias. The high reverse bias causes impact ionization, multiplying the photocurrent and providing internal gain. |
The avalanche photodiode is not commonly used in barcode readers because it is expensive and requires a high bias voltage. However, it is used in applications where extremely weak signals must be detected, such as optical communication and LIDAR. |
The avalanche photodiode operates in a mode that is similar to photoconductive mode, but with much higher bias. The dark current is also much higher, and the noise is correspondingly higher. |
The avalanche photodiode's gain is its main advantage. It can detect signals that are orders of magnitude weaker than those detectable by a standard PIN photodiode. However, for most barcode reading applications, the sensitivity of a PIN photodiode is sufficient. |
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Chapter 28: Photo ICs --- Integration for Compactness |
Photo ICs are integrated circuits that combine a photodetector element with a signal processing IC in the same package. They are smart optical sensors with diverse functions, offering advantages over discrete components . |
Hamamatsu offers Photo ICs that are smaller and lighter, more highly resistant to noise from electromagnetic induction, and more reliable than discrete components . These devices are suitable for compact barcode readers where space is limited. |
The Photo IC integrates the photodiode, the transimpedance amplifier, and the digitizing circuitry on a single chip. This reduces the component count, simplifies the PCB layout, and improves the reliability of the reader. |
The Photo IC is typically designed for a specific operating mode. Some Photo ICs are designed for photovoltaic mode, while others are designed for photoconductive mode. The choice depends on the application requirements. |
Chapter 29: The Comparison of Photodiode Types --- PIN vs. PN |
The PIN photodiode is the most common type for barcode reading, but PN photodiodes are also used. The difference between the two is the presence of the intrinsic layer. |
The intrinsic layer in the PIN photodiode increases the width of the depletion region, reducing the junction capacitance and improving the response speed. This makes the PIN photodiode faster than the PN photodiode. |
The PN photodiode is simpler and less expensive than the PIN photodiode. However, it is slower and has a higher capacitance, making it less suitable for high-speed applications. |
The PIN photodiode is almost always used in barcode readers because of its speed advantage. The PN photodiode may be used in low-cost, low-speed readers where performance is not critical. |
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Chapter 30: The Linearity of Photodiode Modes |
The linearity of the photodiode's response is important for barcode reading. The photodiode's output current must be directly proportional to the incident light intensity to preserve the barcode's contrast information. |
Both photovoltaic and photoconductive modes provide linear response over a wide range of light intensities. The linearity of the PIN photodiode is excellent in both modes . |
The linearity is maintained even at high frequencies, making the photodiode suitable for high-speed scanning. The photodiode's linearity is superior to that of the phototransistor, which can be non-linear at high light levels or high frequencies. |
The linearity of the photodiode's response is essential for decoding algorithms that use the signal amplitude, such as those that use adaptive thresholding. |
Chapter 31: The Rise Time --- A Key Speed Metric |
The rise time is a key metric for the speed of a photodiode. It is the time it takes for the output signal to rise from 10% to 90% of its final value in response to a step change in light. |
The BPW34FAS has a rise time of 20 nanoseconds in photoconductive mode . This is fast enough for barcode reading, where the transitions between bars and spaces occur in microseconds. |
The rise time is determined by the junction capacitance and the load resistance. A lower capacitance and a lower resistance provide a faster rise time. |
The rise time is important for barcode reading because it determines the sharpness of the signal's edges. A faster rise time produces sharper edges, which are easier to decode. |
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Chapter 32: The Fall Time --- Completing the Speed Picture |
The fall time is the time it takes for the output signal to fall from 90% to 10% of its initial value in response to a step change in light. |
The fall time is typically similar to the rise time for a PIN photodiode. The BPW34FAS has a fall time of 20 nanoseconds, matching its rise time . |
The fall time is important for barcode reading because it determines the sharpness of the signal's trailing edges. A faster fall time produces sharper edges, which are easier to decode. |
The rise and fall times together determine the photodiode's ability to track rapidly changing light signals. The photoconductive mode is designed to minimize both the rise and fall times. |
Chapter 33: The Responsivity in Photoconductive Mode |
The responsivity is the same in photovoltaic and photoconductive modes. The reverse bias does not change the quantum efficiency of the photodiode. |
The Hamamatsu S8385 has a responsivity of 0.4 to 0.56 A/W in photoconductive mode . The BPW34FAS has a peak responsivity at 940 nanometers . |
The responsivity is important for barcode reading because it determines the signal strength for a given light level. A higher responsivity produces a stronger signal, making it easier to detect weak reflections. |
The responsivity is typically specified for a particular wavelength. The designer must ensure that the photodiode's responsivity is adequate at the wavelength of the illumination source. |
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Chapter 34: The TIPD176's Bias Circuit --- A Practical Implementation |
The TIPD176 reference design from Texas Instruments provides a practical example of a bias circuit for photoconductive mode. The circuit is designed for single-supply operation, making it suitable for battery-powered barcode readers. |
