Autozero amplifiers have almost zero drift and input-offset values of 1 to 20 µV. You can compensate for the initial voltage offset of an auto-zero amp in sensitive circuits, such as dc amplifiers and integrators, requiring the processing of voltages of 10 µV to 1 mV. Total compensation down to an offset of 0V, however, is an illusion because residual low-frequency output noise is still present in any autozero amp.

The Analog Devices AD8628 autozero amp has a low-frequency-noise value of 0.5 µV p-p at 0.1 to 10 Hz. If your application requires zero drift and low output noise, you can use the circuit in Figure 1. A quad autozero amp develops a gain of almost 1000. The resistor network comprising the R₃ resistors averages the output signals of these amplifiers to create the final output voltage.

The quad autozero amps are the four sections of IC₁, an Analog Devices AD8630 (Reference 1). Quad integrated resistors having one common lead can substitute for the four R₃ resistors. The R₁ and R₂ resistors should be high-quality, precision, film devices with 0.5% or less tolerance. The tolerance of the R₃ resistors should not exceed 1%. The basis for decreasing the circuit’s noise at the output in comparison with a single amplifier of IC₁ is the principle of averaging the signals containing the same deterministic component of random noise. If you assume that the amplifiers of IC₁ represent independent or uncorrelated noise sources that obey the gaussian distribution, then the standard deviation of the average of noise outputs of these sections is:

where σ_X and σ_Y are the standard deviations of noise signals at outputs of the single respective amplifiers. If σ_X=σ_Y—an assumption that you can make without hesitation because the op amps reside in one chip—then:

If you average four amplifiers, you obtain:

If the value of output resistance of the circuit, which is about R₃/4≈38Ω, is too high for your application, place a voltage follower between the output terminal and the next stage.