Zero-drift amplifiers are designed to dynamically correct their offset voltage and reduce noise density, making them ideal for high-precision applications. Two common types are auto-zero and chopper amplifiers, both of which can achieve nV-level offset voltages and exhibit extremely low drift over time and temperature. Additionally, the 1/f noise, often considered a DC error, is also effectively minimized in these devices.
One of the key advantages of zero-drift amplifiers is their ability to eliminate temperature drift and 1/f noise, which are typically challenging to manage in other designs. These amplifiers also offer higher open-loop gain, power supply rejection ratio (PSRR), and common-mode rejection ratio (CMRR) compared to standard amplifiers. As a result, they produce lower total output error when used in similar configurations, making them a preferred choice for precision systems.
Zero-drift amplifiers are particularly well-suited for long-term applications that require stability over many years, such as in high-gain signal chains operating at low frequencies (<100 Hz) and with small signal amplitudes. Common applications include precision weighing systems, medical diagnostic equipment, industrial metering devices, and interfaces for infrared, bridge, and thermocouple sensors.
The operation of an auto-zero amplifier involves two clock phases. During phase A, the offset voltage of the null amplifier is measured and stored on a capacitor. In phase B, the main amplifier’s offset is measured and stored, while the previously stored offset from phase A is applied to cancel out the error during signal processing. This technique allows for continuous correction of offset and noise.
However, because of the sampling nature of the auto-zero process, these amplifiers are prone to aliasing and foldback effects, especially when dealing with higher frequency signals. At low frequencies, where noise changes slowly, subtracting two consecutive samples can lead to effective noise cancellation. But at higher frequencies, this correlation diminishes, causing some broadband noise to fold back into the baseband.
To minimize low-frequency noise, the sampling rate must be increased, but this may introduce additional charge injection. Despite this, the signal path remains simple, allowing for a relatively wide unity gain bandwidth. Overall, zero-drift amplifiers provide exceptional performance in precision and stability, making them a valuable tool for engineers working on demanding analog systems.
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