The bias circuit derives a small bias voltage from the positive supply and applies it to the op-amp's non-inverting input. This prevents the output from saturating at the negative supply rail in the absence of input current . |
The bias circuit is simple and effective, requiring only a few resistors and a capacitor. The bias voltage is typically a few hundred millivolts, which is enough to keep the op-amp out of saturation. |
The TIPD176's bias circuit is a good example of how to implement photoconductive mode in a practical circuit. The design is well-documented, making it easy to replicate. |
Chapter 35: The Noise in Photoconductive Mode --- A Trade-Off |
The noise in photoconductive mode is higher than in photovoltaic mode because of the dark current. The dark current is a source of shot noise, which increases with the square root of the current. |
The noise can be reduced by minimizing the dark current. This can be done by using a photodiode with low dark current, operating at a low bias voltage, and keeping the temperature low. |
The noise can also be reduced by careful circuit design. The TIPD176 reference design uses a bias voltage that prevents saturation without increasing the dark current excessively . The amplifier is also chosen for low noise. |
The noise in photoconductive mode is typically dominated by the thermal noise of the load resistor, rather than the dark current. This is because the dark current of a good PIN photodiode is very low . |
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Chapter 36: The Speed in Photovoltaic Mode --- A Limitation |
The speed in photovoltaic mode is limited by the junction capacitance. The capacitance is higher because the depletion region is narrower, and this capacitance limits the response time. |
The response time in photovoltaic mode is typically in the microsecond range, which is too slow for high-speed barcode reading. The BPW34FAS, for example, has a rise time of 20 nanoseconds in photoconductive mode, but it would be much slower in photovoltaic mode. |
The speed limitation of photovoltaic mode is due to the RC time constant formed by the junction capacitance and the load resistance. This time constant determines the rise time of the output signal. |
For applications where speed is not critical, photovoltaic mode may be acceptable. However, for barcode reading, the speed of photoconductive mode is generally required. |
Chapter 37: The Application in Barcode Scanners --- A Clear Choice |
The choice between photovoltaic and photoconductive modes for barcode scanners is clear: photoconductive mode is the preferred choice. |
The speed advantage of photoconductive mode is essential for barcode reading. The barcode pattern must be captured as it moves past the reading window, and the transitions between bars and spaces must be detected quickly and accurately. |
The noise disadvantage of photoconductive mode is acceptable because the signal-to-noise ratio is still sufficient for reliable decoding. The dark current can be compensated by circuit design, and the noise is typically dominated by other sources. |
The TIPD176 reference design from Texas Instruments is explicitly designed for barcode scanners and uses photoconductive mode . The Hamamatsu S8385 and ams OSRAM BPW34FAS are designed for barcode scanners and are specified for photoconductive mode . |
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Chapter 38: The PIN Photodiode's Dominance in Barcode Reading |
The PIN photodiode is the dominant photodiode type in barcode reading. Its speed, low capacitance, and low dark current make it ideal for photoconductive mode operation. |
The PIN photodiode's intrinsic layer increases the width of the depletion region, reducing the junction capacitance. This low capacitance, combined with reverse bias, provides the speed needed for barcode reading. |
The PIN photodiode's linearity preserves the barcode's contrast information. The linearity is excellent in photoconductive mode over a wide range of light intensities. |
The BPW34 family from ams OSRAM and the S8385 from Hamamatsu are examples of PIN photodiodes designed for barcode reading . |
Chapter 39: The Quantum Efficiency --- A Measure of Performance |
The quantum efficiency of a photodiode is the ratio of the number of electron-hole pairs generated to the number of incident photons. It is a measure of the photodiode's performance. |
The quantum efficiency is typically high for silicon photodiodes, often exceeding 80% . This high efficiency ensures that most of the incident light is converted into a usable electrical signal. |
The quantum efficiency is similar in photovoltaic and photoconductive modes. The reverse bias does not significantly change the quantum efficiency. |
The quantum efficiency is wavelength-dependent. The photodiode's spectral response must match the wavelength of the illumination source to maximize the quantum efficiency. |
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Chapter 40: The Photodiode in Optical Networking --- A Related Application |
Optical networking is a related application where photoconductive mode is essential. In fiber-optic receivers, the photodiode must detect high-speed optical signals. |
The requirements for optical networking are similar to those for barcode reading: speed, sensitivity, and low noise. The photoconductive mode provides all three. |
The TIPD176 reference design from Texas Instruments is designed for both barcode scanners and optical networking applications . This demonstrates the commonality between the two applications. |
The speed of photoconductive mode is essential for optical networking, where data rates can be gigabits per second. The photodiode must respond quickly to the rapidly changing optical pulses. |
Chapter 41: The Hamamatsu Portfolio --- A Wide Range of Options |
Hamamatsu Photonics offers a wide range of photodiodes for industrial applications, including barcode scanners. The company's products feature high sensitivity, low noise, and high reliability . |
