How to achieve chopper stabilization in amplifier design?

“Devices such as accelerometers, angular velocity sensors, and Hall sensors commonly associated with Internet of Things (IoT) applications require efficient post-processing when converting from analog input signals to digital signals. A voltage-to-frequency converter is the solution here, and a key part of the voltage-to-frequency conversion is the precision op amp.

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Author: STEVE TARANOVICH

Since the introduction of semiconductor amplifiers, analog and mixed-signal designers have been challenged with amplifier 1/f noise and DC offset and drift in their Circuit designs. This blog aims to provide designers with the basics on how to implement chopper stabilization in their designs.

According to Jim Williams of Linear Technology, the chopper stabilization method was developed by EA Goldberg in 1948 and uses the input of an amplifier to Amplitude modulation of the AC carrier. This carrier is amplified and synchronously demodulated back to DC and provides the output of the amplifier. Since the DC input is converted to an AC signal and amplified to an AC signal, the DC term of the amplifier does not affect the overall drift. Therefore, chopper-stabilized amplifiers can achieve lower drift over time and temperature than traditional differential types.

Figure 1 Switches implement modulation in a classic chopper-stabilized op amp.Source: Analog Devices

As shown in Figure 1, the switch performs modulation so that the input is 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 This figure shows an improved auto-zero or chopper-stabilized amplifier.Source: Analog Devices

Here, A1 is the main amplifier, where the input signal is always connected to the output, and A2 performs the auto-zeroing amplifier function.

Do’s and Don’ts

Care should be taken to avoid resonance with capacitive loads

Designers need to understand the complex output impedance (Z0) of the op amp and its interaction with capacitive loads. Once the op amp is compensated, the application circuit becomes stable.

Avoid 1/f noise

One way to avoid 1/f noise is to modulate the signal into a region without 1/f noise and then demodulate it. This method, known as chopper stabilization, has been used for many years to move 1/f noise to another frequency band where it can be filtered out. Zero-drift op amps take advantage of this approach to achieve noise levels approaching 100 nV pp (16 nV rms) from 0.1 Hz to 10 Hz, which is primarily due to white noise.

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

However, today’s new generation of chopper amplifiers are useful in a wider range of applications. They have strong offset voltage stability, have virtually no flicker noise, and have performance very close to standard op amps.

Design examples in the real world

Devices such as accelerometers, angular velocity sensors, and Hall sensors commonly associated with Internet of Things (IoT) applications require efficient post-processing when converting from analog input signals to digital signals. A voltage-to-frequency converter is the 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 continuously correct for low-frequency errors across the amplifier’s input.

Chopping op amps are commonly used in industrial and instrumentation applications, especially where low operating power is required. Together with a suitable ADC, reliable performance up to 24-bit precision can be achieved. Typical applications might be chopper-stabilized op amps, precision voltage or current sources used as buffers, or front-end gain amplifiers in sensor applications, or both.

Another common application is where 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 inputs of high-end sigma-delta ADCs often need to be buffered to prevent them from interfering with sensor performance.

A chopper-stabilized amplifier is ideal for this buffer here, as conventional instrument topologies cannot meet noise, voltage offset (VOS), or drift specifications. A separate reference voltage does not normally drive the pressure sensor bridge. Its output must be buffered to ensure that the effective voltage across the bridge sensor remains stable over temperature and time.

The new generation of choppers is much quieter than the earlier ones. Modern choppers contain a switched capacitor filter with multiple notches aligned with the chopping frequency and its odd harmonics. In the frequency domain, this produces a sinc(x) or sin(x)/x filter response whose zeros are precisely aligned with the fundamental and all harmonics of the triangle wave (Figure 3).

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

Since 1/f or flicker noise is just a slow offset voltage that varies over time, the chopper can remove much of this increased noise spectral density in the low frequency range. The chopping action shifts the baseband signal to the chopping frequency, which is well beyond the 1/f region of the input stage. Therefore, the noise spectral density in the low frequency signal range of the chopper amplifier is equal to the noise spectral density in the high frequency range of the amplifier.