When you need to measure a voltage that sits on a different ground than your ADC — say, monitoring a high-side current shunt on a 400V motor drive, or recording ECG signals from a patient — you can't just wire it directly. Common-mode voltages will destroy your op-amp, and ground loops will inject 60 Hz hum into your measurement. The fix is an isolation amplifier: a device that transfers analog signals across a barrier with no ohmic connection.

There are three common isolation mechanisms:

• Transformer-coupled (e.g., AD215): The signal modulates a carrier, crosses a tiny transformer, and gets demodulated. Robust, but bulky and expensive.
• Optical (e.g., HCNR201, IL300): A matched LED-photodiode pair on each side of the barrier. Excellent linearity when used with feedback, but slower (typ. 100 kHz).
• Capacitive (e.g., AMC1300, ISO224): A modulator chops the signal across tiny on-chip caps. This is the modern winner — small, fast (up to MHz), and cheap.

The critical specs to understand:

• Working voltage (V_IOWM): continuous voltage the barrier can withstand. The AMC1300B is rated 1414 V_peak reinforced — enough for 800V DC bus monitoring.
• Transient overvoltage (V_IOTM): short-duration surge rating, typically 7-10 kV.
• Common-mode transient immunity (CMTI): how fast the input can swing without coupling to the output. Need >50 kV/μs for IGBT gate-area work.

Real-world example: motor current sensing. Put a 5 mΩ shunt in the low-side of a 100A motor drive. The shunt sits on a switching node that bounces between 0V and 400V at 20 kHz. Wire the shunt across an AMC1300's ±250 mV input. The chip outputs a differential signal (Vref ± gain × Vin) referenced to the clean MCU side. Now your ADC sees a quiet 0-3.3V signal isolated from the killer transients.

Power isolation matters too. The "hot" side needs its own supply. Either use a tiny isolated DC-DC converter (e.g., Murata MEU1), or pick an integrated solution like the AMC1300 + ISOW7841 combo that bundles signal and power isolation in one package.

Rule of thumb for working voltage derating: Take the datasheet V_IOWM and divide by 2 for design margin. A "1500V" part is really good for ~750V continuous in a noisy environment. Always verify the safety standard (UL 1577, IEC 60747-17 reinforced) matches your application's certification needs.