“In solid-state detectors for light detection, photodiodes are still the basic choice (Figure 1). Photodiodes are widely used in optical communications and medical diagnosis. Other applications include color measurement, information processing, bar codes, camera exposure control, electron beam edge detection, facsimile, laser collimation, aircraft landing assistance, and missile guidance.
“
In solid-state detectors for light detection, photodiodes are still the basic choice (Figure 1). Photodiodes are widely used in optical communications and medical diagnosis. Other applications include color measurement, information processing, bar codes, camera exposure control, electron beam edge detection, facsimile, laser collimation, aircraft landing assistance, and missile guidance.
Figure 1. Photodiode equivalent circuit
The light energy is transmitted to one of the sensors to generate current, which is further processed by the high-precision preamplifier. Analog-to-digital conversion and digital signal processing form the rest of the signal chain. The process of selecting a sensor and designing an analog front end can be reduced to seven steps:
1. Describe the signal to be measured and the design goal.
2. Select the appropriate sensor and describe its electrical output.
3. Determine the maximum gain that can be obtained.
4. Determine the optimal amplifier for the preamplifier stage.
5. Design a complete sensor and preamplifier gain module.
6. Run the simulation.
7. Build the hardware and perform verification.
Step 1: Signals and goals
According to the equivalent circuit in Figure 1, the output current is calculated as:
To convert light into electrical signals for further processing, you need to understand the AC and DC characteristics of the light source, the signal amplitude of the light source, the expected measurement resolution, and the power supply available in the system. Understanding the signal amplitude characteristics and noise level provides a basis for how to select the sensor, the necessary gain in the gain module, and what input voltage range and noise level may be required when selecting an analog-to-digital converter (ADC).
Assume that at room temperature, a light source emits 1 kHz light pulses with a brightness of 50 pW to 250 nW (0.006 lux). This is a very low amount of light and requires very precise signal conditioning and signal processing chains. The goal is to capture and process this signal with 16-bit resolution and precision. Achieving this resolution means that the measurement accuracy needs to reach 3.8 pW.
In addition, assuming that the system can use +12 V and
In solid-state detectors for light detection, photodiodes are still the basic choice (Figure 1). Photodiodes are widely used in optical communications and medical diagnosis. Other applications include color measurement, information processing, bar codes, camera exposure control, electron beam edge detection, facsimile, laser collimation, aircraft landing assistance, and missile guidance.
Figure 1. Photodiode equivalent circuit
The light energy is transmitted to one of the sensors to generate current, which is further processed by the high-precision preamplifier. Analog-to-digital conversion and digital signal processing form the rest of the signal chain. The process of selecting a sensor and designing an analog front end can be reduced to seven steps:
1. Describe the signal to be measured and the design goal.
2. Select the appropriate sensor and describe its electrical output.
3. Determine the maximum gain that can be obtained.
4. Determine the optimal amplifier for the preamplifier stage.
5. Design a complete sensor and preamplifier gain module.
6. Run the simulation.
7. Build the hardware and perform verification.
Step 1: Signals and goals
According to the equivalent circuit in Figure 1, the output current is calculated as:
To convert light into electrical signals for further processing, you need to understand the AC and DC characteristics of the light source, the signal amplitude of the light source, the expected measurement resolution, and the power supply available in the system. Understanding the signal amplitude characteristics and noise level provides a basis for how to select the sensor, the necessary gain in the gain module, and what input voltage range and noise level may be required when selecting an analog-to-digital converter (ADC).
Assume that at room temperature, a light source emits 1 kHz light pulses with a brightness of 50 pW to 250 nW (0.006 lux). This is a very low amount of light and requires very precise signal conditioning and signal processing chains. The goal is to capture and process this signal with 16-bit resolution and precision. Achieving this resolution means that the measurement accuracy needs to reach 3.8 pW.
In addition, assuming that the system can use +12 V and
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