Cover story
How to use zero-drift amplifiers in wider bandwidth applications
By Simon Basilico, Design Engineer, Analog Devices Z
ero-drift operational amplifiers use chopping, auto-zeroing, or a combination of both techniques
to remove unwanted low frequency error sources like offset and 1/f noise. Traditionally, these amplifiers have only been used in low bandwidth applications since these dynamic techniques produce artifacts at higher frequencies. Wider bandwidth solutions can also benefit from zero-drift op-amps’ excellent DC performance as long as high frequency errors such as ripple, glitches, and intermodulation distortion (IMD) are considered in the system design. Chopping is a zero-drift technique that uses modulation to separate offset and low-frequency noise from the signal by modulating the errors to higher frequencies; see Figure 1. Similarly, in the frequency domain, the input signal (Figure 2, blue signal) is (b) modulated to the chopping frequency, processed by the gain stage at fCHOP
,
(c) demodulated at the output back to DC and, finally, (d) passed through the low-pass filter (LPF). The offset and noise sources (Figure 2, red signal) of the amplifier are processed at DC through the gain stage, (c) modulated to fCHOP
by the
output chop switches, and (d) filtered by the LPF. Since square wave modulation is employed, the modulation occurs around odd multiples of the modulation
frequency.There is residual error due to the modulated noise and offset since the LPF is not an ideal brick wall.
Auto-zero background A second zero-drift technique is auto- zeroing, a dynamic correction technique that samples and subtracts low frequency error sources in an amplifier; see Figure 3. During the auto-zero phase, the circuit’s input is shorted to a common voltage and the auto-zero capacitor samples the input offset voltage and noise. The amplifier is unavailable for signal amplification during this phase. For an auto-zeroed amplifier
Figure 2: Frequency domain spectrum of the signal (blue) and errors (red) at (a) input, (b) V1, (c) V2 and (d) VOUT
to operate in a continuous manner, two identical channels must be interleaved, called “ping-pong auto-zeroing”. During the amplification phase, the input is connected back to the signal path and the amplifier is again available for amplifying the signal. The low-frequency noise, offset and drift are cancelled by auto-zeroing, and the remaining error is the difference between the current value and previous sample of the errors. High- frequency noise, on the other hand, is aliased down to baseband and results in an increased white noise floor as shown in Figure 4. Due to the noise folding and the need for an additional channel for
continuous operation, chopping can be a more power efficient zero-drift technique for standalone op-amps. However, although chopping works well
to remove unwanted offset, drift, and 1/f noise, it produces unwanted AC artifacts such as output ripple and glitches. In its recent zero-drift products, Analog Devices has made the magnitude of these artifacts smaller and located at higher frequencies, making filtering easier at the system level.
Ripple artifact
Ripple is a basic consequence of the chopping modulation technique that moves these low frequency errors to
www.electronicsworld.co.uk May 2023 05
Figure 1: Time domain waveforms of the signal (blue) and errors (red) at (a) input, (b) V1, (c) V2, and (d) VOUT
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