Page 11 - EngineerIT November 2022

ELECTRONICS DESIGN

Chopping artifacts 1-3,5-7
Although chopping works well to remove unwanted offset, drift
and 1/f noise, it produces unwanted AC artifacts such as output
ripple and glitches. Analog Devices’ recent zero-drift products
have taken steps to make the magnitude of these artifacts smaller
and located at higher frequencies, which makes 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 odd
harmonics of the chopping frequency. Amplifier designers employ
Figure 3: A basic auto-zero amplifier.
many methods to reduce the effects of ripple, including:
input offset voltage and noise. Note that the amplifier is • Production offset trimming: The nominal offset can be
unavailable for signal amplification during this phase. For an significantly reduced by performing a one-time initial trim, but
auto-zeroed amplifier to operate in a continuous manner, the offset drift and 1/f noise remain.
two identical channels must be interleaved. This is called • Combining chopping and auto-zeroing: The amplifier is first
ping-pong auto-zeroing. auto-zeroed, then chopped to upmodulate the increased noise
During the amplification phase, ϕ 2, the input is spectral density (NSD) to a higher frequency. Figure 4 shows the
connected back to the signal path and the amplifier is resulting noise spectrum after chopping and auto-zeroing.
again available for amplifying the signal. The low frequency • Autocorrection feedback (ACFB): A local feedback loop can be
noise, offset and drift are cancelled by auto-zeroing, and used to sense the modulated ripple at the output and null out
the remaining error is the difference between the current the low frequency errors at their source.
value and previous sample of the errors. Because low
frequency error sources do not change much from ϕ 1 to ϕ 2, Glitch artifact
this subtraction works well. High frequency noise, on the Glitches are transient spikes that are caused by charge injection
other hand, is aliased down to baseband and results in an mismatch from the chopping switches. The magnitude of these
increased white noise floor as shown in Figure 4. glitches depends on many factors including source impedance and
1
Due to the noise folding and the need for an additional the amount of charge mismatch. The glitch spikes not only cause
channel for continuous operation, chopping can be a more artifacts at the even harmonics of the chop frequency but also
power efficient zero-drift technique for standalone op create a residual DC offset, which is proportional to the chopping
amps. 2 frequency. Figure 5 (left) illustrates what these spikes look like

Figure 4: Noise PSD: before chop or AZ, after AZ, after chop, and after chop and AZ.

Figure 5: (L) Glitch voltage from charge injection at V1 (inside the chopping switches) and V2 (outside the chopping switches) in Figure 1; (R) glitches
caused by finite amplifier bandwidth at V1 and V2 in Figure 1.

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