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Auto-zero amplifiers, also known as chopper-stabilized amplifiers, have been around for decades and continue to evolve. In a modern version, as shown in Figure 10-23, two amplifiers are used to check up on each other.
Figure 10-23. Auto-zero op amp.
Each amplifier has a "trim" input—a single node that changes its offset voltage in both directions. A built-in oscillator flips the two switches periodically at a rate of a few hundred to a few thousand Hz.
With the switches set to position A, the inputs of Amplifier 2 are shorted together. Its offset voltage is amplified by the open-loop gain and corrected by feeding the output to the trim input. The required trim voltage is stored in capacitor Ca.
With the switches set to position B, the inputs of Amplifier 2 are connected in parallel to those of Amplifier 1. Its output now feeds Amplifier 1’s trim input. With the charge remaining across Ca, Amplifier 2 continues to be nulled. In this way, it corrects the offset of Amplifier 1.
As the oscillator switches back to phase A, this correction voltage remains across capacitor Cb. Due to the high open-loop gains of both amplifiers, the offset voltage is now reduced to microvolts. Because the correction is done repeatedly, temperature drift is also considerably reduced.
There’s an additional benefit: anything sensed by Amplifier 2 below the switching frequency is treated as an offset. This includes flicker (1/f) noise, which is completely eliminated. Above the switching frequency, the behavior of the auto-zero amplifier is identical to a regular op amp.
Apart from higher current consumption from the additional circuitry, the main drawback of an auto-zero op amp is switching noise. At the switching frequency, there’s a noise peak that also causes (intermodulation) distortion. This effect can be ameliorated by changing the switching frequency at random—in other words, creating a spread spectrum.
