Zero-drift amplifiers are designed to dynamically eliminate their own offset voltage and reduce noise density, making them ideal for high-precision applications. Two common types are auto-zero and chopper-stabilized amplifiers, both capable of achieving sub-nanovolt offset levels and minimal drift over time and temperature. These amplifiers also effectively suppress 1/f noise, which is often treated as a DC error in traditional designs.
One of the key advantages of zero-drift amplifiers is their ability to address two major challenges in precision systems: temperature drift and 1/f noise. These issues are typically hard to mitigate using other methods. In addition, zero-drift amplifiers offer higher open-loop gain, power supply rejection ratio (PSRR), and common-mode rejection ratio (CMRR) compared to standard amplifiers. This means that, under the same configuration, they produce lower total output error, making them more reliable in critical applications.
Zero-drift amplifiers are particularly well-suited for long-term applications where stability is essential, such as systems expected to operate for over 10 years. They are also ideal for signal chains with high closed-loop gains (greater than 100) and low-frequency signals (less than 100 Hz) with small amplitudes. Common applications include precision weighing systems, medical diagnostic equipment, metering devices, and interfaces for infrared, bridge, or thermocouple sensors.
The working principle of self-stabilizing amplifiers involves a two-phase clocking mechanism. For example, in auto-zero amplifiers like the AD8538, AD8638, AD8551, and AD8571 families, the offset voltage is measured and stored during different phases. In phase A, the null amplifier’s offset is captured on capacitor CM1, while in phase B, the main amplifier’s offset is measured and stored on capacitor CM2. The stored offset from phase A is then used to cancel out the offset in phase B, resulting in a more accurate output.
This sampling and holding behavior turns the auto-zero amplifier into a sampled-data system, which can introduce aliasing and foldback effects if not properly managed. At low frequencies, the noise changes slowly, allowing for effective cancellation by subtracting two consecutive samples. However, at higher frequencies, this correlation weakens, leading to residual noise that folds back into the baseband. As a result, the in-band noise of an auto-zero amplifier can be higher than that of a standard operational amplifier.
To minimize low-frequency noise, the sampling frequency must be increased. However, this can lead to additional charge injection, which may affect performance. Despite this, the signal path remains relatively simple, consisting only of the main amplifier, which allows for a larger unity-gain bandwidth. This makes zero-drift amplifiers a powerful choice for applications requiring both accuracy and stability over time.
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