Transmit precision analog signals across a high-voltage isolation barrier. Digital isolation (ADC to isolator to DAC) added quantization noise the client couldn't tolerate. Transformer coupling doesn't pass DC.
The Solution
Linear optocouplers with servo feedback: the feedback photodiode on the transmit side tracks the LED's output and corrects for nonlinearity and aging in real time. Each channel is an independent isolated path. Matched photodiode gains keep channel-to-channel variation tight.
Results
Better than 0.1% linearity from DC through several kHz, with full galvanic isolation. No quantization artifacts, no step transitions, just a clean analog copy on the other side of the barrier.
How the servo loop works
The same light lands on two photodiodes. One stays on the input side and feeds the servo amplifier, which corrects the LED drive for nonlinearity and ageing as it happens. The matched photodiode across the barrier sees the same light, so the output follows the input with no quantisation at all.What the feedback photodiode buys. An optocoupler on its own bows by a few percent across its range, and its gain moves with temperature as the LED and the coupling age. Closing a servo loop around a second photodiode that sees the same light cancels both, keeping the error inside ±0.1% at 25°C and at 70°C, while the loop bandwidth keeps the response flat from DC through several kHz.
Have a Similar Challenge?
Precision analog isolation is part of the regular workload.
