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**Introduction to Linear Optocouplers**
Optical isolation is a widely used method for signal isolation. Common optocoupler devices and their associated circuits are frequently employed in various applications. Due to the simplicity of the optocoupler circuit, it is often used in digital isolation or data transmission systems, such as the 20 mA current loop in UART protocols. However, for analog signals, traditional optocouplers face limitations due to poor linearity between input and output, as well as significant temperature-related variations.
Transformer-based isolation is commonly used for high-frequency AC analog signals but is less suitable for low-frequency or DC signals. Some manufacturers offer isolated amplifiers as an alternative, like ADI’s AD202, which provides up to 0.025% linearity from DC to several kilohertz. These devices typically use voltage-to-frequency conversion, isolate the signal via a transformer, and then convert it back to voltage. While effective, these integrated solutions are complex, large, and expensive, making them unsuitable for mass production.
A more practical solution for analog signal isolation is the use of linear optocouplers. The basic principle of a linear optocoupler is similar to that of a standard optocoupler, but with an added feedback mechanism. This allows for the cancellation of non-linearities in both the forward and feedback paths, resulting in improved linearity. Popular linear optocouplers include Agilent’s HCNR200/201, TI’s TIL300, and CLARE’s LOC111. Here, we will focus on the HCNR200/201 as a case study.
**Chip Introduction and Principle Description**
The internal block diagram of the HCNR200/201 includes pins 1 and 2 for the input signal, 3 and 4 for feedback, and 5 and 6 for the output. The current flowing between pins 1 and 2 is denoted as IF, while the currents through pins 3–4 and 5–6 are IPD1 and IPD2, respectively. The input signal is converted into current (IF), and IPD1 and IPD2 are approximately linear with IF, with coefficients K1 and K2. These coefficients are small (e.g., 0.5% for HCNR200) and vary with temperature, but they are designed to be equal. This balance helps achieve better linearity in the overall system.
The HCNR201 offers higher linearity than the HCNR200, making it more suitable for precision applications. Key specifications for these devices include:
- **Linearity**: HCNR200: 0.25%, HCNR201: 0.05%
- **Linear coefficient K3**: HCNR200: 15%, HCNR201: 5%
- **Temperature coefficient**: -65ppm/°C
- **Isolation voltage**: 1414V
- **Signal bandwidth**: DC to over 1 MHz
While the optocoupler isolates current, achieving true voltage isolation requires additional circuitry, such as an operational amplifier. Below, we analyze a typical circuit configuration for the HCNR200/201, focusing on how feedback and current-voltage conversions are implemented.
**Typical Circuit Analysis**
Agilent's manual provides several practical circuit examples, one of which is shown below. In this setup, the input voltage is Vin, and the output is Vout. The optocoupler has two current transfer ratios, K1 and K2, which play a key role in determining the output.
The preamplifier circuit includes an op-amp with negative feedback, ensuring stability and linearity. Assuming the op-amp is not saturated, its output follows the equation:
$$ V_o = V_{oo} - G \cdot V_i $$
where $ V_{oo} $ is the output when the differential input is zero, and $ G $ is the gain. By applying Ohm’s law and considering the current through resistors R1 and R3, we can derive the relationship between input and output voltages.
Through detailed analysis, it becomes clear that the output is proportional to the input, with the proportionality determined by the ratio of resistors and the optocoupler’s transfer characteristics. Typically, R1 and R2 are set to equal values to avoid amplification and maintain isolation.
**Auxiliary Circuit and Parameter Selection**
To ensure proper operation, the op-amp must be chosen carefully based on its supply voltage, bandwidth, and slew rate. A single-supply op-amp like the LMV321 from TI is suitable for many applications. Additionally, resistor values should be selected to stay within the linear range of the op-amp and the maximum current rating of the optocoupler.
For example, if the supply voltage is 5V, and the input range is 0–4V, the maximum current IF is typically around 25mA. Using this, the resistor R3 can be calculated as 200Ω. For a non-inverting configuration without amplification, R1 and R2 can be matched at 32kΩ. A capacitor is also often added across R2 to filter out high-frequency noise.
**Summary**
This article provides an overview of linear optocouplers, including their design considerations, parameter selection, and reference circuits. It explains the working principles and how to implement a linear optocoupler in practical applications. This information is valuable for engineers working on isolated analog signal transmission systems.
September 08, 2025