How to design analog signal isolation for I/O modules?

The design of analog isolation schemes for I/O modules requires selecting the appropriate technology path based on accuracy, bandwidth, and cost. For DC or low-frequency signals, isolation amplifiers are often used, such as Σ-Δ isolated ADCs employing modulation and demodulation technology. These digitize the analog signal and transmit it through a digital isolator, achieving high accuracy and high common-mode rejection ratio. For intermediate-frequency signals, linear optocouplers can be used in conjunction with external operational amplifiers to form an isolation circuit, but additional circuitry is needed to compensate for the nonlinearity and temperature drift of the optocouplers. Another approach is to use an isolated DC-DC converter to power the field-side sensors, while simultaneously using an isolated ADC or V/F converter for signal acquisition.

Key design considerations include: providing sufficient power and stability to the isolation power supply; ensuring proper grounding and shielding on both sides of the isolation barrier to prevent noise introduction through capacitive coupling; and ensuring that the isolation voltage, creepage distance, and clearance of the isolation devices themselves meet safety regulations. During layout, the isolation devices should be connected across the isolation strip on the PCB, with the circuits on both sides strictly separated. After design, withstand voltage and accuracy tests are required.