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Optocouplers are widely used for signal isolation, especially in digital applications such as UART communication. However, traditional optocouplers have limitations when it comes to analog signal isolation due to their nonlinear behavior and temperature sensitivity. Transformer-based isolation is often preferred for high-frequency AC signals, but not for DC or low-frequency applications. Isolation amplifiers like the AD202 from Analog Devices offer high linearity but come with higher costs and complexity.
A better alternative for analog signal isolation is the use of linear optocouplers. Unlike standard optocouplers, linear optocouplers incorporate a feedback mechanism that helps cancel out nonlinearity, allowing for more accurate signal transmission. Chips like the HCNR200/201 from Agilent, TIL300 from TI, and LOC111 from Clare are popular choices. These devices provide good linearity, wide bandwidth, and reliable isolation.
The internal structure of the HCNR200/201 includes an input stage, a photodiode pair for feedback, and an output stage. The current transfer ratio (CTR) between the input and output stages is crucial for determining the performance of the device. The design ensures that the CTR remains stable across temperature variations, which is essential for maintaining accuracy.
In practical applications, a typical circuit using the HCNR200/201 involves an operational amplifier to manage the feedback loop and ensure linear operation. The op-amp adjusts the output based on the feedback signal, allowing for precise voltage scaling. Proper selection of resistors and op-amps is essential to maintain linearity and avoid saturation.
When designing a circuit with a linear optocoupler, it's important to consider the operating range of the op-amp and the maximum current the optocoupler can handle. Choosing the right resistor values ensures that the circuit operates within its linear range and avoids distortion. Additionally, adding a small capacitor across the feedback resistor can help filter out high-frequency noise, improving overall signal quality.
For example, if the supply voltage is 5V and the input signal ranges from 0 to 4V, the resistor values should be selected to match the optocoupler’s specifications. Using a LMV321 op-amp from Texas Instruments can provide the necessary performance for this application. By carefully selecting components and designing the feedback network, engineers can achieve accurate and reliable analog signal isolation using linear optocouplers.
September 08, 2025