The IL300 Linear Optocoupler is a linear optically isolated device designed for precision analog signal transfer. Its dual photodiode architecture allows implementation of closed-loop feedback circuits for improved gain stability and linearity.
This device is commonly used for isolated voltage monitoring and current sensing in industrial control systems. An automotive-qualified option is also available for high-reliability applications, it’s VOA300.
Figure 1: Typical closed-loop IL300 isolation amplifier configuration
Circuit Description
In this configuration, the IL300 is used as an isolation amplifier, where the gain can be individually adjusted:
- U1 drives the LED input current (IF)
- The primary feedback photodiode current (IP1) is monitored by U1, forming a servo loop that linearizes LED current
- The secondary photodiode current (IP2) is optically coupled and proportional to IP1
- U2 converts IP2 into an output voltage using R2
Due to matched photodiodes, transfer gain is set primarily by the external resistor ratio:
VOUT = (R2 / R1) * K3 * VIN
Where K3 is the photodiode transfer gain ratio specified in the datasheet, as:
K3 = K2 / K1 = (IP2 / IF) / (IP1 / IF) = IP2 / IP1
Input Signal Options (VIN)
The input node can be derived from several measurement sources:
- Voltage drop across a current shunt resistor
- Output of a high-voltage divider
This flexibility allows the same isolation topology to be used for both voltage and current
measurement.
Design Considerations
- Select precision resistors to maintain gain accuracy
- Account for K3 tolerance when calculating system error
- Ensure op amp input bias current is low relative to photodiode current
- Verify isolation voltage rating for the target application