Hamamatsu's photodiode portfolio includes silicon PIN photodiodes, CMOS linear image sensors, and Photo ICs. The S8385 is specifically designed for barcode scanners and other high-speed applications . |
Hamamatsu's commitment to quality and reliability makes it a trusted supplier for barcode reader manufacturers. The company's technical expertise extends to application support, helping designers integrate its components into their designs. |
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Chapter 42: The ams OSRAM Portfolio --- A Broad Range of Photodiodes |
ams OSRAM offers a broad portfolio of photodiodes for barcode reading, including the BPW34 series. The BPW34FAS is a high-speed, high-sensitivity PIN photodiode that is well-suited for photoconductive mode . |
ams OSRAM's photodiode portfolio includes both through-hole and surface-mount packages, providing flexibility for different PCB designs. The BPW34FAS is available in a surface-mount package, making it suitable for compact handheld readers . |
ams OSRAM's technical expertise and product quality make it a leading supplier for barcode reader manufacturers. |
Chapter 43: The Bias Voltage in Practice --- A Design Example |
The bias voltage for photoconductive mode must be carefully chosen to achieve the desired speed without excessive noise. The TIPD176 reference design provides a practical example. |
The TIPD176 uses a single supply voltage and derives a small bias voltage from the positive supply. This bias voltage is applied to the op-amp's non-inverting input, preventing the output from saturating . |
The bias voltage is typically a few hundred millivolts, which is enough to keep the op-amp out of saturation without adding excessive dark current. The bias voltage is stable and well-regulated, ensuring consistent performance. |
The TIPD176's bias circuit is a good example of a simple and effective design. It uses standard components and is easy to implement. |
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Chapter 44: The Future of Photodiode Modes --- Emerging Trends |
The trend in barcode reader design is toward greater integration and digital processing. Photo ICs, which integrate the photodiode with the signal processing circuitry, are becoming more common . |
The Photo IC can be designed for either photovoltaic or photoconductive mode, depending on the application requirements. The choice is made by the IC designer, simplifying the system design for the end user. |
The trend toward integration is likely to continue, with more functions being combined on a single chip. This will reduce the component count, simplify the PCB layout, and improve the reliability of barcode readers. |
However, the fundamental choice between photovoltaic and photoconductive modes will remain. The trade-off between speed and noise is inherent to the physics of the photodiode, and it will continue to shape the design of barcode readers for the foreseeable future. |
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Chapter 45: Summary --- The Photodiode Mode in Perspective |
The choice between photovoltaic and photoconductive modes is one of the most fundamental decisions in barcode reader design. It affects the speed, noise, and linearity of the photodetector, and it has significant implications for the circuit design and overall system performance. |
We have examined how different companies and technologies have approached the challenges of photodiode mode selection: |
Texas Instruments provides the TIPD176 reference design, a practical example of a photoconductive mode transimpedance amplifier. The design is optimized for barcode scanners and other optical networking applications, with a bandwidth greater than 1 MHz and single-supply operation . |
Hamamatsu Photonics offers the S8385, a silicon PIN photodiode explicitly designed for photoconductive operation. The device is specified for barcode scanners, laser radar, and optical switches, with low dark current and fast response . |
ams OSRAM provides the BPW34FAS, a high-speed, high-sensitivity PIN photodiode with a response time of 20 nanoseconds. The device is designed for photoconductive mode and is available in a surface-mount package . |
The physics of photodiodes reveals the fundamental trade-off between speed and noise. Photovoltaic mode eliminates dark current but is slow; photoconductive mode provides high speed but introduces dark current noise . |
The PIN photodiode is the dominant type for barcode reading, optimized for photoconductive mode with its low capacitance and fast response time. |
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The key lessons from our exploration are: |
Speed is paramount in barcode reading. The photodiode must respond quickly to the rapidly changing light signals from the barcode. Photoconductive mode provides the speed needed for high-performance reading. |
Noise is the trade-off. The dark current introduced by reverse bias adds noise to the signal. However, for most barcode reading applications, the speed advantage outweighs the noise disadvantage. |
The bias voltage must be carefully chosen. The bias voltage determines the speed and noise characteristics of the photodiode. A voltage that is too high increases dark current; a voltage that is too low limits speed. |
Integration is the trend. Photo ICs, which combine the photodiode with signal processing circuitry, are becoming more common. These devices simplify the design and improve the reliability of barcode readers. |
The choice depends on the application. For high-speed applications like barcode reading, photoconductive mode is the clear choice. For low-speed, low-light applications, photovoltaic mode may be preferred. |
In the end, the choice between photovoltaic and photoconductive modes is a choice between silence and speed. The photoconductive mode gives the barcode reader the speed it needs to capture the pattern, while the photovoltaic mode provides the silence needed for ultimate sensitivity. For the barcode reader, speed wins, and photoconductive mode is the default. |
cs@easiersoft.com