Volume 44 – March 2010

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Ask The Applications Engineer—39
Zero-Drift Operational Amplifiers

By Reza Moghimi

What Are Zero-Drift Amplifiers?
Zero-drift amplifiers dynamically correct their offset voltage and reshape their noise density. Two commonly used types—auto-zero amplifiers and choppers—achieve nanovolt-level offsets and extremely low offset drifts due to time and temperature. The amplifier's 1/f noise is also seen as a dc error, so it is removed as well. Zero-drift amplifiers provide many benefits to designers, as temperature drift and 1/f noise, always nuisances in the system, are otherwise very difficult to eliminate. In addition, zero-drift amplifiers have higher open-loop gain, power-supply rejection, and common-mode rejection as compared to standard amplifiers; and their overall output error is less than that obtained by a standard precision amplifier in the same configuration.

What Are Good Applications for Zero-Drift Amplifiers?
Zero-drift amplifiers are used in systems with an expected design life of greater than 10 years and in signal chains that use high closed-loop gains (>100) with low-frequency (<100 Hz), low-amplitude level signals. Examples can be found in precision weigh scales, medical instrumentation, precision metrology equipment, and infrared-, bridge-, and thermopile sensor interfaces.

How Does Auto-Zeroing Work?
Auto-zero amplifiers, such as the AD8538, AD8638, AD8551, and AD8571 families, usually correct for input offset in two clock phases. During Clock Phase A, switches labeled φA are closed, while switches labeled φB are open, as shown in Figure 1. The offset voltage of the nulling amplifier is measured and stored on capacitor C_M1.

Figure 1. Phase A of auto-zero amplifier: nulling phase.

During Clock Phase B, switches labeled φB are closed, while switches labeled φA are open, as shown in Figure 2. The offset voltage of the main amplifier is measured and stored on capacitor C_M2, while the stored voltage on capacitor C_M1 adjusts for the offset of the nulling amplifier. The overall offset is then applied to the main amplifier while processing the input signal.

Figure 2. Phase B of auto-zero amplifier: auto-zero phase.

The sample-and-hold function turns auto-zero amplifiers into sampled-data systems, making them prone to aliasing and fold-back effects. At low frequencies, noise changes slowly, so the subtraction of two consecutive noise samples results in true cancellation. At higher frequencies this correlation diminishes, with subtraction errors causing wideband components to fold back into the baseband. Thus, auto-zero amplifiers have more in-band noise than standard op amps. To reduce low-frequency noise, the sampling frequency has to be increased, but this introduces additional charge injection. The signal path includes only the main amplifier, so relatively large unity-gain bandwidth can be obtained.

How Does a Chopper Work?
Figure 3 shows the block diagram design of the ADA4051 chopper amplifier, which uses a local autocorrection feedback (ACFB) loop. The main signal path includes input chopping network CHOP1, transconductance amplifier Gm1, output chopping network CHOP2, and transconductance amplifier Gm2. CHOP1 and CHOP2 modulate the initial offset and 1/f noise from Gm1 up to the chopping frequency. Transconductance amplifier Gm3 senses the modulated ripple at the output of CHOP2. Chopping network CHOP3 demodulates the ripple back to dc. All three chopping networks switch at 40 kHz. Finally, transconductance amplifier Gm4 nulls the dc component at the output of Gm1—which would otherwise appear as ripple in the overall output. The switched capacitor notch filter (SCNF) selectively suppresses the undesired offset-related ripple without disturbing the desired input signal from the overall output. It is synchronized with the chopping clock to perfectly filter out the modulated components.

Figure 3. Chopping scheme used in the ADA4051.

Can the Two Techniques Be Combined?
This is exactly what is done in a new series of amplifiers from Analog Devices. The AD8628 zero-drift amplifier, shown in Figure 4, uses both auto-zeroing and chopping to reduce the energy at the chopping frequency, while keeping the noise very low at lower frequencies. This combined technique allows wider bandwidth than was possible with conventional zero-drift amplifiers.

Figure 4. The AD8628 combines auto-zeroing with chopping to achieve wider bandwidth.

What Applications Issues Are Encountered When Using Zero-Drift Amplifiers?
Zero-drift amplifiers are composite amplifiers that use digital circuitry to dynamically correct for analog offset errors. The charge injection, clock feedthrough, intermodulation distortion, and increased overload recovery time that result from the digital switching action can cause problems within poorly designed analog circuits. The magnitude of the clock feedthrough increases with an increase in closed-loop gain or source resistance; adding a filter at the output or using a lower resistance on the noninverting input will reduce the effect. Also, the output ripple of a zero-drift amplifier increases as the input frequency gets closer to the chopping frequency.

What Happens to Signals at Frequencies Higher Than That of the Internal Clock?
Signals with frequencies greater than the auto-zero frequency can be amplified. The speed of an auto-zeroed amplifier depends on the gain-bandwidth product, which is dependent on the main amplifier, not the nulling amplifier; the auto-zero frequency gives an indication of when switching artifacts will start to occur.

What Are Some Differences Between Auto-Zeroing and Chopping?
Auto-zeroing uses sampling to correct offset, while chopping uses modulation and demodulation. Sampling causes noise to fold back into baseband, so auto-zero amplifiers have more in-band noise. To suppress noise, more current is used, so the devices typically dissipate more power. Choppers have low-frequency noise consistent with their flat-band noise but produce a large amount of energy at the chopping frequency and its harmonics. Output filtering may be required, so these amplifiers are most suitable in low-frequency applications. Typical noise characteristics of auto-zero and chopping techniques are shown in Figure 5.

Figure 5. Typical noise of various amplifier topologies vs. frequency.

When Should I Use Auto-Zero Amplifiers? When Should I Use Choppers?
Choppers are a good choice for low-power, low-frequency applications (<100 Hz), while auto-zero amplifiers are better for wideband applications. The AD8628, which combines auto-zero and chopping techniques, is ideal for applications that require low noise, no switching glitch, and wide bandwidth. Table 1 shows some of the design trade-offs.

Table 1.

What Are Some of ADI's Popular Zero-Drift Amplifiers?
Table 2 shows a sample of zero-drift amplifiers offered by ADI.

Table 2.

References

1. Bridge-Type Sensor Measurements Are Enhanced by Auto-Zeroed Instrumentation Amplifiers
2. Demystifying Auto-Zero Amplifiers—Part 1
3. Demystifying Auto-Zero Amplifiers—Part 2
4. MT-055 Tutorial, Chopper Stabilized (Auto-Zero) Precision Op Amps

Author

Reza Moghimi [reza.moghimi@analog.com] is an applications engineer in San Jose, CA. He received a BSEE from San Jose State University in 1984 and an MBA in 1990—and has also received a number of on-the-job certificates. He has worked for Raytheon Corporation, Siliconix, Inc., and Precision Monolithics, Inc. (PMI)—which was integrated with Analog Devices in 1990. At ADI, he has served in test-, product-, and project-engineering assignments. He has written many articles and design ideas—and has given presentations at technical seminars. His hobbies include travel, music, and soccer. (return to top)

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