Analog Iso
An isolation amplifier delivers a low-distortion analog output that tracks an isolated input voltage. The AD215 and ISO124 are textbook ±10 V devices; modern parts like the AMC1311 carry small-signal voltages from 0–2 V (a shunt's drop) across a kilovolt barrier.
On the input side, the analog voltage is converted to a frequency-modulated or sigma-delta bitstream that crosses the barrier through tiny transformer coils or capacitors fabricated on the IC. The output side demodulates the stream back to an analog voltage with care taken to match the input's gain to within ±0.1 %. Because the encoding is digital across the barrier, low-frequency noise and offset don't leak across.
In plain terms
A pair of identical pendulums on a soundproofed wall: push one and the other swings with exactly the same arc, but no rod, gear, or wire crosses the wall.
Why designers use it
- Sense a current-shunt voltage on the high-side of a 600 V motor drive without a Hall sensor.
- Read patient ECG voltages while floating the front-end thousands of volts from mains earth.
- Measure battery-cell voltages in a high-voltage stack, where each cell's reference floats hundreds of volts above ground.
- Interface a strain gauge in a high-EMI factory environment to a controller without ground-loop hum.
Best for
- Shunt sensing
- Patient ECG
- HV stacks
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- For audio-quality fidelity where 0.001 % THD is required — isolation amps typically deliver 0.05–0.1 %; use a transformer-coupled approach with active-feedback for better.
- For DC-only bias measurements where simpler chopper amplifiers without isolation would suffice.
Common mistakes
- Confusing input-input common-mode rating with input-output (barrier) rating; a 250 V CMRR doesn't mean the part survives 1 kV across the barrier.
- Using a single-supply on the input side when the input voltage swings below ground — the encoder's modulator clips and the output flat-lines mysteriously.
Where you will find it
- An illustrative measurement system can transfer an analog value across an isolation boundary using a suitably rated device.
- Working voltage, transient behavior, accuracy and the complete insulation system require separate verification. An isolation component alone does not establish patient protection.
A short history
Analog isolation amplifiers grew out of the need to safely interface medical instrumentation to patients while eliminating ground loops and common-mode noise. Early optically coupled linear amplifiers appeared in the late 1960s; Burr-Brown (later acquired by Texas Instruments) pioneered integrated isolation amplifiers in the 1970s using modulation-demodulation around a miniature transformer barrier. Modern analog isolators use capacitive or magnetic coupling on a silicon die, achieving bandwidths above 1 MHz while providing kV-class isolation. They remain essential in industrial sensors, motor drives, and medical equipment where hazardous ground potentials must be blocked.