Auto-zero amplifiers solve the same problem as chopper-stabilized amps — eliminating offset voltage and 1/f noise — but through a different mechanism. Instead of modulating the signal up to a chopping frequency and back down, an auto-zero amp uses two amplifiers: a main signal path that's always active, and a nulling amplifier that periodically measures its own offset, stores the correction on a capacitor, and applies it to the main amp's offset-trim port.

The classic architecture has the nulling amp short its inputs during the "zeroing phase," sample its output offset onto a hold capacitor, then during the "amplifying phase" use that stored correction to cancel the main amp's offset. This happens at tens of kHz, fast enough that the user sees essentially DC operation with offsets in the microvolt range (often 1–5 μV) and drift below 0.05 μV/°C.

Auto-zero vs chopper — the real distinction:

• Chopper: modulates the signal itself. Produces visible ripple at the chop frequency in the output spectrum. Excellent low-frequency noise.
• Auto-zero: modulates only the correction loop, not the signal. Smoother output spectrum, but the sampling action aliases wideband noise down into the baseband, raising the noise floor slightly above what a chopper achieves.

Modern parts like the ADA4528, LTC2050, and OPA388 are auto-zero designs. The OPA388 actually combines both techniques: chopping handles the slow drift while auto-zeroing smooths out the chopper switching artifacts — you get 0.25 μV offset with no visible chop ripple.

Real-world example: A 4-wire RTD measuring temperature with 0.1 °C resolution. A Pt100 changes about 0.385 Ω/°C, and at 1 mA excitation that's 385 μV/°C — so 0.1 °C = 38.5 μV. With a standard precision op-amp at 50 μV offset and 0.5 μV/°C drift, you'd see >1 °C of measurement error across a 30 °C ambient swing. Swap in an ADA4528 (2.5 μV offset, 0.015 μV/°C drift) and the drift error becomes 0.0012 °C — completely invisible.

Design rule of thumb: the bandwidth of an auto-zero amp is roughly the clock frequency divided by π. An LTC2050 with internal clock around 3 kHz gives you only ~1 kHz usable bandwidth. Don't use auto-zero amps for audio or fast signals — they shine at DC and low frequencies (sub-100 Hz strain gauges, thermocouples, electrochemistry).

Watch for two gotchas: charge injection from the internal switches creates small voltage spikes at the input pins (use a series resistor to dampen them), and never DC-couple an auto-zero amp's output directly to another high-gain stage without a lowpass filter — the residual switching artifacts will alias in unexpected ways downstream.