The zero-drift amplifier features a unique self-correcting technique that provides ultra-low input offset voltage (Vos) for near-zero and near-zero temperature input offset voltage drift (dVos/dT) for general purpose and precision applications. TI's zero-drift topology also offers other advantages, including no 1/f noise, low-bandwidth noise, and low distortion—simplifying development complexity and reducing cost. This can be done in one of two ways; chopper or auto zero. This technical note will explain the difference between standard continuous time and zero-drift amplifiers.
Applications for zero-drift amplifiers
Zero-drift amplifiers are suitable for a wide range of general-purpose and precision applications, benefiting from the stability of the signal path. The excellent offset and drift performance of these amplifiers make them particularly useful early in the signal path, where high gain configurations and interfaces to microvolt signals are common. Common applications that benefit from this technology include precision strain gages and weight scales, current shunt measurements, thermocouples, thermopiles, and bridge sensor interfaces.
Rail-to-rail zero-drift amplifier
System performance can be optimized using standard continuous time amplifiers and system level auto-calibration mechanisms. However, this additional auto-calibration requires complex hardware and software, increasing development time, cost, and board space. Another more efficient solution is to use a zero-drift amplifier such as the OPA388.
A conventional rail-to-rail input CMOS architecture has two differential pairs; one PMOS transistor pair (blue) and one NMOS transistor pair (red). A zero-drift amplifier with rail-to-rail input operation uses the same complementary p-channel (blue) and n-channel (red) input configurations shown in Figure 1.
Figure 1. Simplified PMOS/NMOS differential pair
The results of this input architecture show some degree of crossover distortion (for more information on crossover distortion, see Zero Crossover Amplifier: Features and Benefits). However, the offset of the amplifier is corrected by its internal periodic calibration, so the magnitude of the offset variation and the crossover distortion are greatly reduced. Figure 2 shows a comparison of the offset between a standard CMOS rail-to-rail and zero-drift amplifier.
Figure 2. Comparison of CMOS and zero-drift input offset voltages
How does zero drift work?
The internal structure of the chopping zero-drift amplifier can have as many stages as a continuous-time amplifier—the main difference is that the input and output of the first stage have a set of switches that are used to invert the input signal during each calibration cycle. Figure 3 shows the first half of the cycle. In the first half of the cycle, both sets of switches are configured to flip the input signal twice, but the offset is flipped once. This keeps the input signals in phase, but the offset errors are opposite in polarity.
Figure 3. The first half of the internal structure
Figure 4 shows the second half of the cycle. Here, both sets of switches are configured to pass signals and offset errors in an unaltered manner. In fact, the input signal never changes phase and remains the same. Since the offset errors from the first clock phase and the second clock phase are opposite in polarity, the errors are averaged to zero.
Figure 4. The second half of the internal structure
A sync notch filter is used at the same switching frequency to attenuate any residual error. This principle is still valid throughout the input, output, and environmental operation of the amplifier. In essence, TI's zero-drift technology provides ultra-high performance and superior accuracy with this self-correcting mechanism.
Table 1 shows the comparison of Vos and dVos/dT for continuous time and zero-drift amplifiers. Note that the zero-drift amplifier's Vos and dVos/dT are three orders of magnitude smaller.
Auto-zeroing requires different topologies but functions similarly. The auto-zero technique has less distortion at the output. Chopping makes the broadband noise lower.
Noise in a zero-drift amplifier
Typically, a zero-drift amplifier has the lowest 1/f noise (0.1 Hz - 10 Hz). 1/f noise (also known as flicker or pink noise) is a major source of noise at low frequencies and can be detrimental to precision DC applications. The zero-drift technique uses a periodic self-correction mechanism to effectively offset slowly varying offset errors (such as temperature drift and low frequency noise).
Figure 5 shows the 1/f and wideband voltage noise spectral density of the zero-drift (red) and continuous-time (black) amplifiers. Note that the zero drift curve has no 1/f voltage noise.
Figure 5. Voltage noise comparison
Again, why choose a zero drift amplifier?
Zero-drift amplifiers provide ultra-low input offset voltage, near-zero temperature and time input offset voltage drift, and no 1/f voltage noise—these design factors are critical for general purpose and precision applications.
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