Page 10 - EngineerIT November 2022

ELECTRONICS DESIGN

How to use zero-drift amplifiers in

wider bandwidth applications

By Simon Basilico, Design Engineer

This article provides a short description of chopping, auto-zeroing, and the sources of zero-drift

artifacts, including a summary of some techniques that amplifier designers can use to reduce their
impact. It also explains how to minimise the impact of these residual AC artifacts in a precision signal
chain, including matching input source impedance, filtering and frequency planning.

Introduction Similarly, in the frequency domain, the input signal (Figure
Zero-drift operational amplifiers use chopping, auto-zeroing, 2, blue signal) is (b) modulated to the chopping frequency,
or a combination of both techniques to remove unwanted low processed by the gain stage at f CHOP, (c) demodulated at the
frequency error sources like offset and 1/f noise. Traditionally, output back to DC, and finally (d) passed through the LPF. The
these amplifiers have only been used in low bandwidth offset and noise sources (Figure 2, red signal) of the amplifier
applications, since these dynamic techniques produce artifacts are processed at DC through the gain stage, (c) modulated to
at higher frequencies. Wider bandwidth solutions can also f CHOP by the output chop switches, and finally (d) filtered by the
benefit from zero-drift op amps’ excellent DC performance LPF. Since square wave modulation is employed, the modulation
as long as high frequency errors such as ripple, glitches, and occurs around odd multiples of the modulation frequency.
intermodulation distortion (IMD) are considered in the system As can been seen in both the frequency and time domain
design. illustrations, there will be some residual error due to the
modulated noise and offset since the LPF is not an ideal brick
ZERO-DRIFT TECHNIQUES wall.
Chopping background 1-7
The first zero-drift technique, chopping, uses modulation to
separate offset and low frequency noise from the signal content
by modulating the errors to higher frequencies.
Figure 1 shows (b) how chopping modulates the input signal
(blue waveform) to a square wave, processes that signal in the
amplifier, then (c) demodulates the signal at the output back to
DC. At the same time, the low frequency errors (red waveform)
in the amplifier are (c) modulated at the output to a square
wave, which is then (d) filtered by the low-pass filter (LPF).
Figure 2: Frequency domain spectrum of the signal (blue) and errors (red)
at (a) input, (b) V1, (c) V2, and (d) V OUT.

Auto-Zero background 1-3,5-7
A second zero-drift technique, auto-zeroing, is also a dynamic
correction technique that works by sampling and subtracting
low frequency error sources in an amplifier.
Figure 3 shows an example of a basic auto-zero amplifier.
It consists of an amplifier with offset and noise, switches to
reconfigure the input and output, and an auto-zero sampling
capacitor.
During the auto-zero phase, ϕ 1, the circuit’s input is shorted
Figure 1: Time domain waveforms of the signal (blue) and errors (red) at
(a) input, (b) V1, (c) V2, and (d) V OUT. to a common voltage and the auto-zero capacitor samples the

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