The essence of chopper stabilization in amplifier designs

Analog and mixed-signal designers have been confronting amplifier 1/f noise as well as DC offset and drift challenges in their circuit designs since the time of the introduction of semiconductor amplifiers. This blog aims to provide designers with a basic understanding of how to implement chopper stabilization into their designs.

According to Linear Technology’s Jim Williams, the chopper-stabilized approach was developed by E. A. Goldberg in 1948, and it used the amplifier’s input to amplitude modulate an AC carrier. That carrier was amplified and synchronously demodulated back to DC and furnished the amplifier’s output. Since the DC input is translated to and amplified as an AC signal, the amplifier’s DC terms do not affect on the overall drift. That’s why chopper-stabilized amplifiers can achieve significantly lower time and temperature drifts than classic differential types.

Figure 1 Switches implement the modulation in classic chopper-stabilized op-amps. Source: Analog Devices Inc.

As shown in Figure 1, the switches implement the modulation, causing the input to be multiplied by a square wave at the chopping frequency. As a result, the amplifier will only pass low frequencies due to the input anti-aliasing filter.

Now let’s look at an improved auto-zero or chopper-stabilized amplifier (Figure 2).

Figure 2 The diagram shows an improved auto-zero or chopper-stabilized amplifier. Source: Analog Devices Inc.

Here, A1 is the main amplifier in which the input signal is always connected to the output, and A2 performs the nulling auto-zero amplifier function.

Do’s and don’ts

Do pay attention to avoiding resonance with a capacitive load

Designers need to understand the op-amp’s complex output impedance (Z0) and its interaction with a capacitive load. Once compensated for the op-amp, an application circuit will get stable.

Do avoid 1/f noise

One way to avoid 1/f noise is to modulate the signal to a region with no 1/f noise and then demodulate it. This method, known as chopper stabilization, has been used for many years to shift the 1/f noise to a different frequency band, where it can be filtered out. Zero-drift op-amps take advantage of this method to get a noise level close to 100 nV p-p (16 nV rms) from 0.1 Hz to 10 Hz, mostly caused by white noise.

Don’t assume that modern chopper op-amps eliminate the need for standard op-amps

However, a new generation of chopper amplifiers is useful today in a much wider range of applications. They provide robust offset voltage stability, virtually no flicker noise, and very close behavior of a standard op-amp.

Real-world design examples

Devices like accelerometers, angular velocity sensors, and Hall sensors commonly associated with the Internet of Things (IoT) applications need efficient post-processing in converting from the analog input signal to a digital signal. Voltage-to-frequency converters are a solution here, and a key part of the voltage-to-frequency conversion is the precision op-amp.

Chopper-stabilized op-amps are precision op-amps that constantly correct low-frequency errors across the amplifier inputs.

Chopper op-amps are generally used in industrial and instrumentation applications, especially when low operating power is required. Coupled with a suitable ADC, reliable performance with up to 24-bit precision is achievable. A typical application might be a chopper-stabilized op-amp used as a buffered precision voltage or current source, as a front-end gain amplifier in a sensor application, or even in both roles.

Another common application is when the output of a pressure-sensing bridge can be digitized using a high-precision 24-bit sigma-delta ADC. The challenge for designers is that the differential input of a high-end sigma-delta ADC often needs to be buffered to prevent it from interfering with the sensor performance.

Here, a chopper-stabilized amp is attractive for use as that buffer because conventional instrumentation topologies cannot meet the noise, voltage offset (VOS), or drift spec requirements. The voltage reference alone typically won’t drive a pressure-sensor bridge. Its output must be buffered to guarantee that the active voltage to the bridge sensor is stable over temperature and time.

A new generation of choppers is far quieter than the early predecessors. Modern choppers incorporate a switched-capacitor filter with multiple notches aligned with the chopping frequency and its odd harmonics. In the frequency domain, this creates a sinc(x) or sin(x)/x filter response with nulls that precisely align with the fundamental and all harmonics of the triangle wave (Figure 3).

Figure 3 The input stage of a chopper op-amp’s filter response shows how a new generation of chopper op-amps incorporates a switched-capacitor filter with multiple notches aligned with the chopping frequency and harmonics. Source: Texas Instruments

Since 1/f or flicker noise is just a slow time-varying offset voltage, choppers can eliminate most of this increased noise-spectral density in the low-frequency range. The chopping action will shift the baseband signal to the chopping frequency, which is well beyond the input stage’s 1/f region. So, the low-frequency signal range of the chopper amp has a noise-spectral density equal to that of the amplifier’s high-frequency range.

See the articles below for more in-depth knowledge of chopper-stabilized amplifiers.

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