Find out how to decrease output noise by a factor of two by simply paralleling four amplifiers of a quad auto-zero amplifier.
Autozero amplifiers have almost zero drift and input-offset values of 1 to 20µV. 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 10µV 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 0.5µV p-p at 0.1 to 10Hz. If your application requires zero drift and low output noise, you can use the circuit in the figure. 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.
Figure: Use this circuit when your application requires zero drift and low output noise.
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 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.
Reference"AD8630 Quad, Zero Drift, Single-Supply, Rail-to-Rail Operational Amplifier," Analog Devices Inc.
This article is a Design Idea selected for re-publication by the editors. It was first published on June 7, 2007 in EDN.com.