Many people find the topic of analog signal separation quite challenging. While there are numerous methods for isolating digital signals, solutions for analog signal isolation are much more limited. However, in many applications, it's essential to isolate analog signals, and this is typically done based on the following requirements:
1. Isolation of interference sources;
2. Separation of high voltage.
There are many ways to isolate digital signals, but when it comes to analog signals, the options are fewer and often more expensive than expected. Especially in precision measurement scenarios, analog signal isolation can be costly and technically demanding. Having worked on such systems for years, I’ve gathered some insights into the common isolation techniques:
Digital Isolation Methods:
1. Optocouplers
2. ADI’s magnetic isolation chips, like the ADuMXXXX series (e.g., ADuM1250 for I2C)
3. Isolation using a transformer
Digital isolation is generally used for unidirectional signals. For bidirectional communication, two isolation units are needed, which increases the size and cost significantly. If the signal speed is below 100kHz, optocouplers like the PS2501 are a good and cost-effective choice, offering high isolation voltage—often above 3000V RMS.
However, for frequencies above 200kHz, optocouplers like the PS2501 may not perform well. High-speed optocouplers such as the 6N137 or 6N136 are available, but they are expensive and take up a lot of PCB space. In contrast, ADI’s magnetic isolation chips, like the ADuM series, offer better performance at a lower cost—around 4-5 RMB per channel. They are also smaller and more efficient, though their isolation voltage is limited to around 1000V, which may not be sufficient for high-voltage applications.
The third method, using an isolation transformer, is rarely used by hobbyists due to its lack of economic benefit. It offers very high isolation voltages, making it suitable for applications like driving IGBTs or inverters where voltages exceed 5000V. But again, it's complex and costly for most use cases.
Analog Signal Isolation Methods:
1. Linear optocouplers;
2. Isolation amplifiers;
3. Frequency conversion and voltage conversion + digital isolation;
4. Flying capacitors;
5. DA/AD + digital isolation for sampling and recovery;
6. Linear isolation using standard optocouplers.
Linear optocouplers, such as the TIL300 (now discontinued), have been replaced by alternatives like the Vishay IL300. These are good for general analog signal isolation, but they require additional op-amps and resistors to improve linearity. The IL300 costs around 10 RMB, and with better op-amps, the total cost can reach about 30 RMB per channel. The isolation frequency is around 200kHz, but the linearity is only about ±0.5%, which is acceptable for many applications but not for high-precision ones.
Isolation amplifiers, such as TI’s ISO124 or ADI’s models, offer excellent linearity—often better than 0.1%—but come with a high price tag. A single channel can cost over 40 RMB, and ADI’s products can go up to $40 per channel, which is impractical for many industrial applications.
The third method involves converting the analog signal to digital, isolating it, and then converting it back. This approach is more complex but offers better accuracy and cost-effectiveness, especially when combined with digital isolation. Using microcontrollers or FPGs for AD/DA conversion can bring the cost down to around 30 RMB per channel, with better performance than linear optocouplers.
Flying capacitors are another technique, though not widely used. They work by charging a capacitor from the signal side and discharging it on the isolated side. This method doesn’t require power isolation and has a simple circuit, but it’s limited to low-frequency signals, typically under 10Hz.
Finally, using ordinary optocouplers with feedback loops is a low-cost solution, but it lacks the stability and consistency required for large-scale applications. It’s best suited for situations where linearity isn’t critical and cost is the main concern.
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