In addition to the initial input offset voltage, other factors can also affect the behavior of this error voltage, such as common-mode voltage, operating voltage, output voltage, temperature, and even time.
Depending on the application, these external factors can determine the best amplifier architecture for each design.
EPROM Tuning:
Some operational amplifiers use non-volatile EPROM fuses to correct the input offset voltage. In many cases, this is done within the package during the final testing stage and is a very economical way to achieve an amplifier with a low initial offset voltage. Since the amplifier is tuned after assembly, any assembly-related offsets can be corrected. Another advantage of this architecture is that the amplifier is tuned by the manufacturer and requires no adjustment by the customer. The drawback is that the EPROM fuse occupies space on the chip, making it difficult to fit EPROM-tuned devices into very compact packages. Like general-purpose amplifiers, this architecture will be sensitive to environmental factors such as temperature, as well as changes in common-mode and operating voltages.
Laser Tuning
Another method often used to increase the accuracy of an operational amplifier is laser tuning. In this process, a laser is used to adjust the resistance value of thin-film resistors integrated into the silicon wafer. The accuracy of this technique can be relatively high because the tuning process is continuous, unlike the series of discrete steps in EPROM tuning. Another advantage is that thin-film resistors are inherently more temperature-stable, thus increasing the amplifier's accuracy over a wide temperature range.
However, laser tuning must be performed at the wafer level and cannot be done within the device package. The processes of cutting the wafer into individual chips, placing the chip into the package, and connecting the chip to the package pins can cause mechanical stress on the wafer, negatively impacting the overall accuracy of the device. These assembly-related changes cannot be evaluated in laser-tuned amplifiers and therefore introduce error into the amplifier.
Similar to non-volatile EPROM fuses, laser tuning is performed only once during the device manufacturing phase, with no possibility of readjusting the device. Changes in external operating conditions, such as temperature and operating voltage fluctuations, will adversely affect the amplifier's accuracy and can directly impact the overall design performance.
Operational Amplifiers with Auto-Zeroing:
The auto-zero architecture is a continuous self-correction architecture that uses a null amplifier to correct the main amplifier's offset voltage. This architecture achieves an ultra-low offset error that can be 100 times better than that of an amplifier with EPROM tuning. It also achieves low offset drift and eliminates 1/f noise while providing superior power supply and common-mode rejection. Because this architecture automatically and continuously corrects the input offset voltage, it is inherently insensitive to the environment. Changes due to temperature and aging, as well as changes in operating and common-mode voltage, will have very little effect on the accuracy of an auto-zero amplifier. Since the self-correction circuitry is integrated into the circuit, no custom input is required. From a system perspective, an operational amplifier with self-zeroing, such as the MCP6V0 shown in Figure 1, looks and functions the same as a standard operational amplifier and adds the advantage of exceptional performance.
Despite all these advantages, the self-zeroing architecture with self-correction has some limitations. The continuous switching of the internal correction circuitry generates switching noise and also results in a higher quiescent current for a given bandwidth. Finally, due to the ultra-high accuracy of this type of device, the verification time can be relatively long, making it more expensive to manufacture.
Integrated Calibration
Another alternative is to use a high-precision operational amplifier with an integrated calibration circuit. Microchip's mCal calibration technology allows operational amplifiers to achieve an initial offset voltage as low as other architectures, but unlike amplifiers with EPROM or laser tuning, calibration is active as soon as power is connected or via an external calibration pin. This allows the user to recalibrate the amplifier whenever desired.
Frequent recalibration can make the amplifier's accuracy insensitive to the environment. For example, if a customer is very concerned about temperature drift, the drift error can be minimized by recalibrating the device every time the temperature changes by five degrees. While this solution can significantly reduce amplifier temperature drift, it also requires the user to actively initiate a calibration routine by connecting the calibration pin to the amplifier.
Conclusions:
Many applications can benefit from higher-precision operational amplifiers, but to choose the right amplifier, designers must understand the strengths and weaknesses of each architecture used to achieve low offset. While all the architectures described above achieve low initial offset voltage, environmental conditions can also affect amplifier accuracy. Using an amplifier with a continuous self-correcting architecture, such as an amplifier with automatic zeroing or one with recalibration capability using mCal technology, minimizes the adverse effects of external conditions. Table 1 outlines the factors to consider when evaluating which amplifier architecture is best for a given application.
